Chapter 5: Preheat

5.1. Theory and Applications. 1

5.2. Commissioning the Preheat6

5.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

5.3. Testing Guidance and Sample Test Forms

5.4. Typical Problems. 10

Figures

Figure 5.1: Make-Up Air Handling Unit with Preheat and Reheat Coils. 3

 

5.1. Theory and Applications

This chapter focuses on heating elements that are in the preheat position in an air handling unit. Preheat elements are the first element in the air stream following the intake and prefilter, which is a position that allows them to protect the rest of the system and building from freezing air. Frequently, this heating element is a coil that uses steam, hot water, or electricity as an energy source. In some situations, the heating element is a fossil fuel fired furnace or an energy recovery coil.

From a psychrometric and HVAC process standpoint, not all heating elements are the same. The specific function they provide depends on:

·       The location of the coil in the system relative to other components.

·       The manner in which the coil is connected to its supply of heating energy (to prevent freezing).

·       The manner in which the coil is controlled.

If the preheat element is to successfully provide the intended function, it is critical that these issues be taken into consideration when the system is configured and the heating element is selected and connected. Failure to do so can result in, at a minimum, the inability to provide the required level of performance and, in the worst case, can damage coils and building elements due to freezing.

The differences between preheat, reheat, warm-up, and heating processes in the air handling unit are also emphasized in this chapter. Many heating elements are labeled as preheat elements, but not all of them are properly configured to perform that function reliably. Preheat is required by an air handling system if it will see operating conditions that will result in supply temperatures that:

·       Are lower than required to maintain the design conditions at the load served.

·       Will subject the system, its components, and/or the loads served to air at subfreezing temperatures and thereby cause damage by freezing.

True preheat applications are typically found on 100% outdoor air systems and on systems with high outdoor air fractions relative to their total supply flow. Unless they are located in an extreme environment, most air handling systems serving office environments will seldom require preheat if their minimum outside air percentage is 20-30% of the supply flow rate and good mixing is achieved. In Section 3.1.2, a discussion on the relationship between minimum outside air and preheat illustrates this concept.

Generally, for 100% outdoor air systems, the coil inlet conditions are set by the worst-case outdoor conditions on record for the area. For recirculating systems, the coil inlet conditions are based on the worst-case mixed air conditions (the maximum anticipated minimum outdoor air requirement and the minimum anticipated return air temperature).

The preceding paragraph made reference to the worst-case outdoor conditions on record for the area. It is important to recognize that these conditions may be significantly different than the heating design conditions for the area. Notice that the ASHRAE 99% design numbers are exceeded for 1% of the hours in a year (about 90 hours). There are some locations where the design condition is above freezing but temperatures occasionally fall below freezing. A design based on the design values would presume that the installation did not have to deal with subfreezing air. The reality of the situation, as represented by the extremes, is that the system would in fact see subfreezing temperatures. A design that did not reflect this contingency could experience significant operating problems or even failures when the subfreezing weather occurred. These problems will be a nuisance in most cases and could be crippling to some facilities serving critical health care or production loads.

In contrast to preheat elements, heating elements that are located downstream of the air handling system’s cooling coils are referred to as being in the reheat position. The summertime cooling coil discharge temperature is typically set based on the amount of dehumidification required to achieve adequate dehumidification for the occupied zone (see Chapter 6: Cooling). The required volume of air, when supplied at this temperature, can overcool the occupied zone under some load conditions. Typically, overcooling can occur in situations where the flow to the occupied zone is set based on air change requirements, ventilation requirements, or make-up air requirements rather than being set by space sensible gains and temperature requirements. Clean rooms and hospital surgeries are good examples of applications where this can occur due to the high air change rates associated with maintaining cleanliness. In these situations, the reheat coil is used to warm the discharge air off the cooling coil as necessary to prevent overcooling of the space while still maintaining the required air flow and space humidity condition. Reheat is an energy-intensive process since it is intentional simultaneous heating and cooling. These functions are discussed in greater detail in Chapter 8: Reheat and are mentioned here in order to distinguish the preheat process from the reheat process. Figure 5.1 illustrates a system that has both a preheat coil and a reheat coil.

Figure 5.1: Make-Up Air Handling Unit with Preheat and Reheat Coils

This 100% outdoor air unit has both a preheat and a reheat coil. The preheat coil first raises the temperature of the air sufficiently to protect the rest of the unit and the area served from sub-freezing temperatures. The integral face and bypass design allows the coil to handle sub-freezing air without danger of freezing the condensate. The reheat coil is located after the cooling coil. The cooling coil discharge temperature is set to deliver saturated air with a specific humidity level as required to maintain the space humidity conditions in the summer. Since the air may overcool the space, the reheat coil warms the air as necessary. Either coil could provide a warm-up function, although it would be an energy intensive process given the system brings in 100% outdoor air.

Some system designs provide the reheat function at the zone location rather than at the central system location. This allows the reheat process to be limited to only the areas requiring it due to the specific needs of the zone while optimizing the central system supply temperature based on the needs of the critical zone.

Systems can also provide the reheat function at both the zone and the central system. Zone reheat coils are often installed in air handling systems that serve a mix of interior and perimeter zones. While the terminal units associated with the perimeter and interior applications are often physically identical and controlled by identical control sequences, there are significant HVAC process differences that need to be considered, since interior zones reheat, but unlike perimeter zones, never have net loss of heat (heating load).

Heating elements located in either the preheat or reheat position can be used for heating in instances where the losses from the zone exceed the internal gains. In this case, the energy that is put into the heating element is used to offset energy losses from the space - a true space heating application. Contrast this with preheat elements, where the energy is required to warm up outdoor air, or reheat elements, where the energy is required to control the HVAC process as necessary to hit the target conditions in the occupied zone. Similarly, elements in either preheat or reheat location can provide the warm-up function often required when a scheduled air handling system is shut down during unoccupied periods and the outdoor conditions result in a net loss of energy from the space.

Many air handling systems will have preheat, reheat, heating, and warm-up requirements for some portion of their operating cycle. Consider the following. If you improved the insulation on the area served by the air handling system, you might lower or even eliminate the heating and warm-up requirements associated with a net energy loss from the space. However, the insulation would not eliminate the preheat requirements or the reheat requirements (although it may modify them) unless you changed the airflow and/or humidity requirements for the zone. These are subtle but important distinctions because the different functions require different control strategies.

Verifying the proper control sequence for preheat elements is an important aspect of commissioning. Table 5.1 contrasts the preheat, reheat, heating, and warm-up processes, summarizing the information in the preceding paragraphs.

Table 5.1 Comparison of the Preheat, Reheat, Heating, and Warm-up Processes

 

Preheat

Reheat

Heating

Warm-up

Function

Offset heating requirements associated with ventilation and make-up air; protect the system and building from sub-freezing air.

Offset unnecessary sensible cooling that was done to provide dehumidification to meet the space design requirement.

Offset space sensible losses through the building envelope associated with the rate of heat transfer exceeding the rate of heat gain in the perimeter zone.

Similar to the heating coil but also must pick up the accumulated loads that occur as the building and its contents cool off during the unoccupied cycle.

Load Offset by Heating Energy

Make up and ventilation air heating load

False cooling load

Perimeter heating and infiltration loads

Accumulated perimeter heating and infiltration loads

Location

First element after the intake for 100% outdoor air systems; first after the mixing box for recirculating systems

After the cooling coil.

Not critical but first after the mixing plenum provides some measure of protection for the rest of the system.

Not critical but first after the mixing plenum provides some measure of protection for the rest of the system.

Installation

Configure to handle air at subfreezing temperatures.2

None that is special to the function.

None that is special to the function.

None that is special to the function.

Control

Controlled to maintain a safe (above freezing) leaving air condition under all operating modes and sequenced with other elements to avoid energy waste. The freezestat must be downstream of the preheat coil if it will see sub freezing entering air temperatures.

The cooling coil discharge temperature setpoint is selected based on design humidity requirements3, while the reheat coil is controlled based on space temperature requirements. The freezestat must be upstream since the reheat coil would not typically be configured for subfreezing air.

Sequence with other system functions to prevent simultaneous heating and cooling and to prevent using heating when the system is not on minimum outdoor air. The freezestat must be upstream since the coil would not typically be configured for subfreezing air.

Sequence with other system functions to prevent simultaneous heating and cooling and to prevent using outdoor air during the warm-up cycle. The freezestat must be upstream since the coil would not typically be configured for subfreezing air.

Notes:

1. Heat transfer elements should always be located downstream of the first set of prefilters in order to protect them from atmospheric dust and dirt and/or dust and dirt returned from the area served.

2. Occasionally, in a moderate environment, preheat is required due to high ventilation rates but the ambient conditions and return air conditions are such that the entering air temperature to the preheat coil will never be below freezing under any condition.

3. In a climate with very low humidity, the cooling coil discharge temperature setpoint may be selected based on temperature requirements, not humidity.

5.2. Commissioning the Preheat

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

5.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

The purpose of the test procedure used with the preheat element will vary with the requirements of the system served. The preheat element control should reliably integrate with the overall system control strategy in a manner that provides the intended function and level of performance.

1 Verify proper sequencing with the other heat transfer elements in the air handling system. This will minimize the potential for simultaneous heating and cooling, thereby saving both heating and cooling energy. Ensuring proper sequencing with the economizer and minimum outdoor air functions will minimize the potential for heating unnecessary volumes of outdoor air, thereby saving heating energy.

2 Verify that the control element range matches the requirements of the control sequence and does not overlap the range of any other elements served by the same signal to prevent unintentional simultaneous heating and cooling.

3 Verify the stroke of control valves to ensure that they close completely. Verifying that control valves close off completely helps ensure that simultaneous heating and cooling do not occur. This will help ensure that there will not be energy waste from the cooling system offsetting heating water leakage. 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.

Addressing items 1 through 3 also ensures that there will be no ripple effects associated with inappropriate preheat element control. One example is unnecessary fan energy triggered by the demand for additional flow from terminal equipment supplied with warm air.

4 Verify the correct shut down sequence for a dedicated heating source serving face and bypass type preheat coils during warm weather. This will eliminate an ongoing waste of heating energy and the associated load to the cooling system.

5 Verify the installation and functionality of the design features targeted at ensuring that the coil can safely and reliably deal with subfreezing air.

6 Verify the coil capacity (required in some instances). 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 test results should be evaluated in the light of the accuracy of instrumentation and the actual conditions at the time of the test.

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

Key Preparations and Cautions

Cautions

1 Testing preheat coils with subfreezing entering air conditions subjects the coil to the danger of freezing if it has not been properly applied and set up. The system and building can be subject to freezing conditions in the event that there is a problem during the test that causes the preheat element to fail to perform. Therefore, testing should proceed in a logical sequence that verifies primary interlocks and safety systems prior to verifying more complex control processes, integrated control functions, and tuning loops.

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. This can pose several problems including:

a Occupant discomfort.

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

c Inadvertent activation of fire dampers, heat detectors, 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 preheat devices. See Section 11.4.2: Fire and Smoke Dampers, and Section 11.4.3: Air Hammer, under the discussion on fusible links.

d Inadvertent activation of heat detectors and rate of rise detectors and subsequent false fire alarm and building evacuation.

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 water or steam hammer problems in piping systems serving the preheat element. If these conditions arise, it indicates that PID control loop is not tuned properly.

4 Functionally testing a large electric preheat element during the summer months while the cooling plant is in operation can cause significant problems including:

a Distribution system load conditions that exceed design and switch gear ratings can trigger trips in the primary switchgear resulting in unscheduled and unanticipated outages.

b Demand peaks well in excess of those that would normally be encountered during normal operation due to the demand that the coil places on the system concurrently with the refrigeration equipment. In locations with high demand charges and ratcheted demand schedules, these peaks can incur a significant cost penalty and lasts for the duration of the ratchet even though the actual period of use was brief, perhaps less than half an hour. A ratcheted demand schedule invokes the peak demand charge seen by the system for a number of months past the month in which the demand was established. In many cases, the ratchet is 11 months; a demand peak set in July will be paid for every month for the next 12 months unless a higher demand peak is set in a subsequent month. For example, if a functional test were to operate a 160 kW preheat coil for 10 or 15 minutes during July when the cooling plant was already operating at peak capacity and the building internal loads were already established, then the coil would probably establish 160 kW demand peak that would not have occurred in a real operating situation and which will probably not be seen again. If this peak occurred on a utility network that had a demand charge of $15 per kW with a 12 month ratchet clause, then the owner would have to pay $2,400 per month (160kW times $15 per kW) every month for the next 12 months (total of $28,800) for that 5 or 10 minutes of electric coil operation during the functional test. As a frame of reference, a 10,000 cfm, 100% outside air make up air handling unit rated for a 50°F temperature rise across the preheat coil could require a 160kW electric preheat coil if that was the method selected to meet the heating load. The demand charges quoted are typical for a project in the Midwest in the late 1990s.

Test Conditions

1 Some of the tests associated with the preheat coil heating source can be accomplished prior to the completion of the air handling unit. For example, pressure tests, flushing, some control valve shut-off test processes, and source side flow tests can all be accomplished with or without air handling unit operation.

2 Other tests, like freezestat testing, interlock testing, discharge control loop testing, and tuning and capacity testing will require that the air handling system be operational and 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 preheat element with the rest of the system will require that the individual components of the system be fully tested and ready for integrated testing. In many cases, the building or at least the portion of the building served by the system must be substantially complete and under load.

4 Simulating a real preheat load in the field is a practical impossibility. In most instances where capacity verification is required, it will be verified based on achieving a target temperature rise above the current ambient conditions, or based on documenting coil performance under the given conditions and then modeling the coil under those same conditions. A capacity test may be limited by the elevation of the actual inlet temperature above the design inlet temperature, the temperature of the heat supply source, and the length of time and conditions under which is it possible to operate the system with an elevated discharge temperature. Often, this condition can be used to simultaneously load test the cooling system although the load is a sensible load rather than a sensible and latent load. An example of one of the more limiting test situations would be load testing a preheat coil on a hot day with a coil supplied by a low temperature water system. Regardless of when an initial test is performed, a preheat coil should always be checked for proper operation under normal seasonal conditions.

5 As an alternative, it is possible to wait for near design conditions and perform the capacity test at that time. This approach is a more realistic test of the coil and also allows the control functions to be evaluated under more realistic conditions. However, it requires that the test process and instrumentation be prepared in advance and that the test team can respond quickly to reach the site and perform the test before the weather changes. This approach also requires that the load served by the system be able to deal with a potentially disruptive test process with little advanced warning.

6 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 tools. A general tool kit is described in 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. This test can be complicated by the need to quickly travel to the site with short notice to run a test while ideal test conditions prevail.

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.

 

5.3. 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

5.4. Typical Problems

Applying the wrong control strategy to the preheat coil can easily produce the desired occupant comfort, but at a significant energy or process control penalty. For example, an economizer-equipped system should be controlled to drive to minimum outdoor air in an effort to maintain discharge set point before the preheat or heating coil is allowed to become active. Failing to ensure this sequencing and simply controlling for a fixed heating coil discharge temperature could result in a significant amount of unnecessary preheat energy consumption. The system would be heating outside air that is actually being brought in for cooling purposes if the economizer is not positioned to minimum outdoor air prior to heating the mixed air stream.

The impact of a misapplied heating coil sequence can ripple out through the rest of the system. A heating coil that was controlled as if it were a reheat coil (based on space temperature and not sequenced with the cooling coil), in an application where reheat was not necessary, could waste an enormous amount of energy due to unnecessary simultaneous heating and cooling. On a VAV system, this effect could ripple out into the fan energy consumption profile if the system supply temperature was raised enough to cause the terminal equipment to demand more flow than was necessary to satisfy the loads.

In contrast, a reheat coil that was controlled in sequence with the cooling coil and economizer dampers (as if it were a heating coil) probably would save energy but at the cost of losing control of the desired space design conditions. Sequencing the reheat function with the other air handling system functions would most likely result in space humidity conditions that were above the required specification. This deviation for the design humidity requirement could have an impact on IAQ, product quality, occupant comfort, and may even result in conditions that degrade the building structural and architectural elements