At-A-Glance
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NA7.5.15 Supply Air Temperature Reset Controls Acceptance |
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Use Document NRCA-MCH-16-A |
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Purpose of the Test |
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The purpose of the test is to ensure that the supply air temperature in a constant or variable air volume application serving multiple zones, according to §140.4(f), modulates to meet system heating and cooling loads. Space conditioning systems must have zone level controls to avoid reheat, re-cool, and simultaneous cooling and heating (§140.4(d)); or, must have controls to reset supply air temperature (SAT) by at least 25 percent of the difference between the design supply-air temperature and the design room air temperature (§140.4(f)2). Air distribution systems serving zones with constant loads shall be designed for the air flows resulting from the fully reset (e.g. lowest/highest) supply air temperature. The requirements for SAT reset apply to both CAV and VAV systems. Exceptions include: Systems with specific humidity needs for exempt process loads (computer rooms or spaces serving only IT equipment are not exempt), Zones served by space conditioning systems in which at least 75 percent of the energy for reheating, or providing warm air in mixing systems, is provided from a site-recovered or site-solar energy source, Systems in which supply air temperature reset would increase overall building energy use, and. Systems with controls to prevent reheat, re-cool, and/or simultaneous cooling and heating Supply air temperature may be reset in response to building loads, zone temperature, outside air temperature, or any other appropriate variable. |
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Instrumentation |
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The instrumentation needed to perform the task may include, but is not limited to: Hand-held temperature probe or temperature data logger, which was calibrated within the last year with date of calibration noted on the Acceptance Document MECH 16-A. |
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Test Conditions |
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Confirm all systems and components are installed and ready for system operation, including: Duct work Terminal boxes Heating and/or cooling coils Outside air dampers and controls Supply air temperature sensor(s) Electrical power to air handling unit Air handling unit start-up procedures should be complete, per manufacturer’s recommendations. If applicable, BAS programming for the operation of the air handling unit and terminal boxes should be complete, including but not limited to: Heating and cooling coil temperature control Terminal box control (including zone temperature sensors and reheat coils) Discharge air temperature sensor Controls for economizer or outside air damper should be disabled during testing to prevent any unwanted interaction. Before testing, ensure all schedules, set points, operating conditions, and control parameters are documented. All systems must be returned to normal at the end of the test. Document current supply air temperature. |
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Estimated Time to Complete |
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Construction inspection: 0.5 to 1 hours (depending on sensor calibration) Functional testing: 0.5 to 1 hours (depending on system control stability) |
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Acceptance Criteria |
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Construction Inspection Criteria: The temperature sensor(s) must be factory calibrated, field calibrated by a TAB technician, or field checked by test technician with a calibrated standard. Calibration certificate or other supporting documentation must be provided. Functional Testing: For each system, the test criteria include: Supply air temperature controls modulate as intended. Actual supply air temperature decreases to meet the new set point within +/- 2°F. Supply air temperature stabilizes within 15 minutes. Supply air temperature and temperature setpoint must be documented in the acceptance form.
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Potential Issues and Conditions |
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Coordinate test procedures with the controls contractor and building staff, if possible, since they may be needed to assist with manipulation of the BAS to achieve the desired operating conditions. Check to make sure that chilled / hot water coils, if used, are not already fully open and calling for maximum cooling / heating. In this case, reverse Steps 1 and 2 and change the set point range as necessary to allow system to operate within acceptable bounds. In general, take care to avoid demand peaks exceeding what would be encountered during the normal operation of the building. Ensure that all disabled reset sequences are enabled upon completion of this test.
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A. Test Application
Newly Constructed and Additions/Alterations: All new supply air temperature reset controls installed on new or existing systems must be tested.
Some of the most common control variables used to reset supply air temperature set point include, but are not limited to: outdoor air temperature; zone or return air temperature; zone box damper position; or number of zone boxes calling for heating or cooling. Examples of each control strategy are provided below:
•Outdoor air temperature. One control strategy is to reset supply air temperature based on outdoor air temperature. For example, cold deck or cooling mode temperature may reset linearly between 55°F and 65°F while the outdoor air temperature is between 50°F and 80°F, respectively.
•Zone or return air temperature. Another control strategy is to reset supply air temperature based on zone temperature or return air temperature. For example, supply air temperature may modulate to maintain a zone temperature dead band between 70°F and 76°F.
•Zones calling for cooling or heating. In a VAV system, the building automation system may reset the supply air temperature based either on the needs of the zone with the highest heating or cooling loads, or based on a certain percent response from the zone boxes for cooling or heating. For example, in a “trim and response” sequence, the air handler supply temperature may reset downwards by 0.5°F when the maximum system demand is above 100 percent, or reset upwards by 0.5°F when the maximum system demand is below 80 percent.
B. Construction Inspection
Reference the supporting documentation when needed.
1. Verify supply air temperature reset controls are installed per the requirements of the 2016 Energy Standards §140.4(f): Multi-zone systems shall include controls that automatically reset supply-air temperatures:
•In response to representative building loads or to outdoor air temperature; and
•By at least 25 percent of the difference between the design supply-air temperature and the design room air temperature.
2. If an exception is taken to these requirements, note the exception, in which case the test is not needed.
3. Document all system air temperature sensor(s) are factory or field calibrated. Attach a copy of the calibration certificate, TAB verification results, or field verification results including results from system air sensors and calibrated reference standards.
4. Document the current supply air temperature.
C. Functional Testing
1. Disable economizer controls and/or outside air damper during testing to prevent any unwanted interaction or effect on air temperature.
2. Check to make sure that chilled and hot water coils, if used, are not already fully open and calling for maximum cooling or heating. If this is the case, reverse Steps 1 and 2 in the test and/or change the set point range as necessary to conduct this test.
3. Document the reset control parameter (e.g. zone air temperature).
Step 1: During occupied mode, adjust the reset control parameter to decrease the supply air temperature (to the lower supply temperature limit).
Override reset control variable to decrease supply air temperature.
For example, temporarily replace outside temperature signal with a high fixed temperature value for outside air temperature, or temporarily override zone damper signals to imitate all zones calling for maximum cooling. For example, if the supply air is currently 65°F, and the control strategy calls for 60°F cool supply air when outdoor air temperature is above 70°F, override the sensor reading to 75°F.
When the reset control variable input cannot be modified, change the limit of the variable around the currently occurring value. For example, if the control strategy calls for 55°F cool supply air when outdoor air temperature is above 80°F, and the current outdoor air temperature is 75°F, adjust the maximum limit from 80°F to 70°F.
Verify and Document
•Supply air temperature controls modulate as intended.
•Actual supply air temperature decreases to meet the new set point within +/- 2°F.
•Supply air temperature stabilizes within 15 minutes.
•Document both supply air temperature setpoint and actual supply air temperature.
Step 2: During occupied mode, adjust the reset control parameter to increase the supply air temperature (to the upper supply temperature limit).
Override reset control variable to increase supply temperature.
When the reset control variable input cannot be modified, change the limit of the variable around the currently occurring value. For example, modify the reset schedule to create an outside air set point low limit above the current outside air temperature, or shift the entire set point range.
Verify and Document
•Supply air temperature controls modulate as intended.
•Actual supply air temperature decreases to meet the new set point within +/- 2°F.
•Supply air temperature stabilizes within 15 minutes.
•Document both supply air temperature setpoint and actual supply air temperature.
Step 3: Restore reset control parameter to automatic control.
Ensure all set points, operating conditions, and control parameters are placed back at their initial conditions. Remove any system overrides initiated during the test.
Verify and Document
•Supply air temperature controls modulate as intended.
•Actual supply air temperature decreases to meet the new set point within +/- 2°F. Document both supply air temperature setpoint and actual supply air temperature.
•Supply air temperature stabilizes.