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

Figures

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.

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.

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.

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.

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