Chapter 8: Reheat

8.1. Theory and Applications

8.2. Commissioning Reheat Equipment

8.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

8.2.2. Design Issues Overview

8.3. Typical Problems

8.3.1. Minimum Ventilation Rate Set Too High

8.3.2. Ineffective Use of Reheat for a Net Heating Load

8.4 Testing Guidance and Sample Test Forms

Figures

Figure 8.1: Cooling coil discharge conditions necessary to meet the two extremes in the ASHRAE surgery HVAC requirements

Figure 8.2: Load Conditions for a Perimeter Office

Figure 8.3: Load Conditions for a Perimeter One Person Office

Figure 8.4: Effect of Increasing Air Flow Rate at a Reheat Coil.

Tables

Table 8.1 ASHRAE Surgery HVAC Requirements. 2

 

8.1. Theory and Applications

Reheat is one of the many processes that can be provided by a heat transfer element in an HVAC air handling process. Not all heating coils are reheat coils due to their location in the system, their method of control, and the way they are connected to the source of heat (to prevent freezing). In contrast to preheat elements which are located before the air handlers cooling coil, heating elements that are located downstream of the air handling system’s cooling coils are referred to as being in the reheat position. Chapter 5: Preheat includes a summary table comparing the various operations that place heat in an air handling unit’s air stream. Commissioning efforts during design and construction, as well as retrocommissioning efforts for existing buildings have great potential to save energy at the cooling plant, at the heating plant, and at the air handling unit due to improved reheat.

The reheat process is employed when the discharge temperature required to dehumidify an air stream results in air that will overcool the area when delivered in the required volume. Therefore, reheat is required to maintain space temperature control. The reheat process allows for precise control of relative humidity levels, but since reheat processes involve simultaneous heating and cooling, they are especially energy intensive. As a result, many building codes and efficiency standards limit or prohibit the use of reheat for some applications. However, there are situations where the process is unavoidable if the required space conditions are to be maintained. Common examples include:

·       Surgeries, Labor and Delivery Rooms, and other Health Care Applications Air flow rates, temperature, and humidity specifications are often determined in a health care facility by hospital code and licensing requirements.

·       Laboratories The need for large exhaust quantities leads to the need for large make-up air quantities in laboratories. In the summer, this make up air must be cooled sufficiently to dehumidify, then reheated for space temperature control.

·       Clean Rooms Semiconductor manufacturing, pharmaceuticals, and advanced technology fabrication areas frequently face challenges presented by high air change rates set by cleanliness requirements and high exhaust rates set by safety requirements. As a result, the reheat loads for these types of facilities can be significant.

·       Loads with High Ventilation Requirements and Relatively Low Cooling Requirements This category of reheat load is the most common one encountered in commercial buildings. Conference rooms, theaters, auditoriums, and other large places of assembly are particularly prone to requiring reheat when loads are low. But, office zones may also need reheating, especially where one zone serves a lightly loaded common office space.

Surgeries are good examples of why reheat is necessary in some applications. Table 8.1 illustrates ASHRAE recommendations for surgeries, which are typical of the requirements that are found in many hospital licensing standards.

Table 8.1: ASHRAE Surgery HVAC Requirements

Item

Requirement

Total air change rate

25 per hour2

Outdoor air change rate

5 per hour

Design temperature capability1

Capable of maintaining 62 to 80ºF

Minimum humidity level

45% relative humidity

Maximum humidity level

55% relative humidity

1. The system needs to be capable of maintaining any temperature with-in the specified range. The surgeon usually determines the set based on the procedure or personal preference.

2. As a frame of reference, most office building loads can be satisfied with 6 to 10 air changes.

Figure 8.1 illustrates a psychrometric analysis of the requirements in Table 8.1: ASHRAE Surgery HVAC Requirements. Since the space temperature set point is determined by the requirements of the procedure and preference of the surgical team, system serving more than one operating room during the summer months (when there is a dehumidifying requirement) may need to provide the cooling coil discharge air temperatures for the 62º room while simultaneously serving the 80º room. With a typical summer (dehumidifying) design relative humidity level of 50%, the supply temperature for a typical office with a normal latent load will be in the 54 to 56ºF range. However, achieving the lower humidity requirements for a surgery will require a much colder discharge temperature at the cooling coil to ensure adequate dehumidification, as can be seen in Figure 8.1. Since the airflow rate cannot be varied and is relatively high, the high temperature room will require a significant amount of reheat to keep from overcooling. In fact, at the high air change rates required by the licensing standards, even the low temperature room may require some reheat if there are minimal internal gains to the space.

Figure 8.1: Cooling coil discharge conditions necessary to meet the two extremes in the ASHRAE surgery HVAC requirements

In industrial and health care applications, it is not uncommon for the reheat coil to be located in the central air-handling unit with supplemental coils provided at the terminal location. In commercial buildings, reheat is nearly always provided at the terminal location. The primary exception to this is for systems serving theaters and places of assembly.

When reheat coils are located at the terminal equipment location, there is a very subtle but important point to recognize. For all air systems, reheat coils serving perimeter zones will frequently provide space heating in addition to the reheat function. In the winter months, when there is a net loss of heat from the space, the coils must first reheat the air from the supply temperature to the space temperature to offset unnecessary cooling. Then the coils must heat the supply air to a level above the space temperature to make up for the losses from the space.

When the internal heat gains begin to exceed the losses through the building envelope (and for interior zones year round) the reheat coil heats the supply air to the room temperature or less[1]. Although we think of this as a “summer” operating mode, the transition point for many buildings is at a temperature much lower than what would traditionally be called summer. Figure 8.2 illustrates that the transition point to using reheat is above 50°F.

The difference in perimeter and internal load requirements over the course of a year is a good argument for segregating perimeter zones from interior zones so they can be served by different air handling systems. This will allow the perimeter air handling system supply temperature to be reset to a much greater extent during winter months. Concurrently, the supply temperature for the system serving interior zones can be maintained at a lower level to ensure that their temperature needs, which are relatively immune to the outdoor environment, are met.

Figure 8.2: Load Conditions for a Perimeter Office

Figure 8.2 compares load conditions for a typical 120 square foot office. Note the difference between the “design” cooling load, which assumes everything operates at name plate, 24 hours per day with full occupancy at all hours, and the actual peak cooling load. Also note that any time the zone flow demand is less than the minimum flow setting required for ventilation (black horizontal line), the zone will be operating at this constant minimum air volume with reheat. For example, the zone transitions from a net heating load to a cooling load at about 50°F (the purple line crosses 0 Btu/hr at about 10 am). While this zone is a net cooling load, it requires reheat since the minimum airflow for ventilation delivers more cooling than the zone requires. This extra cooling is reheated away until the cooling load in the zone exceeds the cooling delivered by the minimum airflow. Therefore, between 50°F and 65°F outdoor temperatures, the zone is in reheat mode assuming the 56F inlet temperature to the terminal unit is necessary for other zones with cooling loads. When outdoor temperatures are above 65ºF, the zone no longer requires reheat. Notice that the minimum flow setting is critical in determining the required reheat.

Where reheat loads are inevitable and significant, they can often be served relatively easily with recovered energy via low temperature hot water. Many times the heat that would otherwise be thrown away in the cooling towers can provide the necessary energy for reheat during summer and moderate weather. For additional information, see Making Energy Intensive HVAC Processes More Sustainable via Low Temperature Heat Recovery from the proceedings of the 2002 ACEEE Summer Study on Energy Efficiency in Buildings.

8.2. Commissioning Reheat Equipment

The following sections present benefits, practical tips, and design issues associated with commissioning the reheat system. 

8.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

The purpose of the test procedure used with the reheat element will vary with the requirements of the system served.

1 Verify the proper control sequence and integration of that sequence into the air handling unit sequence and terminal equipment sequence. This will minimize reheat, thereby saving both heating and cooling energy.

2 To prevent unintentional simultaneous heating and cooling, the reheat control valve range should match the requirements of the control sequence and not overlap the range of any other elements served by the same signal.

3 Reheat will, by definition, simultaneously heat and cool. The goal of the design and performance tests should be to ensure that this occurs only when necessary.

4 With terminal units, verify that the minimum flow rate on the terminal unit reflects the actual occupant load rather than a design value based on the number of seats or some other criteria. This will lead to greater VAV turn down and minimize reheat thereby saving heating energy, cooling energy and fan energy. See Figure 8.2 for an illustration.

5 Verify that reheat control valves close off completely helps ensure that simultaneous heating and cooling do not occur unless reheat is truly necessary. This will also ensure that there will be not ripple affects associated with pumping extra heating water through the distribution network, and extra cooling water to offset its effect, and extra cooling and heating plant loads.

6 Control valve leakage testing should reveal no detectable leakage. Some of the larger globe valve designs, especially balanced double-seated designs, are not capable of complete and total shut-off. Most valves of this type have specifications for maximum leakage tolerances. Valves that are rated bubble tight should be capable of producing no detectable leakage when stroked fully closed.

7 Optimize discharge temperature reset schedules to maximize AHU discharge temperatures within the constraints of dehumidification and zone cooling requirements. This, too, will minimize the reheat burden.

8 In some instances, flushing and pressure testing of the coil may be required.

9 If noted in the specifications, verifying the coil capacity may be required. Tests targeted at verifying that the installed conditions will allow the coil to perform as intended may prove to be more cost effective that tests targeted at documenting absolute capacity. Capacity tests results should be evaluated in the light of the accuracy of instrumentation and the actual conditions at the time of the test.

Key Preparations and Cautions

Cautions

1 It is important that the design and test plan recognize the difference between preheat, reheat, heating and warm-up elements and functions. Occasionally a reheat coil may be found in the central air handling unit, especially if the cooling coil is being controlled to maintain a specific humidity setpoint. This is discussed in detail in Chapter 5: Preheat. In this type of application, the reheat coil may not in fact be able to safely handle subfreezing air and may require protection of its own in environments where such exposure could occur. This is not an issue if the reheat coil is located at the zone level.

2 Some test procedures, either by design or by failure of the element under test to perform as intended, can cause air handling system discharge temperatures to become significantly elevated above normal. These high discharge temperatures can pose the following problems:

a Occupant discomfort.

b Disruption of the process served by the system and potential damage to product

c Inadvertent activation of fire dampers, heat activators, and/or rate of rise detectors. Fire dampers may shut as well as trigger false fire alarm and building evacuation. This is of greater concern with steam or electric reheat devices. See Section 11.4.2: Fire and Smoke Dampers, and Section 11.4.3: Air Hammer, under the discussion on fusible links.

Test plans should provide for these contingencies by taking steps such as disabling key fire detection elements for the test and ensuring that fusible links have been selected to tolerate any temperature that can be produced in the system.

3 Overly rapid stroking of valves and dampers during a test process can cause air, water, and steam hammer problems in the duct and piping systems serving the reheat element. If any of these conditions arise, it indicates the PID control loop is not tuned properly.

4 Functionally testing an electric reheat element during the summer months while the cooling plant is in operation may cause problems if multiple elements are tested simultaneously, including:

a Distribution system load conditions that exceed design and trip the primary switchgear resulting in unscheduled and unanticipated outages.

b Demand peaks well in excess of those that would normally be encountered during the normal operation of the building due to the demand that the reheat coil places on the system concurrently with the refrigeration equipment. See Chapter 5: Preheat, Potential Problems and Cautions for more discussion of demand peaks.

5 In efforts to increase the reheat capacity to meet a perimeter heating load, it is common to increase the airflow to the zone. By increasing the airflow, the reheat coil must reheat more air up to the room temperature before the heating load can actually be met.  In addition, more air flow at a cooler temperature may be perceived by the occupants as a comfort issue.  Supplying less flow of hotter air to the space may result in more comfortable conditions, reduced reheat energy, and reduced fan energy.  However, the air flow should not drop below the minimum flow required by the diffuser to maintain an adequate discharge pattern.  If the air flow is too low, the air will not be distributed to the zone properly and stratification can occur.  Typically, the air flow should be sufficient to maintain the discharge air temperature 10°F to 20°F above the zone ambient temperature. 

Test Conditions

1 Some testing of the heating source that serves the reheat element(s) can be accomplished prior to the completion of the air handling unit and terminal system. For example, pressure tests, flushing, some control valve shut-off test processes, and heating source flow tests can all be accomplished without air handling unit and terminal equipment operation.

2 Other tests, like discharge control loop testing and tuning, VAV sequence testing and tuning, and capacity testing will require that the air handling system and its terminal equipment be operational and capable of moving the design volume of air, but not necessarily fully under control. Safety systems should be operational to protect the machinery and occupants in the event of a problem during the test sequence.

3 Testing the integrated performance of the reheat element with the rest of the system will require that the individual components of the system be fully tested and ready for integrated testing. To truly test integrated performance, the building or at least the portion of the building served by the system should be substantially complete and under load.

4 Simulating a full reheat load in the field is typically possible under most outdoor ambient conditions by manipulating the other elements of the air handling system as necessary to produce the required flow rates and entering air conditions at the reheat element. Often, this condition can be coordinated with a simultaneous load test of other elements in the unit such as the preheat or the cooling systems although the sensible and latent components of the load may not be correct for a true cooling capacity test.

5 Valve leakage tests and tests that are targeted at verifying valve stroke, spring range, and sequencing should be conducted with the pumping system operating at its peak differential pressure. The differential pressure across the valve plug can have a significant impact on the close-off rating and shift the operating spring range of the valve.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will include the following in addition to the standard tool kit listed in the Functional Testing Basics:

1 Inclined manometers, Magnahelics, Shortridge Air Data Multimeters, and other instruments capable of measuring and documenting low air static and velocity pressures. This equipment can also be used to verify flow rates.

2 A stethoscope or similar sound sensitive device can be useful for listening for valve leakage sounds when verifying that the valve is fully closed.

3 For capacity testing, flow measuring equipment capable of measuring the flow of the heating energy source to the necessary degree of accuracy will be required.

Time Required to Test

Some of the simpler tests like an interlock test or a valve shut-off test can be accomplished in an hour or less with one or two people.

More complex tests like a capacity test can require several hours and several team members to set up and monitor all of the necessary functions, especially if multiple operating points are to be evaluated.

Tests that require referencing back to a model require some time to either develop or support the development of the model that will be used to evaluate the coil's performance. If the modeling capability does not exist in-house, then it may be necessary to retain the coil manufacturer's services if the modeling requirement has not been included in the pricing package.

Field-testing to lab or factory standards is expensive and a practical impossibility in many instances.

 

8.2.2. Design Issues Overview

The Design Issues Overview presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand, even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing.

Is the operation of the reheat element properly integrated with the terminal and air handling unit control sequences?

Reheat, by its nature is an energy intensive process. To minimize this intensity, it is essential that the operation of any reheat elements be fully integrated into the operation of the terminal equipment and fan system.

Integration requires making sure that the minimum flow setting is appropriate to the load served. The sequence should be arranged to ensure that reheat only occurs under minimum flow conditions. See Figure 8.3 for an illustration. Increasing the air flow rate while reheating will often work against the intended heating effect as can be seen from Figure 8.4.

Is the design robust enough to allow the system and its components to be tuned to different, less than design load variations encountered in the field, thereby minimizing reheat and fan energy requirements?

The side bar in Section 8.3 Typical Problems illustrates how important providing a robust flexible design can be when this design is coupled with proper tuning in the field. Providing a system that has flexible capacity beyond the current requirements may be a desirable feature, but failing to tune the minimum zone flow to match the current loads it serves can waste a lot of energy.

Has the design taken all possible steps to minimize the reheat load while meeting the needs of the process?

Minimizing the load is a good first step in any design process, but it can have double or triple savings implications in reheat systems. Not only is heating energy eliminated, cooling energy during the summer months is also impacted. Both of these savings can show up as lower pumping distribution energy requirements. Finally, lower reheat loads often go hand in hand with lower flow rates and thus less fan energy.

Have efforts to minimize the reheat load compromised the ability of the air handling system to properly dehumidify?

It is important not to get carried away with reheat savings. As Einstein once said “Everything should be made a simple as possible, but not simpler.” Similarly, the reheat load should be made as small as possible, but no smaller. Elevating cooling coil discharge temperatures too much can result in inadequate dehumidification, which can lead to poor IAQ, reduced comfort, and loss of product, especially in hot and humid environments.

Does the reheat load lend itself to being met by recovered energy that is otherwise being discarded on the site?

Many times, a little creative thinking will reveal ways to serve the reheat load with recovered energy that is currently being rejected to the atmosphere. The ACEEE paper referenced previously provides some examples of how to use recovered energy for reheat in new and existing construction.

Have electric reheat coil interlock, safety, and control systems been designed to deliver reliable performance over the range of load conditions that could be encountered by the system?

Many energy codes prohibit the use of electric reheat coils. However, even where prohibited in new construction, commissioning providers often encounter them on retrocommissioning projects. Where electric reheat is not prohibited, it is often applied in an effort to lower first cost at the cost of higher operational costs. When encountered, they can often present the commissioning provider with an array of operational problems that are not encountered with systems that use steam or hot water. Common problems include:

·       Minimum flow rates driven by the capabilities of the airflow safety switches rather than the requirements of the zone served.

·       Defeated operating schedules resulting from nuisance high limit safety trips that occur from the residual heat that remains at the coil location when the fan system is shut down with the coils operating at full capacity instead of cycling the coils off and allowing them to cool down for a few minutes before cycling the fan off.

·       Poor zone temperature control as the result of the staged capacity control strategies typically provided unless more costly SCR controls are specified and correctly installed and commissioned.

 

8.3. Typical Problems

8.3.1. Minimum Ventilation Rate Set Too High

On the scoping study for a retro-commissioning project, the commissioning provider noticed that the boilers serving an office complex were firing at a 30% to 50% duty cycle during the summer months. Since the loads served were primarily offices, this seemed like a good indicator of a retro-commissioning opportunity. Further investigation revealed that the complex was being ventilated for an occupant level that was approximately 3 times greater than the actual level. A contributing factor was that virtually all of the air handling systems on the site were being operated around the clock to serve a few zones on each system. By adjusting ventilation rates to match the actual occupant levels and implementing scheduling at the zone level, over $60,000 per year in operating costs were avoided in the form of reheat energy at the boilers and chillers and fan energy at the air handling units. The payback for the effort was well under one year.

Minimum airflow rates for VAV boxes are often in the design stages, based on the design occupancy levels. While this practice seems reasonable, in reality there is often a large difference between the design loads and the actual loads. Design loads are often based on variables like the number of chairs in the office, without taking into account that all chairs will not be filled simultaneously. The actual minimum flow setting required is generally better represented by only one person being in the office most of the time. Setting the minimum airflow rate higher than necessary leads to high reheat loads when the minimum level would overcool the space. As long as the flow demanded by the zone is below the flow supplied by the minimum airflow, the system will act as a constant volume reheat system.

The story in the side bar is one example of how taking steps to ensure that the design and implementation of a reheat system is matched to the actual loads can save a significant amount of energy. Additional discussion of this topic can be found in the Document Review Design Brief, which can be downloaded free of charge from the Energy Design Resources web site at www.energydesignresources.com.

Figure 8.3: Load Conditions for a Perimeter One Person Office

This plot shows the loads for the office illustrated in Figure 8.2 if it was an interior zone. Without the influence of solar and transmission effects, the difference between the actual design day load and the design load is even more significant.

In Figure 8.3, both the minimum flow setting with design occupant levels and the minimum flow rate with actual occupant levels are shown. With the design minimum flow, almost the full daily operation relies on reheat to meet the desired space conditions. With the actual minimum flow required, the zone does not need reheat during the entire day.

8.3.2. Ineffective Use of Reheat for a Net Heating Load

You may have encountered a system that could not meet the perimeter heating requirements because the same air handler served the perimeter zones as the interior zones, which required cold air all year. In efforts to increase the reheat capacity to meet this perimeter heating load, it is a common mistake to increase the airflow to the zone. In fact, increasing the airflow does not lead maximum capacity. This result stems from the difference between the reheat function and the heating function, discussed previously. The reheat function increases the delivered air temperature from the air handler’s discharge air setpoint (51.6°F in the example below) to the room temperature. The heating function increases the delivered air temperature above the room temperature in order to meet the heating load. By increasing the airflow, the reheat coil must reheat more air up to the room temperature before the heating load can actually be met. Supplying less flow of hotter air to the space results in more comfortable conditions, reduced reheat energy, and reduced fan energy.

Figure 8.4: Effect of Increasing Air Flow Rate at a Reheat Coil.

This drawing illustrates a reheat coil serving a perimeter zone where the minimum flow rate was adjusted to match the current occupant load. The control sequence was modified to heat at minimum air flow, rather than increase air flow along with the reheat function. Notice how the modified performance delivers much warmer air while at the same time, the reheat burden has bee minimized. In the design condition, the coil delivered a lot of air but all of the available capacity was used up in simply offsetting unnecessary cooling (reheat). Serving a space with a design temperature of 71ºF using a high volume of air at 73.5ºF provided very little heating capacity and made for a drafty environment.

 

8.4. 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

 



[1]   If there are not heat losses from the space and only the potential for internal heat gains, then if all of the internal gains are off (lights, computers, office equipment) or not present (occupants) then delivering air at the space temperature would be the only way to maintain space temperature. Of course, if this is the case, then you may want to consider a way to simply shut down the zone and save the fan energy in addition to the reheat energy. The one exception to this is for a warm-up cycle, where elevated supply temperatures may bring a space up to occupied temperature more quickly.