Functional Testing for Air Handlers

Key Commissioning Test Requirements

General

These test requirements mostly address the air handling system, rather than component-level tests. For additional information on component-level tests, refer to the Air Handling System Reference Guide Functional Testing Field Tips sections.

The purpose of the air handler system-level tests is to ensure that the individual components are integrated to operate on a system level per the design intent and sequence of operations. The air handling system must also integrate effectively and efficiently with other systems such as chillers, boilers, distribution pumps, and terminal units.

Safeties, Interlocks, and Alarms

Verify that all safeties, interlocks, and alarms are programmed (or hard-wired, if applicable) and function correctly in both automatic and manual operating modes.

Sensors

Verify that sensor installation and calibration is sufficient to achieve the design control strategies.

Unit Capacity

Verify that the pre-heat, heating, and/or cooling coil(s) meet the manufacturer's stated part load performance under the actual test conditions. In some instances, verifying coil capacity at peak load may be required. However, creating a peak load operating condition and testing the system at design inlet air conditions may be difficult, especially if the system is tested during off-peak months, so verifying part-load performance can be a more cost-effective solution.

Actuation and Sequencing

Verify proper stroke for outdoor, return, and exhaust dampers to ensure that they open, close, and modulate as intended per the specified economizer and building pressure control strategies. Refer to Chapter 3: Economizer and Mixed Air of the Air Handling System Reference Guide for testing procedures.

Verify that the outdoor air ventilation is maintained at or above the minimum specified value as supply air flow varies to meet system load in variable-air-volume systems.

Verify proper stroke for control valves to ensure that they open, close, and modulate as intended in order to maintain stable air temperature control under all flow conditions. Refer to Chapter 5: Preheat and Chapter 8: Reheat of the Air Handling System Reference Guide for testing procedures. Verify that control valves close off completely.

Verify proper control of heating coil, economizer, and cooling coil in each air handler. Depending on the control programming, each component may have its own PID control loop or all three elements may be controlled by a single loop. Regardless of the strategy employed, testing each function will minimize the potential for unnecessary simultaneous heating and cooling.

Verify proper supply and return fan control per specified sequence of operations, including (but not limited to): morning warm-up; building pressure control; heating and cooling mode; economizer mode; and fire/life safety mode. Refer to Chapter 10: Fans and Drives and Chapter 13: Return, Relief, and Exhaust of the Air Handling System Reference Guide for component-level testing procedures. Typically, supply air flow rate in a variable air volume system is modulated based on maintaining discharge air pressure setpoint in the supply duct as described below. The return fan is controlled so that the desired building pressure is maintained, either directly—by measuring building pressure—or indirectly—by tracking and following supply fan speed. Maintaining the desired pressure relationship between inside and outside conditions can have a large impact on infiltration. Unwanted air migrating into the building can result in significant energy waste as well as lead to comfort and potential moisture problems.

Setpoints and Reset Controls

Verify that the system operates and maintains discharge air temperature setpoint in all modes including Morning Warm-up, Occupied/Unoccupied mode, and Night Low Limit mode.

Verify proper supply fan VFD control static pressure setpoint in variable air volume systems. Typically, the supply fans in variable air volume air handling systems utilize a VFD to modulate fan speed in order to deliver air flow that matches system loads. Supply fan VFD modulation control can be based on maintaining the static pressure setpoint either at the discharge of the fan itself or out in the loop. The static pressure setpoint is based on the pressure required to provide adequate flow through the worst case load (for example, the load with the highest overall pressure drop). Oftentimes, the setpoint (whether specified by design engineer or estimated by controls contractor) is set artificially high, wasting fan energy for the life of the building if not corrected. Actual system pressure requirements and setpoints should be determined during initial system set-up and commissioning to improve system control and minimize fan energy.

Verify the static pressure reset control strategy in variable-air-volume systems. The supply fan static pressure setpoint may be reset downward based on dynamic load requirements to further reduce fan energy. Various indicators can be used to signify reduced load on the system, such as terminal unit damper position. The control strategy resets the system pressure setpoint higher or lower in order to maintain one terminal unit primary air damper at near full-open to minimize system pressure drop and still be in control (95% open, for example). There may be problems with individual VAV boxes that drive the entire system to operate at the high end of the static pressure reset schedule, wasting significant fan energy.

Verify that the maximum setpoint does not exceed the specified design pressure value.

Verify the discharge air temperature reset control strategy. Typically, the discharge air temperature setpoint will be reset based on a parameter(s) that characterizes system load (such as outdoor air temperature, terminal unit valve, or damper position).

Verify proper coordination between individual setpoints and reset strategies. For example, the discharge air temperature reset and discharge static pressure reset control strategies are coordinated. Close coordination between the chilled and hot water temperature reset and the discharge air temperature reset is needed to prevent the air handler from trying to satisfy a setpoint that is impossible with the respective water temperature. This lack of coordination can result in distribution pumps operating at full flow even if there is little load on the system, wasting significant pumping energy. (See examples in the Boiler module - Preparations and Cautions, and the Chiller module - Preparations and Cautions.)

Control Accuracy and Stability

Verify proper control sequence and integration over all associated components (including start-up/shut-down procedures and time delays as well as setpoints and reset strategies.)

Verify that control algorithms generate the proper setpoints based on the reset parameters.

Verify that all PID control loops achieve stability within a reasonable amount of time (typically 2 to 5 minutes) after a significant load change (such as start-up, or automatic or manual recovery from shut down).

Verify that operation of the air handling unit being tested does not create an unstable operating condition in any other system serving, or operating adjacent to, the unit (such as chillers, boilers, distribution pumping, terminal units, and other air handling units). This is typically best addressed through system trending under normal operating conditions.

Key Preparations and Cautions

Prefunctional Checklists and Start-up

Prefunctional checklists should be completed throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor and actuator calibration is typically considered to be part of prefunctional checkout procedures. Prefunctional checks include, but are not limited to: factory start-up procedures have been completed; smoke-fire dampers are operational; control system checkout and programming is complete; duct access doors are closed; and hydronic systems have been flushed and properly filled.

In addition to the prefunctional checklists, all component start-up procedures must be complete in order to conduct functional test procedures. Both the air-side and water-side TAB must also be complete prior to functional testing.

Test Conditions, Considerations, and Cautions

Typically, individual components of the air handling system require testing under specific atmospheric conditions (for example, test reheat/heating coil in winter, cooling coil in summer, and economizer during swing months). Therefore, functional performance testing encompasses all seasons in order to fully observe the entire system under normal operating conditions. However the construction and occupancy schedule may necessitate testing parts of the system during off-peak conditions. System operation and performance may be verified by either creating false loads on the equipment or through manipulation of setpoints to accommodate existing atmospheric conditions. For example, a load can be simulated by adjusting all setpoints to be 10 ºF above or below current ambient conditions and allowing the system to respond accordingly. Also, the preheat/heating coil may be activated to actually put a load on the cooling coil. However, care should be taken to ensure that atypical temperatures or operating conditions do not have an adverse impact on other building systems or the spaces served by the equipment being tested.

The following points should be noted to avoid testing complications:

1 Successful execution of the air handling system functional tests is dependent on the operation of ancillary equipment (including terminal units, chillers, boilers, and distribution pumps). At a minimum, the prefunctional checklists should be completed on the respective components/systems serving the air handling system and should be capable of safe temporary operation.

2 Discharge air temperature setpoint and reset schedules must be coordinated with the chilled and hot water temperature setpoints. Uncoordinated reset schedules can force the distribution pumps to operate at full flow even though there is little load on the system, wasting significant pumping energy.

3 All resets, except the one being tested, should be overridden to prevent unwanted system interaction during testing. Once the individual reset control strategy has been verified, trending should be used to verify proper interaction and integration between the various control strategies. Once the specific reset control strategy has been verified, the remaining resets should be restored. System operation must then be monitored to ensure all control processes remain stable.

4 Depending on when functional testing occurs, one or more control strategies may need to be tested during off-peak conditions (i.e. heating in summer and cooling in winter). Be aware of the potential impact that atypical air temperatures delivered by the unit being tested may have on the building and occupants within the respective zones. Construction work in these areas could be adversely affected if the space temperature deviates significantly from a tolerable level.

5 Proper execution of the supply fan flow control functional test requires functioning terminal units. Yet the terminal units cannot be completely tested until the supply fan is functioning correctly. Ensure that the minimum and maximum design air flow values for each terminal unit are programmed correctly prior to supply fan testing. This should help mitigate the time necessary to verify terminal unit performance and overall integration between the air handling unit and the terminal units being served. Improper terminal unit control can lead to significant control and performance problems at the air handling unit, which can result in excess energy usage. For example, if the minimum flow rate value is incorrectly set too high, the volume of air delivered by the supply fan will not change significantly as the zone cooling load decreases. This could result in the air handling unit acting more like a constant volume system rather than a variable air volume system. In addition, the individual terminal unit will use more reheat in order to prevent overcooling of the zone due to the excess air flow.

6 Indoor air quality may not be maintained without attention to achieving design outside air flow at part-load conditions. As the supply fan slows down to meet reduced demand, the amount of outdoor air introduced into the supply air stream may also reduce (depending on the pressure in the mixed air plenum). Proper outdoor air flow can be achieved by a variety of "active" control methods, including modulation of the economizer dampers, modulation of dedicated outdoor air dampers, or through the use of an independent ventilation fan system.

7 A lack of attention to maintaining the desired pressure relationship between inside and outside conditions can have a large impact on infiltration. Unwanted air migrating into the building can result in significant energy waste as well as lead to comfort and potential moisture problems. Typically, return air flow rate in a variable air volume system is modulated to directly or indirectly maintain proper pressure relationships between inside and outside or adjacent zones. Many control strategies are used to control return fan flow rate including, but not limited to: signal/speed tracking, volume/flow tracking, building static pressure control, return discharge plenum pressure control, or mixed air plenum control. A speed/signal tracking control strategy is the simplest and would take the least amount of time to execute; however, the control strategy may not result in adequate outside air control and building pressure relationships during part-load conditions. The other return fan control strategies are more involved, and testing time is commensurate with complexity.

8 Safety and interlock tests, as well as some test procedures and loop tuning efforts (for example, high/low limit cut-out set points, emergency shut-down procedures, and failure/back-up system operation), could place the system at risk if the sequences do not function as intended. Appropriate precautions and procedures should be in place to protect personnel and machinery, including plans for quickly aborting the test if necessary.

Instrumentation Required

Instrumentation requirements will vary from test to test and typically will include, but are not limited to, the following:

Temperature measurement devices (hand-held devices to calibrate existing sensors and determine coil capacity)

Differential pressure measurement devices (to test installed flow meters and verify fan static pressure control)

Amperage and voltage measurement devices (these verify measured values don’t exceed nameplate)

Tachometer (for verifying fan speed, if necessary)

Flow measurement devices (installed or hand-held devices to measure water flows)

Data loggers (these supplement existing sensors to verify system operation)

Time Required to Test

Overview

The time necessary to execute functional tests on an entire air handling system depends greatly on the size and complexity of the installation and specified control sequences.

The number of system components (such as preheat/heating/cooling coils, outdoor/return/relief dampers, supply/return fans, humidification elements, and terminal equipment), as well as complexity of the sequence of operation (including discharge air temperature and pressure reset strategies, supply and return fan VFD flow controls, economizer/ventilation control parameters, and safeties/alarms) will significantly impact the time associated with testing the entire system. Time to execute system-level tests also depends greatly on the interaction with other systems like chillers, boilers, distribution pumps, and other air handling units.

Therefore, time estimates have been separated out by component on a per unit basis as well as on an overall system level. Component-level tests typically refer to discrete functions of each piece of equipment (such as start/stop procedures, safeties, operational and failure interlocks, and alarms), whereas system-level tests focus on evaluating proper integration of each component to satisfy the desired control strategy (such as staging, set points and reset strategies).

Component Level Testing

Coils: refer to Chapter 5 of the Air Handling System Reference Guide for an estimate of component testing time

Dampers: refer to Chapter 3 of the Air Handling System Reference Guide for an estimate of component testing time

Fans/Drives: refer to Chapter 10 of the Air Handling System Reference Guide for an estimate of component testing time

Humidification: refer to Chapter 7 of the Air Handling System Reference Guide for an estimate of component testing time

Terminal Units: refer to Chapter 12 of the Air Handling System Reference Guide for an estimate of component testing time

System Level Testing

Two to four hours to verify proper discharge air temperature control.

Several hours to a day to verify proper integration between discharge air temperature control and the chiller, boiler, and distribution pumping system control strategies. As coil valves modulate to meet the discharge air temperature control strategy, instability in chiller, boiler or distribution pump staging control loops can occur (refer to the Preparations and Cautions section for further discussion). Both trend analysis and physical observation of system operating parameters will assist in verifying proper system integrated operation.

Two to three hours to verify proper supply fan flow control. Testing generally entails verifying that the reset parameters are optimized for the system and the PID loop(s) maintains setpoint without instability or hunting.

Several hours to days to verify proper integration between discharge air pressure control and the terminal units served by the air handling unit. Proper supply fan flow control is dependent on functioning terminal units, yet the terminal units cannot be completely tested until the supply fan is functioning correctly. Depending on the number of terminal units that require testing, verification of acceptable integration between the air handling unit and terminal units may require a significant time commitment (refer to the Preparations and Cautions section for further discussion). Verification is accomplished by using both trend analysis and physical observation of system operating parameters.

One to four hours to verify the minimum outdoor air control, depending on the complexity of the control strategy. This test can be performed simultaneously with the supply fan control test. Testing generally entails verifying that the control parameters are optimized for the system and the PID loop(s) maintain setpoint without instability or hunting.

Two to six hours to verify proper return fan flow control. This test can be performed simultaneously with the supply fan control test, if applicable. Testing generally entails verifying that the control parameters are optimized for the system and the PID loop(s) maintains setpoint without instability or .hunting.

Discussion

The time necessary to develop a specific functional test or to adapt a generic test procedure to meet the specific needs of the current project has not been included in the estimates above. A rough estimate is two to four hours for each component type.

The time associated with completing prefunctional checklists has not been included in the estimates above. These checks should be made throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor calibration is typically considered to be part of completing the prefunctional checklist.