Chapter 9: Warm-up

9.1. Theory and Applications

9.2. Commissioning the Warm-Up Process

9.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

9.2.2. Design Issues Overview

9.3. Typical Problems

9.3.1. Unacceptable Cycle Length During Extreme Weather

9.3.2. Warm Up During Moderate Weather

9.3.3. Warm Up Load Control Problems

9.4. Testing Guidance and Sample Test Forms



Figure 9.1: Coil Performance with Elevated Supply Temperatures


9.1. Theory and Applications

The warm-up function is 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. In most instances, the warm-up process can be provided by heating elements located in the system for some other purpose such as preheat, reheat, or space heating. Occasionally, a heating element dedicated to the warm-up function will be encountered. The topic of preheat is covered in Chapter 5: Preheat and reheat is covered in Chapter 8: Reheat. Space heating is discussed in both of these chapters. Chapter 5: Preheat also contains a summary table comparing the various heating processes commonly encountered in air handling systems.

There is more to the warm-up process than simply bringing the air temperature in the space up to the desired set point. All of the mass in the space including the walls, floors, ceilings, and furnishing must also be warmed up. When a building is shut down and allowed to cool off, it begins to lose the thermal energy stored in its mass. [1] The thermal mass will delay the rate at which the building temperature drops off relative to the outdoor temperature. Once the mass is cooled off, it will also delay the rate at which the building mass regains that heat relative to the indoor temperature once the systems are restarted.

Most warm-up processes occur prior to occupancy. Thus, it is possible to minimize the energy consumption for the cycle by locking out any ventilation air requirement associated with occupancy and operating the system in full recirculation mode. The exhaust systems that operate in conjunction with the ventilation air/minimum outdoor air requirement should also be locked out during the warm-up cycle when their associated make up air is not available. Locking out the exhaust systems will save fan energy and help prevent the building from becoming negative relative to atmosphere and creating an unnecessary infiltration load. It is essential that sequences that eliminate ventilation air during the warm-up process for energy conservation purposes be configured, adjusted and controlled in a manner that guarantees that appropriate ventilation and make up air will be provided when the spaces are occupied and/or the exhaust systems are placed in operation.

Alternatively, some other method of warm-up may be desirable such as a sequence using an independent perimeter heating system that does not rely on the make up air system for warm-up.

9.2. Commissioning the Warm-Up Process

The following sections present benefits, practical tips, and design issues associated with commissioning an air handling system’s warm-up process.

9.2.1. 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

Because the heating elements used for warm-up usually perform other functions in the HVAC system they serve, the purpose of the test procedure for the warm-up process will verify that the warm-up function is properly integrated with the other heating functions and the overall operation of the system. Where dedicated warm-up elements are provided, additional testing to verify valve stroke, pressure, flushing, and capacity may also be required. The functional testing tips associated with these tests will be similar to those outlined under this topic in Chapter 5: Preheat and Chapter 8: Reheat.

1 Verify the proper control sequence and integration with both the air handling unit sequence and terminal equipment. This will minimize reheat, thereby saving both heating and cooling energy. Also, properly coordinated and applied warm-up will ensure that scheduled operation of the fan systems is maintained because the operating staff will be able to count on comfortable zones at the beginning of the occupied cycle without resorting to setting extended hours of operation.

2 If warm-up uses the terminal unit reheat coils for warm-up, verify that the flow rate used by the terminal unit maximizes the heat available for warm-up and minimizes the cycle length without sacrificing energy efficiency. Higher flow rates at the terminal reheat coils do not necessarily translate into quicker warm-up times and more warm-up capacity, as can be seen from Figure 8.4 in Chapter 8: Reheat. When ventilation loads cannot be eliminated, the terminal unit flow rate should be at minimum.

3 Where appropriate, verify that make up air, ventilation air, and exhaust systems are not operated during the warm-up cycle (typical sequence of operations). This will result in energy savings due to lower make-up air heating requirements and less fan energy.

4 Verify that heating control valves close off completely with no leakage past the valve. This helps ensure that simultaneous heating and cooling do not occur unnecessarily. Where coils serving other functions in the system provide warm-up, this will typically be checked as a part of the testing of the other functions. Dedicated warm-up coils will require that this function be checked independently.

5 Verify that the warm-up cycle length is not too long during extreme cold weather. This is best verified through trending. If the warm-up cycle is too long, the night setback control sequence and setpoints may need to be adjusted.

6 Verify that heat is available for warm-up during moderate weather (i.e. heating plant is locked out based on atmospheric conditions). The lockout setpoint may need to be overridden during testing.

Addressing control valve close off also ensures that there will be not ripple affects associated with pumping extra cooling and heating water through the distribution network due to heating leakage and the effort made by the cooling system to offset it.

Key Preparations and Cautions


1 It is important that the design and test plan recognize the difference between preheat, reheat, heating and warm-up elements and functions. This is discussed in detail in Chapter 5: Preheat.

2 Reference Chapter 5: Preheat and Chapter 8: Reheat for other potential problems and cautions.

Test Conditions

Simulating a warm-up load can be difficult during moderate weather and summer months. It may be possible to over-cool the building by running the cooling equipment overnight or over the weekend. Then test the warm-up cycle by shutting down the system and verifying the normal morning restart. This test may require temporary manipulation of the outdoor air temperature input, as well as the cost of overcooling. These actions may be marginally beneficial due to the less critical nature of the warm-up process. For systems that must be tested during warm weather, a more palatable approach might involve verification of the control sequence supplemented by trend analysis during the first heating season.

In most cases, the fact that the warm-up element performs other functions in the system probably will mean that the necessary utility and support processes were in place when the primary function provided by the element was tested. However, for dedicated warm-up elements, many of the considerations outlined under this topic in Chapter 5: Preheat and Chapter 8: Reheat will also apply.

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 other sound sensitive device is useful for listening for valve leakage sounds in 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

Considerations are similar to those outlined in Chapter 5: Preheat and Chapter 8: Reheat.


9.2.2. 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.

Can the warm-up cycle be accomplished without ventilation air to minimize energy consumption?

Significant energy savings may be realized by eliminating ventilation during the warm-up cycle, especially for systems with large outdoor air percentages. This can only be done if the area served is unoccupied during the warm-up process. For systems with large outdoor air percentages, an alternative warm-up process such as independent perimeter heating may be beneficial.

Will elevated heating water supply temperatures during the warm-up cycle provide improved performance and limit the warm-up cycle time and pump energy?

Heating water systems can be quite flexible. The water temperature available for warm-up minimizes the cycle length without sacrificing energy efficiency. Elevating the supply temperature of the water serving a coil can increase its capacity significantly.

Does the configuration of the distribution network for the heating medium necessitate special considerations during warm-up?

For air handling units served from a central system, unregulated warm-up processes can cause operational problems in both the distribution network and at the heating plant. For buildings served with multiple air handling systems that use electric heat for the warm-up process, an unregulated process can lead to excessive demand charges.

Do zoning and system selections need to take the differences between interior and perimeter zones into account to improve the benefits of set back and accommodate the needs of a warm-up cycle?

Different requirements for interior and perimeter zones, or other zoning requirements should be accounted for. The location of a zone can make a significant difference in its response to temperature setback during the unoccupied mode and the requirements to warm-up it when the occupied mode is reinitiated. On large buildings or buildings with significantly different envelope conditions at various points on the perimeter, it may be wise to segregate zones with different warm-up and setback characteristics to different HVAC systems if the building will spend a significant amount of time unoccupied and the climate varies. Remember, most office buildings only will have tenants in them for 2,600 to 3,000 hours per year out of a possible 8,760 hours per year.

Does the configuration of the distribution network for the heating medium necessitate special considerations during warm-up?

• Turn off all make-up air, ventilation air, and exhaust fans during warm-up. Note that some jurisdictions may require a pre-occupancy purge, in which case ventilation air may be required. But it may be better to warm up the building first and then start to slowly ventilate over an extended period of time to minimize energy usage.

• Prevent system from going to maximum output unless load is not being met within a reasonable amount of time.

• To prevent overheating and creating a false cooling load, command VAV box dampers closed on units serving interior zones if these zones do not require warm-up.

• Ensure warm-up mode cannot be enabled when the building is occupied, especially if ventilation air and exhaust fans are turned OFF as part of the sequence of operation.


9.3. Typical Problems

The following problems are often encountered when dealing with warm-up processes.

9.3.1. Unacceptable Cycle Length During Extreme Weather

The HVAC system may be able to raise the ambient air temperature to the desired set point quickly and hold it there with relative ease, but the occupants may still perceive the space as cold because all of the surfaces around them are still at a temperature below the ambient space temperature. If the surfaces are massive, this condition will persist for some time. [2] Thus, it may be desirable to modify setback temperatures based on ambient outdoor conditions and the location of the space.

Buildings frequently handle ambient conditions that are outside of the typical design day. For some buildings, a long shutdown for a holiday or weekend during extreme weather may result in an excessively long warm-up cycle. This can lead to several problems including:

·       The building is not brought up to temperature by the time of occupancy, resulting in occupant discomfort. To combat this problem, scheduled operation is often changed to start much earlier. If the earlier start persists in the long term, energy is wasted.

·       For some systems, the ‘learning process’ used by the optimized start-stop sequence will become distorted by the extended warm-up requirement. This distortion can lead to unnecessarily long warm-up cycles during less extreme weather and energy waste.

Modifying the night set-back temperatures as a function of outdoor conditions may eliminate the need for continuous operation. Initially, the set points for warm-up and set-back can be set at the building’s design day values or parameters agreed to with the design team. The operators can then be trained to optimize the settings as they gain experience with the building’s characteristics. Consider the following issues when adjusting the set back temperature:

·       Outdoor temperature A few degrees of difference in set back temperature setpoint can make a significant difference in how often a zone or system operates in set back mode and how long it takes the zone or system to warm back up. Trending the building decay in temperature over time relative to ambient temperature may reveal a condition that merits a change in set back temperatures for certain outside air temperatures.

·       Zone location The location and envelope conditions for any given zone may influence the setback temperature selected for it. Interior zones will tend to cool of more slowly than perimeter zones. The presence or lack of a roof load and the how well the zone is sealed from infiltration will also impact the rate at which it loses energy.

·       Wind and day time sky cover Wind can be a significant factor in how a building loses energy during an unoccupied cycle, especially if the building does not have a tight envelope.

A project engineer for a control contracting firm was caught off guard one Monday morning when his company received a string of irate phone calls regarding a project that had just been occupied. The client’s new building was ‘freezing cold’ after being off for a holiday weekend in the fall. The problem occurred because the building heating plant was programmed to shut down when ambient temperatures rose above 65ºF since the load calculations indicated that there was no heating load above that temperature due to the insulated envelope and the internal gains in the building. Over the weekend, the weather had been above 65ºF, but never above 70º. As a result, the internal temperatures in the building had drifted back into the upper 60’sºF. When the fans started and dutifully entered their warm-up cycle on Monday, there was no heat for them to warm up the building with. The heating plant was not in operation due to the outdoor temperatures being above its activation temperature. The problem was solved by adding a line to the control program that started the heating plant if a warm up cycle occurred, in addition to starting the plant based on outdoor temperature.

9.3.2. Warm Up During Moderate Weather

In some climates a short warm-up cycle may be necessary after a weekend or long holiday shutdown during moderate weather. But, if the weather conditions are moderate enough that the building heating plant is not in operation since it is not ‘heating season’ then there may be no heat to warm-up with. Without a source of supplemental heat, it can take a very long time to bring a cold building up to temperature using only the internal gains. This lack of heating can be prevented triggering the operation of the heating plant if the warm-up process calls for heating. Be sure that the warm-up process and night setback temperatures are tuned to the building. See the sidebar for an example of this problem.

If the building’s heating plant is difficult to start-up and shut down, then delaying the shut down until ambient conditions are such that a warm up cycle will not be required may be a more practical alternative. However, this alternative will probably be more costly due to the parasitic loads associated with keeping the heating plant operating to serve a small, infrequent load.

9.3.3. Warm Up Load Control Problems

One common approach to warm-up for systems served by hot water or steam is to simply open all of the heating valves to full capacity and run that way until the system comes up to temperature. This approach should be used with caution for the following reasons:

·       On systems with significant diversity, the flow or heating capacity imposed by the wide-open valves can exceed the installed capacity of the plant. Even if the capacity is available, wide-open valves can cause problems related to the sudden change it imposes on the system.

·       In water systems, loads that are closest to the plant can short circuit the loads at the end of the pumping network especially if balancing valves have not been installed and set to provide design flow with a wide open control valve and/or flow regulating valves are not provided.

·       The wide-open valves may also cause the distribution pumps to run at peak capacity, which adversely impacts electrical demand in large plants. This high flow rate produces little added benefit since the rate of increase in heat transfer drops off as hot water flow rate increases towards 100%.

·       For loads served by steam plants, wide-open valves can adversely affect the pressure in the distribution header. In extreme situations, water can be drawn out of the boilers and into the distribution system due to excessive velocities through the outlet nozzle on the boiler, which can lead to problems like water hammer.

·       Systems that are served by electric heat need to consider the demand implications of having all of the heating elements go into the full heat mode simultaneously. The demand spike created could result in utility charges that exceed the costs associated with staging the start-up of the systems over a longer time interval, even though such an approach would involve more fan operating hours.

Instead of fully opening the heating valves (or staging electric heating coils fully on), the following methods can be used to help control the warm-up cycle:

·       Design the warm-up sequence so that the heating valves at the zone level are controlled to maintain zone set points. Controlling the valves will also provide some commissioning and operational flexibility by allowing the warm-up temperature supplied by a system to be tuned to the loads it serves.

·       If the heating coil is located at the air handler, control the heating valve to produce about 120°F discharge air. This control process will reduce overheating compared to letting the coils run wild, especially for air handlers located closest to the hot water pumps. Using 120°F discharge air in a warm-up cycle instead of 70°F discharge air significantly increases a coil’s capacity as seen in Figure 9.1. This approach takes advantage of standby heating capacity to shorten the warm-up cycle without a commensurate increase in flow requirements and pump energy. The method has the added benefit of exercising the stand-by capacity in the system, thus verifying the reliability of the system.

Figure 9.1: Coil Performance with Elevated Supply Temperatures

The example in Figure 9.1 shows the benefits of elevated hot water supply temperatures for a low temperature hot water system (from an energy recovery process). The low temperature water increased the length of the warm-up process. Programming the steam heat exchanger control system to briefly increase the supply temperature for the warm up cycle during extreme weather significantly lowered the warm-up cycle time with minimal disruption to the hot water system’s operation. Note that the flow is unchanged despite the significant increase in leaving air temperature.


9.4. 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]   In humidified environments, the building materials can represent a significant humidification load if the relative humidity in the building drops during the off cycle and moisture is lost to the surroundings.

[2]   You can paint yourself a picture of this by deploying a data logger on a project with a set back cycle during the winter months. Bury one probe in the middle of the files in the middle drawer of a full filing cabinet. Dangle another one in free air, next to the space temperature sensor if it has one. Put a third probe outside where it can see the true outdoor air temperatures. Gather samples once every 30 minutes and look at simultaneous plots of the three temperatures over the weekend in the spring or fall compared to a weekend in mid winter.