Chapter 3: Economizer and Mixed Air

3.1. Theory and Applications

3.1.1. Minimum Outdoor Air

3.1.2. Economizer Free Cooling

3.1.3. Building Pressure Control and Return Air Heat Recovery

3.2. Commissioning the Economizer and Mixed Air Section

3.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

3.3. Testing Guidance and Sample Test Forms

3.4. Typical Problems

3.4.1. Control Loop Instability

3.4.1.1. Damper Oversizing

3.4.1.2. High Turn Down Ratio

3.4.2. Poor Mixing

3.4.3. Excess Minimum Outside Air

3.4.4. Terminating the Economizer Cycle

3.4.5. Setting Damper Interlocks

3.4.6. Nuisance Freezestat Trips

3.4.7. Coil Freeze-ups

3.5. Economizer Control Strategies

3.5.1. Operating Control

3.5.2. Operational Interlocks

3.5.3. Limit Control

3.5.4. Safety Interlocks

3.5.4.1. Freezestat Control Sequences

3.5.5. Static Pressure Switches

3.5.6. Ambient Condition Interlocks

3.5.6.1. Outdoor Temperature Based Interlock

3.5.6.2. Enthalpy Based Interlock

3.5.7. Alarms

3.6. Supplemental Information

 

Figures

Figure 3.1: Air-side economizer

Figure 3.2: Economizer Operating Curves (75°F Return Air)

Figure 3.3: Mixed Air Stratification Carried Through a DWDI Fan

Figure 3.4: Mixed air temperature vs. outdoor air temperature

Figure 3.5: Typical damper limit switch installation

Figure 3.6: Typical static pressure safety switch

Figure 3.7: Cooling minimum outdoor air vs. 100% outdoor air

Figure 3.8: Determining temperature-based economizer changeover

 

3.1. Theory and Applications

Outdoor air that comes into a building generally falls into three categories:

·       Air that is brought in for ventilation/indoor air quality and make up air.

·       Air that is brought in by the economizer cycle to meet cooling loads.

·       Air that is brought in to pressurize the building and control infiltration.

The economizer and mixing section includes the outdoor air dampers, return air dampers, and mixing box as shown in Figure 3.1.   These components provide an important energy conservation function by allowing the system to mix return air with outdoor air to minimize mechanical cooling and heating.

Figure 3.1: Air-side economizer

The design, performance, and operation of the economizer cycle are directly related to the overall make-up and exhaust flow pattern for the system.  Therefore, it is necessary to consider the economizer cycle in the context of the requirements for minimum outdoor airflow and exhaust airflow.  The minimum outside air is intended to provide ventilation to ensure satisfactory indoor air quality (IAQ) and make up for any exhaust that is taken from the space by process functions like a lab exhaust hood.  The minimum outdoor air requirement often includes an additional component to provide for building pressurization. 

The economizer cycle operates if there is a cooling load and the outdoor air temperatures are low enough.  Extra outside air is brought in instead of running refrigeration equipment to cool the mix of return air and minimum outdoor air.   

If outdoor air was simply introduced into the building without regard for how it would be removed, then there would be a tendency for the supply fan system to pressurize the building.  Eventually, the pressure in the building would become so high that the doors would be blown open or the flow of outdoor air into the building would be restricted.  While these problems seem obvious, they are often not addressed by the design documents or are misinterpreted in the field, resulting in commissioning and operational problems.  Problems with bringing in outdoor air fall into the following categories:

·       Pressure relationship problems between various spaces in the building.

·       Building pressurization problems.

·       Temperature control problems when operating in the economizer mode.

The additional outdoor air that is brought in by the economizer cycle for temperature control purposes is generally removed from the building by some sort of relief system.  Relief systems are discussed in 3.1.3 Building Pressure Control. 

Despite the significant energy savings that can be achieved by proper application of economizers, many economizer sections never achieve their design intent in the real world operating environment.  Thus, proper functional testing and adjustment of the economizer and mixed air section is essential to achieving design intent, efficient operation, and good indoor air quality.  Common economizer problems are presented in Section 3.4

While simple in concept, the successful operation of an economizer is dependent upon the dynamic interaction of a variety of components, all of which must be properly designed, installed, and adjusted.  The information in this chapter promotes successful economizer systems by providing technical information about the process and its components as well as information about functional testing. 

The following subsections give more detail on the three main functions of the economizer and mixed air section operation:

·       Minimum Outdoor Air

·       Economizer Free Cooling

·       Building Pressure Control and Return Air Heat Recovery

3.1.1. Minimum Outdoor Air

As stated previously, minimum outdoor airflow must be introduced in most air handling systems to ensure good IAQ and to pressurize the building.   In general, this flow rate should match or exceed the amount of exhaust taken from the area served by the system unless the area served is required to operate at a negative pressure relationship relative to the surrounding areas.  Building pressure control functions are discussed further in Section 3.1.3

For energy efficiency, the system must be set up to only ventilate as necessary for the real occupant load and only supply air as cool as necessary for the worst case cooling requirement.  The flow required to meet the actual load is often below the design minimum flow setting when the minimum flow rate is based on an overestimation of the number of occupants.  With excess ventilation air, the air handling system serves an unnecessary heating load during cold outside conditions and an unnecessary cooling load during hot and humid conditions. 

The following approaches can be used to provide minimum outside airflow, sometimes with varying degrees of success:

·       Provide a Limit Signal for Minimum Outdoor Air:  In this arrangement, the minimum outdoor airflow is often neglected or is set based on a percentage of the output signal to the outdoor air damper.  For instance, if the system is designed for 20% minimum outdoor air, then a minimum position signal equal to 20% of the actuator span is sent to the economizer dampers.  This method assumes a linear relationship between actuator stroke and airflow, which is often a bad assumption, especially if the dampers are oversized. The two most likely problems are: 

1   The minimum flow may be much higher than required because of the non-linear relationship between flow and damper stroke.  Review the damper sizing curves depicted in Figure 3.10, Section 3.6.1.2.  A high minimum flow wastes energy due to treating excessive quantities of outdoor air.

2   The minimum flow is not positively set (the amount of outside air is not regulated).  Older or lower quality dampers may still provide 5-10% minimum outdoor air when they are closed due to leakage, but newer, low leakage dampers may not be counted on to provide this air.  Without adequate minimum outdoor airflow, the building can experience indoor air quality problems (IAQ) and/or problems with pressure relationships.

The limit signal approach can work for constant volume systems where the economizer dampers have been properly sized and the pressure relationships tend to remain fixed.  The approach may not provide the required flow under all operating conditions in systems where the pressure and flow relationships vary with load, like a VAV system.  As the load decreases, less outdoor air may be brought into the system as the VAV system turns down, depending on the pressure in the mixed air plenum.  If the load change is not proportional to the occupancy change, then inadequate minimum outside air may result from the VAV system turn down.  In any case, it is critical that the system be set up to provide the required minimum outdoor air flow rate and then maintained in a manner that ensures this.  The commissioning process can pay a key role by:

1   Functionally testing and coordinating with the balancer at start-up to ensure proper minimum outdoor air flow rates and building pressure relationships.

2   Training the operating staff to help them understand the initial settings and ensure their persistence.

3   Document the initial settings as well as the procedure used to obtain them, thereby further ensuring their persistence.

·       Provide an Independent Non-Regulated Minimum Outdoor Air Damper: This approach is an improvement over a limit signal because an independent damper dedicated to the minimum outdoor air function can provide more reliable outside air regulation.  Typically the damper is interlocked to open when the unit is in operation and in an occupied cycle and closed when the unit is shut down. 

Achieving the desired flow rate involves an effort on the part of the start-up team to measure and adjust the system to meet the design requirements.  Sometimes, an independent manual balancing damper is provided in series with the automatic minimum outdoor air damper so that it can be used to set the flow while the automatic damper provides the on/off function.  In other instances, the required flow is achieved by adjusting the automatic damper’s crank radius or limiting its travel so that it only opens to the point required to deliver the design minimum outdoor air flow. 

Because this method does not measure and regulate for a specific flow rate it is still subject to the same problems as the first approach on systems where the pressure and flow relationships vary with the load.  As with the first case, a good commissioning process is a key step in achieving and maintaining design operation.

·       Provide an Independent Regulated Minimum Outdoor Air Damper This approach adds flow measurement and control to the independent non-regulated minimum outdoor air approach discussed previously.  In some applications, a dedicated minimum outdoor air fan is also provided.  When properly implemented and commissioned, this design provides one of the most effective ways to:

1   Ensure that the required minimum outdoor air flow rate is delivered under all operating conditions.

2   Allow the minimum outdoor air flow rate to be adjusted to match current occupancy levels.

The minimum outdoor air set point can be a fixed value set for the design minimum flow rate or can be a variable based on occupancy, CO2 level or some other parameter.  The commissioning issues are similar to the previous options discussed, but have the added complication of a flow control loop.  The commissioning provider should take steps to ensure that the control loop is properly set up and tuned and that the input signal it is receiving accurately represents the actual flow rate.  These checks often require attention during the design and construction phase to the inlet conditions at the flow sensor to ensure a good velocity profile.

·       Provide an Independent Make Up Air Handling System This system treats all of the make-up air and supplies it to recirculating air handling systems. This approach is frequently seen in systems like clean rooms where pressure relationships and cleanliness requirements make an airside economizer cycle difficult and/or cost prohibitive (due to filtration requirements) to implement.  The commissioning issues are the same as previously discussed; the outdoor air flow needs to be set correctly, and, if the recirculating system operates with variable flows and pressures, then it may be necessary to regulate the make-up air connection. 

During commissioning, it may be desirable to determine the outside air percentage to make sure minimum damper position is correctly set.  In addition, the commissioning provider may want to predict the mixed air temperature that should occur with a known outside air percentage.  The predicted temperature can be compared to the measured average mixed air temperature.  Significant deviations from the prediction may indicate that the system minimum outdoor air flow is improperly set.

When the supply, return, and outdoor air temperatures are known Equation 9-1 can be used to calculate outside air percentage or the mixed air temperature.  This equation assumes that perfect mixing occurs, the temperature or moisture content of the mixed air stream will be the same regardless of where they are measured in the air stream.  This approach works best when there are significant differences between the outdoor air temperature and the return temperature.  Accurate measurement of the temperature is critical.  Using the same temperature sensor to measure all temperatures will help eliminate temperature sensor calibration errors.  Several mixed air temperature readings may need to be taken and averaged to accurately reflect the true mixed air temperature. 

Equation 3.1

Ultimately, the control of minimum outside air needs to be integrated with the economizer functions.  This integration is most critical during extreme weather conditions when the system is not operating on an economizer cycle.  At other times, the extra outdoor air brought in by the economizer cycle usually mitigates any IAQ issues and provides more than enough air for building pressurization requirements.

3.1.2. Economizer Free Cooling

The primary design intent behind most economizer systems is to provide free cooling any time the outdoor temperature is below the required system supply temperature.  The economizer cycle will also reduce the mechanical cooling load when the outdoor temperature is higher than the required supply temperature but the outdoor air enthalpy (or total heat content) is less than the enthalpy of the return air.  They key is to minimize energy – a task that relies on accurate control system and component operation. 

The outdoor air damper modulates from minimum position, when the full cooling load can be met by the minimum outside air volume, to the 100% outdoor air position as the outdoor air temperature approaches the required supply air temperature.  When outdoor air temperature (or enthalpy) is greater than the return air (or enthalpy), the outdoor air damper should revert back to the minimum setting for ventilation. 

Figure 3.2 illustrates the operating curves for an economizer section serving a system with 75°F return air when operating at a 55°F supply air set point and a 65°F set point.  The curves are based on the conservation of mass and energy relationships. 

Figure 3.2: Economizer Operating Curves (75°F Return Air)

Figure 3.2 conveys two important points:

1   The economizer operating curve is non-linear.  A 10°F outdoor air temperature change at low temperatures only requires a 4% change in the amount of outdoor air brought in to maintain the set point (Circle 2).  At higher outdoor temperatures, the same change in temperature requires a requires a 23% change in the amount of outdoor air brought in to maintain set point (Circle 1). 

At low outdoor air temperatures, a change in outdoor air temperature requires a much smaller change in the percentage of outdoor air brought in to maintain set point compared to higher outdoor air temperatures.  This introduces a non-linearity into the control loop that can make the loop more difficult to tune.  A loop that was tuned and stable when it is cold outside may become unstable (hunting or oscillations) when the outdoor air temperatures warm up or visa-versa.  This instability can cascade into other control loops in the system.  These issues are discussed further in 3.4.

2   Typical office environment minimum outdoor air percentages will not require preheat until it is fairly cold outside.  Air handling systems serving office environments typically operate with an outdoor air percentage in the range of 10-30% and discharge temperatures in the 55-65°F range.  The operating curves in Figure 3.2 show that it must be quite cold before the desired mixed air set point cannot be achieved by mixing outdoor air with return air.  For instance, on a system with a 20% minimum outdoor air requirement and a 55°F discharge set point, it will be approximately 28°F below zero before the mix of minimum outdoor air and return air will result in a temperature below 55°F (Circle 3).  This implies that most systems should not use preheat until the outdoor temperatures are extremely cold if everything is functioning properly. 

For an air-side economizer system to function well, it must mix the supply and return air streams effectively.  Often economizer sections cannot mix the air streams in their designed configuration.  In some cases, there can be a 50° - 60°F temperature difference between the warmest and coldest point in the mixed air plenum.  Poor mixing causes numerous operational problems, which are discussed in Section 3.4.2

3.1.3. Building Pressure Control and Return Air Heat Recovery

The temperature control function of operating the outdoor air and return air dampers in an economizer cycle generally results in the need for building pressure control to allow the extra outdoor air that was brought into the building for cooling purposes to exit the building.  Additionally, the minimum outdoor air requirement often includes an additional component to provide for building pressurization.  In most cases, operating a building at a positive pressure relationship relative to the outdoors is desirable with benefits that include:

·       Improved occupant comfort due to reduced drafts and infiltration Pressurizing the building tends to cause air to exfiltrate through the leaks and openings in the building envelope rather than infiltrate through them.  Occupants in perimeter spaces will be more comfortable if conditioned air from inside the building moves past them to exit the building rather than having unconditioned air from the exterior of the building move past them into the building. 

·       Avoided IAQ and condensation problems Pressurizing the building ensures that outside air does not infiltrate into the building, and all air entering the building is positively conditioned.  During the summer, the potential for condensation in the building envelope is minimized since all of the air inside the building is brought in through the air handling system and has been cooled and dehumidified.  If hot, humid air is allowed to infiltrate through the building envelope, then it is likely that moisture will condense when it comes in contact with air or surfaces inside the building that have been cooled below its dew point.   At a minimum, this can cause damage to finishes and materials.  Frequently this situation will lead to mold growth and IAQ problems.

·       Recovered heat from the return air Pressurizing the building results inreduced perimeter heating loads because instead of infiltrating air at the perimeter, the air is exfiltrated at the perimeter. This exfiltrated air has been heated by the internal gains in the building either at the mixed air plenum where return air and outdoor air are blended or via the heat gains in the space as the supply air warms up to the space temperature.  In most cases, any cold air infiltration load on the perimeter system is compensated using energy from a boiler system.  Reduced return fan energy consumption results because the return fan moves less air back to the relief damper location.  Through experience with retro-commissioning projects, fan energy use has been reduced as much as 20% to 40%.

Recovering energy from the return air stream is limited when the outdoor air quantities are great enough and/or the outdoor temperatures low enough that preheating the mixed air is necessary to achieve the required supply temperature. However, in many parts of the country, these are extreme conditions, not the norm. 

The desired level of building pressurization will be accomplished via one of the following methods:

1   Barometric Dampers This is generally the simplest approach to controlling building pressurizations and works well if the building is reasonably tight, not geometrically complex or tall enough that chimney effect becomes a factor, and if the path from the occupied space to the relief damper location does not have much pressure drop at design flow.  The dampers generally consist of blades and a frame that are mounted and pivoted in such a way as to allow gravity to close the damper and small, positive building pressures to open the damper.  Some dampers are counter balanced and can be adjusted to control at specific pressures. 

2   Modulating Relief System using the Same Signal as the Outdoor Air and Return Air Dampers This approach is a common way to control building pressure, especially on older systems.  The approach works well for constant volume systems.  It also can provide reasonable performance for VAV systems serving relatively small non-geometrically complex low-rise buildings.  However, problems can occur because the signal controlling the relief dampers (a pressure control function) is also the same signal as the one controlling the economizer dampers (a temperature control function).  As a result, building pressure control problems can be aggravated in high rise buildings, leaky buildings, and buildings with relief fans (as compared to return fans) serving VAV systems.  Consider what happens to the partially occupied building in the following example on a 55°F overcast day. 

The building’s air handling system is VAV with an economizer cycle equipped with relief fans.  The relief fans and dampers are controlled by the same signal that is used by the economizer cycle to control the outdoor air and return air dampers to maintain a 55°F discharge temperature.  On this particular day, since the outdoor air temperature is equal to the required discharge temperature, the economizer cycle opens the outdoor air dampers and relief dampers to 100% and the closes return dampers.  The relief fans are being commanded to run at full speed.  But, since there is no solar load and the internal gains are low due to the low occupancy level, the VAV function of the air handling system is meeting the supply flow requirements by operating the supply fan at 50% of its design flow rate.  As a result, there is a serious mismatch between the air being brought into the building through the wide open outside air dampers by the supply fan running at 50% capacity and the air being removed from the building through the wide open relief dampers by the relief fans running at 100% capacity.  In this particular case, the difference was so significant that most people attempting to enter the building could not open the entry doors due to the forces placed on them by the severe negative pressure difference relative to atmosphere. 

3   Modulating Damper System Controlled by Some Other Signal such as Building Static Pressure This approach solves the problem described in the example above.  The relief dampers (and relief fans if the system is configured that way) are operated based on building static pressure instead of by the economizer signal.  As a result, the system only begins to relieve air when the building becomes slightly positive.  Only as much air is relieved via the relief system as necessary to maintain the slight positive pressure relationship in the building relative to atmosphere.

This approach is especially beneficial in older, leaky buildings, large complex buildings and high rises regardless of the HVAC system type.  In these situations, the chimney effect and other factors can become significant influences on the pressure relationship between the inside and outside of the building.  By controlling the relief damper off building static pressure, problems with significant positive or negative pressure from the economizer mode can be avoided. 

For systems equipped with return fans, the relief system is usually downstream of the return fan location.  On systems not equipped with return fans, there may be relief fans if the pressure drop through the relief system with the economizer on 100% outdoor air is greater than the desired positive pressure in the building.

3.2. Commissioning the Economizer and Mixed Air Section

The following sections present benefits, practical tips, and design issues associated with commissioning an air handler’s economizer and mixed air section.

3.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements lists practical considerations for functional testing. Key Preparations and Cautions address potential problems that may occur during functional testing and ways to prevent them.

Key Commissioning Test Requirements

An operating economizer can save significant cooling energy compared to operating mechanical refrigeration (Section 3.1.2). Exactly how much energy varies with climate and building operating hours, and precise calculations require computer modeling. Approximate solutions can be arrived at by using programs such as EZSim® to simulate a virtual building with and without an economizer. Spreadsheets can also be used to develop approximate solutions based on bin weather data and estimated loads at low ambient temperatures. However, it is reasonable to assume that if an economizer has been provided by the design then the effort necessary to make it work is justified. If there are problems with the economizer that require major capital outlays to correct, then additional evaluation regarding the value of the economizer cycle vs. mechanical refrigeration may be necessary. For example, a system that would not function in economizer mode because the relief air was directly recirculated into the outdoor air duct may require some quantification of costs and benefits to prove that it is cost effective to correct the problem.

The purpose of functionally testing an economizer cycle is to verify that the process and its related functions perform satisfactorily under all building operating conditions to provide free cooling using outdoor air quantities beyond ventilation requirements.

1 Verify that the control process provides reliable free cooling when conditions are appropriate under all building and system operating modes (including automatic and manual control modes) and under all climate conditions including seasonal extremes outside the statistical design envelope. (Section 3.5.1)

2 Verify the cycle is integrated properly with other building processes and systems in both normal and emergency control modes. Building processes and systems include, but are not limited to, building pressurization requirements, zone pressurization requirements, minimum outdoor air requirements, normal and emergency operating modes, and scheduled operation. (Section 3.1.1, Section 3.1.3).

3 Verify that interlocks return the economizer dampers to safe and efficient positions when the air handling system is shut down. (Section 3.5.2)

4 Verify that interlocks disable the economizer cycle when it no longer provides energy savings benefit. The set points of these interlocks are appropriate for the loads served and the local environmental conditions. (Section 3.5.2)

5 Verify that interlocks protect the air handling system and building areas served by the economizer from damage in the event of a failure of the economizer control process or a component of the system. These interlocks include low temperature cut-outs, high and low static pressure cut-outs, pressure relief doors, and limit switches. (Section 3.5.3, Section 3.5.4)

6 Verify that alarms are provided to alert the operating staff to economizer operating conditions that indicate an ezonomizer control failure that could lead to energy waste and/or the failure or unscheduled shut down of the air handling system served by the economizer. (Section 3.5.7)

Key Preparations and Cautions

Cautions

Like most functional testing process, economizer test procedures generally force the system to operate at the extremes of its design and performance envelope during portions of the test cycle. When everything is working properly, systems operating at these extremes generally will be exposed to the greatest risk of failure, energy waste or other undesirable outcomes. Thus, the testing team needs to have evaluated the system for the test to be performed. Many of these issues are covered in Functional Testing Basics. Specific areas of concern for economizer testing include:

1 Safety interlocks such as the low temperature cut-out and the mixed air plenum low static pressure cut-out and permissive interlock functions should be verified early in the test sequence to protect the system from experiencing problems or catastrophic failures during functional testing.

2 If economizer testing occurs in extreme weather, the heating and cooling/dehumidification functions associated with the systems should be functional. This will help to protect the system and building from temperature and humidity extremes and their related freezing, overheating, or condensation potential if problems occur with control of the economizer while under test.

Test Conditions

Ideally, an economizer cycle should be tested several times during the year to allow its functionality to be confirmed under different seasonal conditions. This is because the dynamics of an operating economizer will vary with the seasonal conditions and loads, and because failure to function properly under different seasonal conditions can often lead to energy waste and IAQ problems. If time or budget do not allow for several test cycles, then it is best to test the economizer under extreme winter and extreme summertime conditions. If only one test sequence can be performed, then it should be performed under extreme winter conditions. Extreme winter conditions typically are the times when most of the problems encountered with economizers become apparent as operational issues. Failure of important control cycles and interlocks to function properly, such as ambient condition interlocks and minimum outdoor air regulation typically are easiest to detect under seasonal extremes. If undetected, these failures will result in significant energy waste that is often masked by other processes in the air handling system.

Instrumentation Required

Instrumentation requirements will vary with the specific test sequence selected. In general, having the following instruments available while testing an economizer process will be beneficial.

1 Temperature measurement instrumentation

2 Sling psychrometer and psychrometric chart

3 Digital camera

4 Tape measure or folding ruler

5 Pneumatic pressure gauge and gradual switch baumanometer to allow pneumatic actuators to be stroked independently from the control system

6 Air pressure gauge capable of measuring very low air pressures and differential pressures in the range of 0.01 to 0.25 inches w.c. (An inclined manometer or magnehelic gauge is typical of this type of instrument)

7 Airflow measuring device capable of measuring very low velocities in the range of 50 to 1,000 fpm (A Shortridge airflow multimeter is ideal for this and also provides a low pressure and temperature measurement capability. Rotating vane anemometers represent a less costly approach.)

8 Multipoint data logger with several temperature probes can be very helpful if there is not a building automation system with trending capabilities available

Time Required to Test

The time required to test an economizer cycle will vary with the complexity of the system and the level of rigor, but will be fairly significant. At a minimum, 2 to 4 labor hours will be required to verify interlocks and integrated function for the small, simple, packaged systems often provided with small tonnage rooftop equipment if the functions are only going to be tested one time at start-up. At the high end, 3 to 5 labor days can be required over the course of the first year of operation if the economizer-equipped systems are large and complex and can interact with other systems in the building, and if the economizer performance is to be evaluated under a variety of seasonal conditions.

 

3.3. Testing Guidance and Sample Test Forms

Click the button below to access all publicly-available prefunctional checklists, functional test procedures, and test guidance documents referenced in the Testing Guidance and Sample Test Forms table of the Air Handler system module.

AHU Testing Guidance and Sample Test Forms

3.3.2. Economizer Evaluation Checklist

It is important for the commissioning agent to follow through on these items as the project moves from design into construction by reviewing shop drawings and verifying proper installations during site inspections.  In the end, these efforts will be rewarded by a system with fewer commissioning issues to address at start-up and during the first year of operation.

Items to watch for include:

·       Make sure someone is taking responsibility for sizing the dampers.  Sizing can either be done by the designer on the actual contract documents or through assignment to the controls contractor.  If damper sizing is assigned to the controls contractor, then the commissioning agent should review the sizing calculations and other details included in the control submittals.  Additional discussion about damper sizing can be found in Section 3.4.1.1 Damper Oversizing and in the Supplemental Information in Section 3.6.1.2 Damper Sizing.

·       When necessary, be sure that a mixed-air, low-limit cycle is included in the control sequence.  Additional information on this sequence and its benefits can be found in Section 3.5.3 Limit Control.

·       Be sure the design provides sufficient distance for the air to mix between the mixing dampers and the first coil in the air handling system.  Be sure that the design reflects an economizer damper configuration that promotes mixing both by the arrangement of the dampers relative to each other as well as by the way the dampers rotate to close.  Ideally, the designer should detail the required arrangement on the construction documents.  However, if the engineering time or budget does not support this, then the details can be delegated to the control contractor.  When delegated, the contract documents should require that the control contractor include the necessary detailing as a part of the controls submittal package and the commissioning agent should review this information.  These issues are discussed in Section 3.4.2.

·       Make sure the documents reflect installing the mixed air sensors and freezestats in a manner that fully covers the mixed air plenum and allows the mixed air sensor to accurately reflect the conditions that the freezestat will see.  This may require multiple sensors for larger systems.  Running the sensing elements for the mixed air sensor and freezestat together helps to ensure consistent system performance by subjecting these two related sensors to identical conditions.  The support system detailed for the sensors should ensure that they are only affected by the air stream conditions, not frame or coil radiant effects. 

·       Make sure the documents detail the installation of actuation systems in a manner that assures linear or near linear relationships between actuator stroke and blade rotation (and thus flow if the dampers are sized).  Additional information regarding this topic can be found in Figure 3.16 in Section 3.6.2.5.

·       Make sure that the designer has considered the impact of flow reductions in VAV systems on damper performance at part load. Damper performance and linearity are velocity dependent, as is discussed in Section 3.4.1.2.  The problem is much easier to address at the design phase when the necessary adjustments are made on paper.

The checklist that is linked to the button below uses some rules of thumb and quick evaluation techniques to help evaluate a new or existing system and determine if the system is likely to have economizer-related operating problems.

Bevel: Link to Check List

 

Link to a checklist that helps assess the susceptibility of a new or existing economizer to operating problems. Includes evaluation criteria, links back to the Guide and directs the user to appropriate follow-up actions.

 

The checklist summarizes many of the concepts presented in detail in this chapter in a form that is usable as a field tool.  There are several ways that this checklist can be used:

·       As a design review tool The checklist can be used during the design phase to evaluate the economizer design information shown on the plans.  Issues identified can be targeted for further review and evaluation by the design team.

·       As a field tool for new construction The checklist can be used in new construction during the construction observation phase to allow the installation to be evaluated as it is installed.  Problems identified can be flagged and resolved prior to start-up and the functional test sequence can be structured to ensure successful resolution.

·       As a field tool for retro-commissioning The checklist can be used during the initial assessment phase of the project to help identify potential problem areas with energy conservation and performance improvement potential.  The results can be used to target functional tests and identify solutions.

3.4. Typical Problems

The following typical problems with economizer and mixed air operation are described briefly in this section.  When each problem is described, references are provided to educational information contained in the remainder of this chapter and in Section 3.6 Supplemental Information.

3.4.1. Control Loop Instability

With regards to economizer operation, two system characteristics can lead to discharge air temperature control loop instability: outdoor and return air damper oversizing and operation at a high turn down ratio.

3.4.1.1. Damper Oversizing

Dampers that are oversized or not properly actuated can cause control linearity and mixing problems.  If the damper is oversized, the air may be moving too slow through the damper to result in a significant pressure drop.  Damper pressure drop relative to the system pressure drop has a major impact on achieving a linear relationship between damper flow rate and damper stroke. Refer to Figure 3.10 in Section 3.6.1.2 for a damper characteristic curve (% flow vs. % damper stroke) and discussion about how to achieve good damper performance.  The linkage arrangement used to connect the actuators to the dampers can also have an impact on achieving this linear relationship.  Without a linear relationship between flow and damper stroke, the economizer cycle is difficult to control – instability in the economizer cycle can cascade into other control loops. 

Where independent loops are used to control each heat transfer element, instability in the economizer loop causes an unstable input to the control loop for the next heat transfer element.  If the economizer control loop starts to hunt, it will introduce pressure variations into the air handling system.  In variable volume systems, these pressure variations can cascade into the fan capacity control loop and cause it to become unstable. In turn, fan control instability can result in unstable supply static pressure at the inlet to the terminal unit, which can trigger instability in the terminal equipment flow regulation control loops.  The bottom line is that a hunting system can waste energy due to simultaneous heating and cooling, wear out valve and actuator seals and other mechanical components, and create operational and comfort control problems.

3.4.1.2. High Turn Down Ratio

VAV systems can pose particularly challenging economizer control problems if the system must operate at a high turn-down ratio.  VAV systems provide a wide variation in flow rate - it is not uncommon for systems with the ability to shut off flow to an unoccupied zone to see a flow variation of 10:1 or more (design flow vs. flow with one tenant area occupied at low load).  The linearity of the dampers as well as the mixing performance are related to velocity of the air through the damper.  The reduced flow rates in VAV systems at part load translate into reduced damper velocities.  Specifically, at 50% flow, the damper velocities are 50% of design flow, and the pressure drop across the damper is 25% of the design value.  This reduced pressure drop can cause a significant decay in the linearity of flow through the damper vs. damper stroke.  A VAV system with reasonable damper flow vs. stroke linearity at higher load conditions can become less linear at low load, resulting in poor control.  An oversized damper exacerbates the problem, since even at design flow, the velocity is lower than desired. 

When economizers fail to perform reliably at high turn down ratios, the solutions typically involve either preventing the system from turning down as much or giving up on the economizer and running mechanical cooling in its place.  These solutions can be energy intensive and have some operational problems of their own at the central plant.  But, many times, they are the only viable solutions for an existing system, especially if the system doesn't spend a lot of time running at the low turn-down ratio. 

To address high turn-down ratios on larger systems with multi-section dampers, each damper section of the multi-section outdoor air, return air and relief air dampers is equipped with an independent actuator.[1]  The output signal to each damper assembly is split so that half of the sections are controlled by one output and the other half are controlled by the other.  The control logic is arranged to disable and force closed half of the damper sections in each assembly when the system reaches 50% turn-down.  This will reduce the available damper area by 50%, which increases the velocities at low loads.  As a result, the damper pressure drops increase and the flow linearity at low load improves.  In addition, the higher velocities also promote mixing.  This solution is easier to implement in a new design rather than as a retrofit, but retrofits can be feasible if the existing dampers are multi-sectioned.

3.4.2. Poor Mixing

The configuration of the mixing plenum and the damper sections play important roles in ensuring the outdoor air and return air streams are thoroughly mixed by the economizer process.  Economizers with poor mixing can have excessively stratified air streams. The problems occur partly because the momentum and turbulence that promote mixing drop off significantly as the velocities in the mixing box and through the dampers drop. 

In one instance in a retro-commissioning environment, performing the temperature traverse test revealed a temperature difference of 60°F across the plenum with nearly the subfreezing outdoor air temperature in one layer at the bottom of the plenum and nearly the return air temperature in a layer at the top.  The freezestat element was located in a manner that did not expose it to the layer that was subfreezing.  The averaging sensor controlling the economizer was doing what it was designed to do (averaging the temperature across its element) and indicated a mixed condition of 53°F.  As a result the economizer control loop thought it was doing a fine job, the freezestat sensed that nothing was wrong, and the coil froze several of the bottom rows where the subfreezing air in the bottom layer contacted it.  VAV systems require special attention in this regard as is discussed in Section 3.4.1.2 High Turn Down Ratio and Section 3.5.1 Operating Control.

Occasionally, poor mixing conditions can have far reaching and unexpected implications.  Figure 3.3 is an example of a particularly interesting problem related to vertical stratification that was set up by a bad mixing arrangement and carried itself through the DWDI fan to the discharge duct.  For a significant distance down the discharge duct, zones on one side were hot and zones on the other side were cold. 

Figure 3.3: Mixed Air Stratification Carried Through a DWDI Fan

Additionally, a high turn down ratio can cause a system that exhibits good mixing characteristics at design flow rates to degrade to poor control at low flow rates. Variable air volume systems are particularly prone to this issue. 

Problems that result from poor mixing:

·       Turn down capabilities are defeated. 

·       The economizer function is disabled and the chilled water plant is operated all year to serve the cooling loads.

·       Outside air that stays at the bottom of the duct trips the freezestat. 

·       Systems cannot cool spaces during winter months because the mixed air controllers have to be set to extremely high temperatures to prevent nuisance freezestat trips.

Identifying a mixing problem:

·       Take a temperature traverse of the mixed air plenum in cold weather to better understand the problem.  The results of this test can be used to direct the remaining steps in this list.

·       Calculate the turn down ratio (see 3.3.2 Economizer Evaluation Checklist).  High turn down ratios lead to low velocity through the dampers and poor mixing.

Methods for improving mixing:

·       Disable and permanently close some of the damper blades to improve the velocity through the damper.  This makes the damper characteristic more linear and gives the air streams momentum to mix.

·       Rotate the damper sections so that the damper blades direct the air streams into each other as they close.  This creates turbulence and helps to promote the mixing process.

·       Add baffles to divert the air stream several times before it reached the coils.  This also creates turbulence, which promotes mixing.  If the baffles are arranged so that the velocity through them is low (800-1,000 fpm) then significant benefits can be realized without significant additional pressure drop.  Applying these techniques to solve a problem is described in Section 3.6.6.

By taking some or all of these steps on problem systems or by focusing attention during design phase commissioning, it is possible to obtain systems that have a 5°F or less temperature variation between the warmest and coldest point in the mixing section, even in extreme winter weather. 

3.4.3. Excess Minimum Outside Air

The design minimum outdoor air flow setting at the air handler ensures that adequate outside air is distributed to the building for occupant ventilation and building pressurization.  This design outside air flow is often well above the actual flow required, since design flow is routinely based on an overestimation of the number of occupants.  With excess ventilation air, the air handling system serves an unnecessary heating load during cold outside conditions and an unnecessary cooling load during hot and humid conditions. 

Regulating the outside air flow is a challenge.  Placing a limit signal on the outside air dampers may not correctly regulate flow, since this method assumes a linear relationship between actuator stroke and airflow, which is often a bad assumption, especially if the dampers are oversized.  Another option is to provide an independent minimum outside air damper that modulates open or closed.   This independent damper can also be regulated, which adds flow measurement, control, and sometimes a dedicated minimum outside air fan.  An alternative way to bring in outside air is to use a dedicated outside air make-up unit in conjunction with recirculating systems.  These issues are discussed in detail in Section 3.1.1 Minimum Outdoor Air.

However, it is not unusual to find office air handling systems using preheat when the outdoor temperatures are over 40°F.  There are a variety of reasons for this including:

·       Systems do not have a mechanism in place to regulate the minimum outdoor airflow rate regardless of system flow rate.[2]

·       Figure 3.4 illustrates the mixed air temperature will be achieved by systems with good mixing operating at different minimum outdoor air percentages as the outdoor air temperature varies.  Notice that Figure 3.2 shows the same information as Figure 3.4 but with different axes.

Figure 3.4: Mixed air temperature vs. outdoor air temperature

3.4.4. Terminating the Economizer Cycle

The combination of outdoor temperature and humidity will reach a point where the economizer provides no energy savings benefit.  At this point, an interlock disables the economizer and changes the system over to a recirculating mode with minimum outdoor air.  There are three main operational interlocks for the economizer cycle:

·       Fixed dry bulb Compare outside air dry bulb temperature to a fixed setpoint to control the economizer cycle.

·       Differential dry bulb Compare return air dry bulb temperature to outside air dry bulb temperature to control the economizer cycle.

·       Differential enthalpy Compare return air enthalpy to outside enthalpy to control the economizer cycle.

These operational interlocks are often incorrectly set, changing over to purely mechanical cooling when the outside air can still provide energy savings.  With a fixed dry bulb setting, ASHRAE 90.1 and California’s Title 24 require that the setpoint be 75°F in dry climates, 70°F in intermediate climates, and 65°F in humid climates.  These setpoints were determined from building energy modeling to minimize energy use.  In dry climates, the differential drybulb control may not improve energy savings enough to justify the added cost.  Terminating the economizer cycle based on the enthalpy of the outside air provides the maximum amount of free cooling.  However, the additional maintenance of the enthalpy sensor must be weighed against the incremental hours of economizer cooling.  The improvement in performance of differential enthalpy can be justified for humid climates if the sensors are kept in calibration. 

For more details on selecting an outdoor temperature interlock setting or an enthalpy-based interlock setting, refer to Section 3.5.6 Ambient Condition Interlocks.  Enthalpy switches and sensors are discussed in Section 3.6.3.3.  

3.4.5. Setting Damper Interlocks

The operation of the economizer cycle needs to be interlocked with the operation of the system it serves so that the outdoor air and relief dampers are closed when the system is shut down, thereby preventing problems with unconditioned air entering the building during unoccupied hours.  There are a number of issues that often show up as commissioning or operational problems if they are not addressed during design.  Interlocks need to operate reliably in all system modes, and on systems with large fans capable of high static pressures, interlocking the return dampers to close when the unit is off for smoke isolation purposes sets the system up for a failure if both the outdoor air and the return dampers fail to open prior to fan operation.  These issues are described in detail in Section 3.5.2 Operational Interlocks.  

3.4.6. Nuisance Freezestat Trips

Nuisance freezestat trips can be challenging to troubleshoot because they often occur under very specific conditions that are difficult to duplicate or simulate.  Furthermore, it is often difficult to restart the system and return to normal operation after the trip.  In an extreme case, some systems simply will not run below certain outdoor temperatures due to freezestat trips. 

Nuisance freezestat trips can become frustrating to operators, who usually solve the problem by either defeating the scheduling program, preventing the economizer operation and using mechanical cooling, manually cracking the hot water valve open so that there is always enough heating to protect the freezestat, or by jumping out the freezestat.  The first three solutions will waste energy.  The last solution can lead to a failure of the water coil or coils due to freezing, and in extreme cases can freeze the entire building plumbing system.   If the unit operates on a schedule, then the problem will show up during the warranty year and there is some hope that it will be addressed and corrected via the warranty process.  However, for systems that operate 24 hours per day, 7 days per week, the problem will most likely occur the first time there is an extended power outage during cold weather.  This may not occur for years after the unit is installed.  When a nuisance freezestat trip occurs, it is quite likely that a less than optimal solution will be employed in an effort to return the critical system back to operating status.

The commissioning provider can alleviate the likelihood of nuisance freezestat trips through a well-designed inspection and functional testing program focusing on the following areas:

·       Mixed air low limit control Sometimes it is necessary to use a low limit control strategy for economizers in cold climates.  When the economizer is controlled based on the discharge air temperature from the supply fan, employing mixed air low limit control during operation will reduce freezestat trips.  If the economizer is controlled off the mixed air temperature, then the mixed air low limit control strategy is not necessary. 

Consider the following scenario.  During an extended shut down, the air in the vicinity of the discharge sensor as well as the various components in the unit will tend to stabilize at the temperature of the surroundings, typically 65°F - 75°F range.  When the system restarts, the discharge temperature is significantly above set point (usually in the 55°F - 60°F range) and begins to drive the economizer dampers toward the 100% outdoor air position.  Since there is some distance between the mixed air plenum and the discharge temperature sensor, and since the air from the mixed air plenum will be warmed up as it cools down the coils, casing, fan housing and other components of the air handling unit, it will take some time for the discharge temperature to drop to match the mixed air temperature.  As a result, the unit will tend to drive toward and stay at the 100% outdoor air position until the temperature at the discharge sensor drops toward the discharge temperature set point.  However, if the outdoor air temperature is below freezing, the mixed air temperature can drop below the set point of the freezestat.  The outside air trips the freezestat, which typically requires a manual reset.  In most cases, it will take 5-10 attempts to start the unit before the discharge temperature will catch up with the mixed air temperature fast enough to prevent the problem. In some cases, it is impossible to get out of this mode by repeated freezestat resets; the thermal inertia of the system is too great when combined with the dynamics of the intake system, control dampers, and control system.

·       Freezestat Location Sometimes, systems experience nuisance freezestat trips simply because the freezestat element has not been located properly.  The purpose of the freezestat is to protect water coils that have not been designed to handle subfreezing air by shutting down the system if the air that reaches them is approaching freezing.  The freezestat element should be at a location in the system that is never expected to see air near freezing temperatures under all normal operating conditions.

·       Sensor Calibration Some freezestat problems to be traced to sensor calibration either directly with the mixed air sensor or freezestat or with the relative calibration of the other sensors in the system with reference to the mixed air sensor or freezestat.  If the individual heat transfer elements are controlled by independent control loops and there are relative calibration problems between the control loops, then the loops will tend to fight each other.  Any instability in one could cause the other control loops to hunt.  If the hunting becomes pronounced enough, a temperature swing may trip the freezestat.  The Relative Calibration Test in Chapter 18 can be used to check new and existing systems for problems in this area. 

·       Mixing Poor mixing is also a common cause of nuisance freezestat trips.  Section 3.4.2 discusses key system parameters and design features that can help promote good mixing. 

·       Transient Condition Many nuisance freezestat trip problems can be traced to the inability of the system to respond to a transient condition.  The most common transient condition is a system start-up.  Other transient conditions include: 

1   Changes in the operating mode of a system from continuous operation to scheduled operation. 

2   Changes in load profile that increases the turn-down ratio of a VAV system.

3   Climate extremes at or beyond design which were not encountered in the previous operating life of the system.

4   Changes in the performance characteristics of one or more of the heat transfer elements due to fouling, component wear, or some other age related factor.

5   Unanticipated outages in a system that normally operates continuously due to a power outage in extreme weather.

3.4.7. Coil Freeze-ups

The direct cause of coil freezing is typically that the freezestat failed to function and did not provide the intended freeze protection.  The indirect cause is a malfunction upstream of the freezestat that caused the coil to be subjected to subfreezing air.  These malfunctions include poor mixing, improper damper sizing, and damper actuation problems.  Most coil freeze-ups can be attributed to one of four causes:

1   Coils subjected to subfreezing air by design but were not installed and controlled properly or the protecting mechanism failed.  Steam coils that have not specifically been designed to handle subfreezing air can freeze, especially at low steam flow rates.  At low steam flow rates, the steam condenses to a liquid at some point early in the tube circuit.  It then must flow as condensate (liquid water) to the end of the tube and out of the coil.  If the air is subfreezing, the coil is often capable of cooling the condensate to the freezing point before it exits the coil, eventually rupturing the tube.  Steam coils that are not designed to handle subfreezing air need to be protected by a freezestat, just like any other water coil. 

2   Coils subjected to subfreezing entering air temperatures by the operation of a life safety control function (intentional or false trip).  This failure is discussed in Chapter 15: Management and Control of Smoke and Fire, Section 15.4.5 Coil and Water Piping Protection, Condensation Protection

3   Coils subjected to subfreezing entering air temperatures due to failure of a preheat system or failure of a control or limit system.  Failures in this category usually relate to malfunctions in the economizer system, especially poor mixing and damper oversizing.

There are instances where a coil has frozen and the freezestat never tripped.  This can occur for a number of reasons.

· Freezestat Mechanism Failure Freezestats can fail within the switching mechanism as well as a failure of the sensing element.  For this reason, a functional test that targets the entire mechanism, including the sensor, is a better test than one that targets just the electrical switching mechanism. 

· Freezestat Disabled If the economizer has not undergone a thorough commissioning process that ensured that the economizer functioned reliably, then the economizer may plague the operators with nuisance problems, particularly, the start-up problems associated with a lack of a mixed air low limit control sequence.  In frustration, the operators may have disabled the freezestat.  Unfortunately, this approach places the system at risk.

· Outside air dampers Failure of the outside air dampers to close when the air handler is not operating can result in cold air entering the system.  This situation is not a freezestat failure since a conventional freezestat installation merely shuts down the fan when the freezestat trips.  This situation is discussed in Section 3.5.4 Safety Interlocks.

3.5. Economizer Control Strategies

Economizer cycles typically have several different control requirements associated with them including:

· The operating control strategy that governs the cycle in normal operation.

· Interlock strategies designed to terminate the economizer cycle when it no longer provides any useful benefit or the system it serves is not in operation.

·Limit control strategies designed to accommodate transient conditions such as start up or sudden load changes.

·Alarms designed to detect problems.

3.5.1. Operating Control

The operating control of the economizer system is designed to modulate the supply and return dampers to maintain either the mixed air temperature, or in sequence with the heating and cooling coils to maintain the discharge air temperature or zone temperature.  In addition to these temperature control related functions, the control of the relief dampers must also be integrated with the operation of the return and outdoor air dampers.  Relief dampers need to be controlled in a manner that allows the system to deal with the extra outdoor air brought in by the economizer cycle.  This issue is discussed in Section 3.1.3 Building Pressure Control and Return Air Heat Recovery.

In many ways, a start-up is one of the most difficult and complex operational modes for an HVAC system.  Every component and control loop in the system is subjected to a significant step change in its input (going from the shut down state to the operating state).  These upsets result in the control loops modulating their outputs in an effort to find a stable control point in the new operating mode.  The unstable output of one control loop can become an unstable input to the other, causing instability in the second control loop in the cascade. 

The following common problems related to operating control are presented in Section 3.4:

·   Control Loop Instability

·   Excess Minimum Outside Air

·   Poor Mixing

·   Terminating the Economizer Cycle

3.5.2. Operational Interlocks

The operation of the economizer cycle needs to be interlocked with the operation of the system it serves so that the outdoor air and relief dampers are closed when the system is shut down, thereby preventing problems with unconditioned air entering the building during unoccupied hours.  However, there are several details that need to be addressed in order to provide the most robust and reliable system.  These issues often show up as commissioning or operational problems if they are not addressed during design.

1   The interlocks that shut down the economizer dampers need to function regardless of the position of any hand-off-auto selector switches at the DDC controller, fan drives or starters.  Interlocks that depend on the operation of the DDC system are easy to implement.  If the system is being operated manually, either for commissioning purposes, temporary heat purposes or in an emergency, perhaps caused by the failure of the DDC controller, then the interlock function can inadvertently be aborted.  In these cases, the system runs without freeze protection.

2   Interlocks that are based on pressure or current signals need to be set to function reliably at all system operating points.  As a basis for the operating interlocks on a system, proof of operation must be determined.  Fan motor current or pressure differential across a filter bank, cooling coil, or fan are good indicators, but they need to be set up to provide reliable information in all operating modes.

Differential pressures vary as a function of the square of the flow rate, and motor horsepower (and thus, amperage) varies with the cube of the flow rate.  As a result, when a variable volume system unloads, the signals available for differential pressure-based and motor current-based proof of operation inputs can decrease quickly. In order to ensure operation of the economizer and other features that use this information as an interlock, the functional testing process should verify that a reliable signal is provided at design flow as well as at the minimum flow.

Some system differential pressure signals prove fan operation but do not necessarily prove flow.  Consider a system where the fan has started but the discharge smoke damper has failed to open for some reason.  A proof of operation input from based on fan differential pressure would indicate that the system was running since the fan does not have to move air to generate a pressure difference.  A proof of operation input based on coil or filter bank differential pressure would not indicate the system is running since there must be flow to generate a pressure difference across these elements.

3   Any damper that is used for fire or smoke control functions must be listed for that service.  On some larger systems, the outdoor air and return air dampers are selected and controlled to provide code dictated smoke isolation of the air handling equipment from the duct system[3].  These installations are less flexible for field modifications to address economizer problems.  In addition, any repairs or component replacements made must be done with components designated and installed in a manner dictated by the manufacturer in order to retain the U.L. rating.

If the return dampers are being used for smoke isolation purposes, then they will need to be interlocked to close rather than open when the unit is shut down.  This is contrary to the conventional operating mode for an economizer, which closes the outdoor air and relief dampers and opens the return dampers when the unit is shut down.

4   Interlocking the return dampers to close when the unit is off for smoke isolation purposes sets large systems up for a failure if both the outdoor air and the return dampers fail to open prior to the fan start.  Under a no-flow condition, a fan will generate its rated shut-off static pressure.  If the inlet side of the fan is closed off and the outlet is reference to atmosphere via the open but inactive duct system, then the fan will attempt generate its rated static as a negative pressure on its inlet.  Many large fans can generate static negative static pressures in this manner that are well above the rating of the intake systems, mixing boxes and air handling unit casings.  To prevent damaged to the air handler, there are several measures that can be taken.

·    Fabricate the duct and plenums on the intake side of the fan for a pressure class rating in excess of the fan’s rated shut-off static pressure.  This may be the most viable and trouble-free approach on systems with fans rated for modest static pressures at shut-off.

·    Install manual reset type static pressure limit switches and wire them into the safety circuit to shut down and lock out the fan in the event such a condition were to occur.  This may not be the best course of action since the switch needs to respond quickly enough to shut down the fan before it can do damage. A large fan wheel can take several minutes to decelerate (spin-down time).

·    Install pressure relief doors.  These devices are discussed in Section 11.4.3 Air Hammer.

·    Install limit switches wired in a permissive interlock circuit so that the fan will only be allowed to start after the dampers are open to the point where no damage to the ductwork and air handler casing can be done.  The switches should monitor blade position, not shaft or crank arm position since crank arms and shafts can come loose from the blades they serve.  Figure 3.5 illustrates a typical installation of this type of switch. 

Figure 3.5: Typical damper limit switch installation

Note that the switch is sensing blade position and that the unistrut mount makes it easy to adjust the switch for the desired trip point.

If limit switches are used for an economizer-equipped air handling unit, then both the return and outdoor air dampers need to have limit switches, and the limit switches need to be wired and adjusted so that if either damper is open sufficiently, then the system will be allowed to start (or remain in operation).  If the limit switches were installed only on the return damper, then the system would “think” it had a problem when the economizer drove to the 100% outdoor air position.  The start-up and commissioning process needs to include a Permissive Interlock Test to verify that the adjustment of the switches on the different damper assemblies is coordinated and will not inadvertently shut down the system when the economizer is under modulating control or the outdoor or return dampers are closed.

3.5.3. Limit Control

Sometimes it is necessary to use a low limit control strategy for economizers in cold climates.  When the economizer is controlled based on the discharge air temperature from the supply fan, employing mixed air low limit control during operation will reduce freezestat trips.  If the economizer is controlled off the mixed air temperature, then the mixed air low limit control strategy is not necessary. 

A properly employed mixed-air low-limit cycle can prevent freezestat trips and not reduce the performance of the system.  A control loop is created based on mixed air temperature that overrides the normal economizer control sequence to prevent the mixed air plenum from dropping below some limit, like 40°F.  This limit cycle will hold the mixed air temperature at a safe level until the temperature at the discharge sensor drops toward the discharge temperature set point. 

3.5.4. Safety Interlocks

Like operational interlocks, safety interlocks need to be arranged to function regardless of the position of any hand-off-auto or inverter-bypass selector switches associated with the air handling system’s fan motors.  In many instances, it is also desirable to have them trigger some sort of alarm to bring the problem to the attention of the operators.  This section discusses control issues related to two safety interlocks: freezestats and static pressure switches.

3.5.4.1. Freezestat Control Sequences

The most common safety interlock associated with the mixed air section is the low temperature cut-out, typically referred to as the freezestat.  Control strategies are discussed below, while freezestat installation details are discussed in Section 3.6.3.2.

There are some situations where a properly installed and functioning freezestat will not protect the coil it is associated with from freezing.  In this situation, the outdoor air dampers fail to close or seal completely when the unit shuts down during subfreezing weather.  The dampers can fail to close for a variety of reasons including actuator failures, blade and jamb seal failures, linkage failures, operator errors, or binding of the damper blades.  If damper failure combines with a pressure difference between the inside and outside of the building, like stack effect in a high rise, or an operating exhaust system with no make-up air, then outdoor air can flow through the open dampers and unit into the building.  Since the flow is not directly related to the operation of the fan, a conventional freezestat installation (where the freezestat is wired to shut down the fan when it trips) will afford little protection.   There are several approaches that can be used to address this problem.

1   Arrange a freezestat trip to cause full flow to the heating and cooling coils.  In addition to shutting down the air handling unit, the freezestat can be arranged to fully open the valves on the heating and cooling coils and to start the chilled and hot water distribution pumps.  These safety interlocks provide several layers of protection.  The most obvious is that activating the heating coil will provide heat inside the unit and warm up the subfreezing air stream until the operating staff can respond to the alarm.  Moving water through the coils provides a second layer of protection.  The thermal energy stored in the piping circuit, even if the water is at ambient building temperatures, will protect the system.  If subfreezing air entering the unit persisted long enough, the heat transfer out of the water system to the subfreezing air stream would eventually drop the temperature of the water system to dangerous levels. 

Some control sequences vary this approach by commanding the coil valves fully open any time the unit is shut down.  While this strategy accomplishes the same intent as the interlock of flow to the coils with the freezestat, there are some operating difficulties associated with it.  If there is no air flow through the unit when it is off, then the air and air handler casing in the vicinity of the coil will approach the cooling or heating water temperature.  In the case of the heating coil, this slug of very hot air, in addition to the other start-up transient conditions, can make the system difficult to start.  Wide-open valves also can put a significant parasitic burden on the heating plant, with energy losses through the air handling unit casing.  Problems associated with a cooling coil operated with a wide-open valve when the unit is off are less severe.  The lower tube and fin temperatures mean that the coil has a lower apparatus dew point (a measure of its ability to dehumidify).  As a result, the air inside the unit casing is cooled and dehumidified more than it would be if the unit were in operation.  The open valve wastes energy and can cause condensation and water damage problems, especially for rooftop equipment in hot and humid environments.

2   Arrange the freezestat to start the return fan when it trips.  This approach provides temporary protection for the coils by using the warm air in the building to pressurize the mixing plenum until the operators can respond to the alarm.  It is a reasonable temporary measure to prevent frozen coils, but can create problems if it persists long term since the air that the return fan is moving is drawn from elsewhere in the building.[4]

3   Include a control sequence that uses the hot water coil to hold the air handling unit casing temperature above freezing any time the unit is not operating.   In this approach, an independent control loop is set up to modulate the heating valve to hold the mixed air plenum or coil plenum at some safe value, like 40°F - 45°F when the unit is off.  With modulating the hot water valve, the system will be easier to start than if the valves were commanded fully open since it will not have a large amount of very hot air that accumulated in the unit while it was off.  In addition, heating energy will be saved: heating energy in the form of lower losses due to lower temperatures at the unit casing, and pumping energy (assuming a variable flow system) since the valve will be modulated, probably nearly closed, most of the time rather than wide open.  Using a mixed air low limit cycle when the system is not operating also makes a central heating plant easier to control since the wide open valve(s) serving a coil with no air flow represent a short circuit on the hydronic system.  The short circuit artificially raises the system return water temperature and creates flow conditions in variable flow systems that look like design load conditions without the corresponding thermal loads.  This condition is similar to the overflow problem on a variable flow chilled water system.  Modulating the hot water valve to hold the air handler above freezing will also make the start-up swings of the entire system, including the economizer dampers, less pronounced.

Obviously, the best approach to guarding against freezing air entering a unit due to damper failure is to prevent the failure.  On new construction projects, the issues that cause damper failure are addressed by good field inspection practices followed up by a thorough functional testing plan. Field inspections should target:

·       Verification of the blade and jamb seals and general damper construction.

·       Verification that the damper assembly is installed in a manner that prevents racking and binding.

Functional testing should target:

·       Verification of damper actuator stroke and smooth operation.

·       Verification of proper interlocks.

·       Verification of any special safeties and cycles incorporated into the system to counteract operating problems like a unit shutdown with the outdoor air dampers stuck open.

In existing systems, the issues typically are related to Operations and Maintenance (O&M) practices that can be addressed by targeting the items listed above as a part of the ongoing O&M program.

3.5.5. Static Pressure Switches

Figure 3.6: Typical static pressure safety switch

(Image courtesy of the Dwyer web site)

Another common safety associated with the mixed air section is a low limit differential pressure switch (Figure 3.6).  This type of interlock is applied to systems where the fan is capable of significant static pressures and a control or damper limit switch interlock failure would make it possible for a fan to attempt to start or remain in operation with both the return and outdoor air dampers closed.[5] 

Static pressure switches do not provide absolute protection against duct system collapse.  The air hammer phenomenon discussed in Section 11.4.3 happens too quickly for this switch to provide protection. 

In addition, large centrifugal fans can take some time to spin down to a stop after power is removed due to the inertia of the fan wheel.  They continue to generate pressure and flow during this spin down cycle.  The setting of this switch needs to be adjusted to shut down the system before the dangerous conditions exist so that the conditions created as the fan spins down do not cause the duct to collapse.  It is important to coordinate closing the outdoor air and return dampers with the fan spin down time on systems where both dampers are closed when the fan is off.[6]  Otherwise, the pressures created as the fan spins down and the dampers stroke closed can cause nuisance trips of this safety device.

3.5.6. Ambient Condition Interlocks

In most locations, the ambient conditions will reach a point where the economizer provides no energy savings benefit. To address this issue, an interlock needs to be provided to disable the economizer and return the system to a recirculating mode with minimum outdoor air.  There are two main ways to accomplish this: outdoor temperature based interlocks and enthalpy based interlocks.

3.5.6.1. Outdoor Temperature Based Interlock

Interlocks based on outdoor dry bulb temperature is usually the least complex and least costly to implement.  Typically, the economizer cycle is terminated based on some ambient outdoor air temperature, also called the changeover setting.  The trick is to select a temperature that will maximize the energy savings obtained from the economizer.  The temperature will vary by location depending on the local climate.  In dry climates with low mean coincident wet bulb temperatures it is possible to set the changeover setpoint near the design space temperature.  However, in a hot and humid climate, this approach can place a significant energy penalty on the air handling system since it will have to do much more dehumidification to cool the outdoor air at or near the design space temperature compared to cooling the return air from the space mixed with the minimum outdoor air requirement. 

Figure 3.7: Cooling minimum outdoor air vs. 100% outdoor air

Figure 3.7 shows that cooling and dehumidifying 100% outdoor air to 54°F requires 3.44 Btu of energy be removed from every pound of air (17.96 Btu/lb – 14.52 Btu/lb).  Cooling the mix of 70% return air and 30% outside air requires that 5.26 Btu/lb be removed (19.78 Btu/lb – 14.52 Btu/lb).  Even though the outdoor temperature is above the required supply temperature, the system will use less energy if it remains on the economizer cycle using 100% outdoor air.  Mechanical cooling is still necessary, but not as much as would be required if return air were used.

The energy needed to cool the air depends on the temperature and moisture of the incoming air, which must be taken into account when determining a dry-bulb economizer changeover setting.  Figure 3.8 illustrates one possible method of determining an energy-efficient dry bulb changeover temperature.  The steps are listed below:

1   A line representing the statistical average of a specific location’s climate conditions is plotted on a psychrometric chart.  In the example, bin data from the Air Force Engineering Weather Data Manual was plotted using the Mean Coincident Wet Bulb (MCWB) temperature for each dry bulb temperature bin. 

2   A second line is plotted that is a constant enthalpy line for the state of the air in the space at its design condition. 

3   The economizer change over controller is set for the dry bulb temperature where the two lines intersect. 

Figure 3.8: Determining temperature-based economizer changeover

From a statistical standpoint, it will take less energy to cool the outdoor air than the return air at the conditions to the left of the intersection for the particular location being analyzed.  Conditions to the right will take more cooling energy to cool the outdoor air compared to the return air.  Therefore, the dry bulb controller should be set up to shut down the economizer for temperatures above the intersection point and allow it to function for conditions below the intersection point. 

As can be seen from Figure 3.8, the exact value for this point can vary significantly from location to location.  In the very hot and humid climate of Key West, the change-over point is somewhere in the 66-67°F range.  In the mild climate of Portland Oregon, the changeover point is 75-76°F.  The lines on this graph were developed from bin weather data in the Air Force Engineering Weather Data Manual but could also be developed from other readily available sources such as NOAA.

It is important to understand that the statistical weather data reports average conditions.  The changeover temperature selected by plotting this data on a psychrometric chart will be correct most of the time, but probably not all of the time.  However, this method provides more insight and better criteria for the correct changeover set point selection compared to simply shutting down the economizer function when the outdoor temperature is warmer than the required supply temperature or warmer than the space design temperature.  When properly applied and then verified by the commissioning process, the optimized set point selection saves energy.

3.5.6.2. Enthalpy Based Interlock

When properly applied, using an enthalpy-based interlock has the advantage of providing an exact solution to determining the changeover point, while the preceding approach provided an approximate solution.  Unfortunately, enthalpy is a property that is more difficult to understand and measure than temperature.  Several approaches to applying enthalpy-based economizer control can provide reliable performance if they are properly implemented, adjusted, and maintained. 

The importance of proper adjustment and maintenance of enthalpy sensors cannot be overemphasized.  Verification of operation requires a basic understanding of psychrometrics and access to a sling psychrometer or some other reliable indicator of atmospheric moisture content.  Most of the maintenance problems are related to the portions of the equipment that sense humidity and often can be traced to contamination of the sensing element by dust and water or failure due to exposure of the plastics used to the direct or reflect rays of the sun and their associated ultraviolet component.  The Enthalpy Change Over functional test included later in this chapter as well as the information in the following paragraphs and in the Sensing Elements section of this chapter are targeted at providing guidance for an enthalpy-based economizer interlock.

There are two main ways to implement an enthalpy based change-over from economizer to non-economizer operation.

·       The most common approach simply assumes an enthalpy state of the return air based on design conditions and then allows the economizer to function only if the measured outdoor air enthalpy is less than the assumed return air state.  This approach avoids the cost of an enthalpy sensor or switch for the return air and allows the change over decision to be made by one master switch for the building.  The approach will provide the desired result as long as the fundamental assumption regarding the constant enthalpy state of the return air is valid.  In projects where constant return enthalpy is not a good assumption, a differential enthalpy-based strategy is often employed.

·       Differential enthalpy-based economizer change-over cycles require at least one enthalpy switch or sensor in the outdoor air stream for the building or system and another switch or sensor in each air handling system’s return air.  The control strategy is arranged to change over from economizer mode to non-economizer mode if the actual measured enthalpy of its return air stream is less than the current outdoor air enthalpy.  This approach provides the most precise solution to determining the changeover setting, but it also adds first cost for the sensors and commissioning, as well as a significant ongoing maintenance burden compared to an outdoor temperature-based interlock.  Enthalpy sensors tend to require more attention than temperature sensors if they are to remain reliable.  This is discussed in greater detail later in this chapter under Sensing Elements.  Thus, the decision to use differential enthalpy to optimize energy consumption should be weighed against the added resources required to implement it.

To measure enthalpy, two position switches and enthalpy transmitters.  As an alternative, it is possible to use a temperature transmitter and a humidity or dew point transmitter and calculate enthalpy based on ASHRAE psychrometric equations. 

3.5.7. Alarms

There are several alarms and smart alarms that can provide benefit for economizer-equipped systems.   The minor commissioning costs to verify the alarms are typically small relative to the potential benefits if the alarms are properly applied.  The alarm requirements should be identified before the system is programmed, allowing the logic to be developed in conjunction with the rest of the operating software.  Alarms to consider include:

·       A mixed air low limit alarm set to provide a warning of an impending freezestat trip.

·       Low and high alarms on the set point used by the system for the mixed air low limit control as well as the normal mixed air control cycle.  These alarms will alert the operating staff to inadvertent changes to these set points that could cause operational problems.

Smart alarms require the development of program logic as opposed to simply entering a value in a point parameter screen.  Thus, there can be some cost associated with implementing them. Options to consider include:

·       Alarm if the system is not on minimum outdoor air but is using preheat.

·       Alarm if the system is using mechanical cooling but is not on maximum outdoor air when the conditions are suitable for using outdoor air.

·       Alarm if the outdoor or return air dampers are hunting.

3.6. Supplemental Information

There are many hyperlinks throughout Chapter 3 that reference supplemental information regarding components of an economizer.  In addition to accessing this information by clicking the hyperlinks, the supplemental information document can be accessed using the link below.

Bevel: Economizer and Mixed Air Supplemental Information



[1]   This has some other advantages since it is often better to have a small actuator on each section rather than a large actuator with a jackshaft running multiple sections to ensure that the dampers achieve their leakage rating.

[2]   Without such regulation, variations in the system pressure relationships that occur with variations in flow on VAV systems can change the minimum outdoor air percentage as the sensible load changes.  This change in minimum outdoor air may not be desirable depending on the relationship between occupancy and sensible loads.  If the sensible load is closely coupled to occupancy, then it is desirable to reduce the minimum outdoor air flow rate as the system flow rate drops off, which tends to maintain a constant percentage of minimum outdoor air.  Without flow regulation, many systems will tend to maintain a constant minimum outdoor air flow rate which increases the minimum outdoor air percentage at low flow.  This situation can cause a system to preheat when it is unnecessary.

[3]   See 1999 NFPA 90A paragraph 2-3.9.2 for and example of this requirement.  Other codes contain similar language or simply refer to NFPA.

[4]   Since the return fan will pressurize the mixing plenum in this operating mode, it is likely that air will be blowing backwards (out) of the intake louver.  Since this means air is leaving the building at the intake louver, then air must be entering the building at some other location, probably through leakage around cracks or through other air handling systems.  Eventually, this could cause freezing problems at those locations.

[5]   Centrifugal fans will generate their rated pressure on their inlet or their outlet or both.  If the inlet connection is totally closed off, as would occur if a control failure caused both the outdoor air and return air dampers to close, then the fan will generate its rated no-flow static pressure as a negative pressure on the inlet side since the discharge is essentially referenced to atmospheric pressure via the supply duct system.  If this no-flow or shut off static pressure rating exceeds the negative pressure class of the intake duct or the negative pressure rating of the air handling unit casing, the casing or duct could collapse.  Note that the positive and negative pressure ratings for any given duct construction is usually not the same.  Typically a duct will be capable of withstanding more positive pressure than negative pressure.

[6]   While not the standard configuration for most systems, this arrangement will often be found on systems where the return damper provides the smoke isolation function required by NFPA or other building codes and in systems with parallel air handling units where the return damper is used to isolate the off-line system from the ductwork and other operating systems to prevent backflow.