At-A-Glance
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NA7.5.6 Supply Fan Variable Flow Controls |
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Use Document NRCA-MCH-07-A |
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Purpose of the Test |
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The purpose of the test is to ensure that the supply fan in a variable air volume application modulates to meet system airflow demand. In most applications, the individual variable air valve (VAV) boxes serving each space will modulate the amount of air delivered to the space based on heating and cooling requirements. As a result, the total supply airflow provided by the central air handling unit must also vary to maintain sufficient airflow through each VAV box. Airflow is typically controlled using a variable frequency drive (VFD) to modulate supply fan speed and vary system airflow. The most common strategy for controlling the VFD is to measure and maintain static pressure within the duct. |
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Related acceptance tests for these systems include the following: |
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NA7.5.1.1 Variable Air Volume Systems Outdoor Air Acceptance |
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Instrumentation |
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The instrumentation needed to perform the task may include, but is not limited to: |
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Differential pressure gauge (must be calibrated within the past year) Pitot tube Drill |
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Test Conditions |
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If applicable, supply air temperature reset should be disabled during testing to prevent any unwanted interaction. All systems and components must be installed and ready for system operation, including: Duct work VAV boxes Static pressure sensor(s) (note multiple sensors with separate control loops are often used on large systems with multiple branches) Electrical power to air handling unit Air handling unit start-up procedures are complete, per manufacturer’s recommendations BAS programming for the operation of the air handling unit and VAV boxes must be complete, including but not limited to: |
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Supply fan motor control, either VFD or ECM motor control |
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VAV box control (including zone temperature sensors and maximum/minimum flow rates) |
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Before testing, ensure all schedules, setpoints, operating conditions, and control parameters are documented. All systems must be returned to normal at the end of the test. |
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This test can and should be performed in conjunction with NA7.5.1.1 Variable Air Volume Systems Outdoor Air Acceptance test procedures. |
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Estimated Time to Complete |
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Construction inspection: 0.5 to 1.5 hours (depending on sensor calibration and minimum VFD speed verification) |
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Functional testing: 1 to 2 hours (depending on how total fan power at design airflow is determined and system control stability) |
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Acceptance Criteria |
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Static pressure sensor(s) is field calibrated to within 10 percent of reference sensor, with differential pressure gauge and pitot tube. |
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For systems without DDC controls to the zone level the pressure sensor setpoint is less than 1/3 of the supply fan design static pressure. |
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For systems with DDC controls with VAV boxes reporting to the central control panel, the pressure setpoint is reset by zone demand (box damper position or a trim and respond algorithm or other method that dynamically reduces duct static pressure setpoint as low as possible while maintaining adequate pressure at the VAV box zone(s) of greatest demand). |
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At full flow: |
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Supply fan maintains discharge static pressure within ± 10 percent of the current operating control static pressure setpoint |
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Supply fan control stabilizes within 5 minute period. |
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At minimum flow (at least 30 percent of total design flow): |
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Supply fan controls modulate to decrease capacity. |
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Current operating setpoint has decreased (for systems with DDC to the zone level) |
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Supply fan maintains discharge static pressure within ± 10 percent of the current operating setpoint. |
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Potential Issues and Cautions |
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Ensure that all disabled reset sequences are enabled upon completion of this test. |
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Coordinate test procedures with the controls contractor since they may be needed to assist with manipulation of the BAS to achieve the desired operating conditions. |
A. Test Application
Newly Constructed and Additions/Alterations: All new VAV fan controls installed on new or existing systems must be tested.
B. Construction Inspection
Instrumentation used to perform the test may include a calibrated differential pressure gauge, a pitot tube, and a drill. Note the date of calibration for the differential pressure gauge on the NRCA-MCH-07-A Document; calibration must be within the past year.
1. Check that the static pressure sensor location, setpoint, and reset control meet the requirements of §140.4(c)2, as follows:
•Location: For a multi-zone system with a static pressure sensor located downstream of major duct splits, multiple sensors must be installed in each major branch while controlling fan capacity controlled to satisfy the sensor furthest below its setpoint.
•Setpoint: Setpoint of must be no greater than one-third of the total design fan static pressure. Note the design total static pressure and the setpoint in I.W.C. on the NRCA-MCH-07-A document.
•Setpoint Reset Control: For systems with direct digital control of individual zone boxes reporting to the central control panel, static pressure set points shall be reset based on the zone requiring the most pressure; i.e., the set point is reset lower until one zone damper is nearly wide open.
2. Verify that the supply fan includes a means to modulate airflow such as a variable speed drive.
3. Discharge static pressure sensor(s) shall be field calibrated. Performing a field calibration check requires measuring static pressure as close to the existing sensor as possible using a calibrated hand-held measuring device and comparing the field measured value to the value measured by the BAS. When the value measured by the BAS is within 10 percent of the field-measured value, the sensor is calibrated. Attach supporting documentation to the NRCA-MCH-07-A document.
C. Functional Testing
Supply air temperature reset should be disabled during testing to prevent any unwanted interaction.
Step 1: Drive all VAV boxes to achieve full design airflow.
•The intent is to verify proper supply fan operation at or near full flow condition. This typically occurs when all of the VAV boxes are operating at maximum cooling flow rate. There are a variety of ways to force the VAV boxes to a maximum cooling position depending on the building automation system capabilities and control strategies used, for example:
Command all VAV boxes to maximum flow position (may be accomplished by a global command or it may have to be done per individual box or zone thermostat).
Space temperature setpoint can be lowered below current space conditions to force the VAV box into maximum cooling (may be accomplished by a global command or it may have to be done per individual box or zone thermostat).
For this test, you cannot simply adjust the fan VFD to a maximum speed since the purpose of the test is to show the stability of the pressure control loop that automatically controls the fan speed. The fan speed must be set to AUTO.
Verify and Document
Record system full design airflow in cfm (e.g. from design documents).
Check that supply fan speed modulates to increase capacity. For VFD, record fan motor frequency (Hz).
For multi-zone systems, check that supply fan maintains discharge static pressure setpoint within ±10 percent of the current operating set point. Verification can be accomplished by simply reading the value measured by calibrated pressure sensor and comparing it to setpoint.
When tests depart from NA7.5.1 (document NRCA-MCH-02-A), check if another method was used for verifying VFD operation (besides commanding to maximum flow and cooling).
Verify system operation and supply fan control stabilizes within 5 minutes. The intent is to ensure the PID control loops are tuned properly to prevent the system from hunting excessively.
Step 2: Drive all VAV boxes to a low airflow condition.
The intent is to verify proper supply fan operation when the system is at or near minimum flow conditions. There are a variety of ways to force the VAV boxes to a minimum position depending on the building automation system capabilities and control strategies used, for example:
•Command all VAV boxes to minimum flow position (using global command or per individual box).
•Set maximum flow setpoint to be the same as minimum flow setpoint (using a global command or per individual box).
•Space temperature setpoint can be raised above current space conditions to force the VAV box into minimum cooling or heating mode (using a global command, per individual box, or per zone thermostat).
Again, you cannot simply override the VFD as doing so negates the purpose of the test.
Verify and Document
•Supply fan speed decreases to meet flow conditions. For VFD, record fan VFD frequency (Hz).
•For systems with DDC to the zone level, check that current operating static pressure setpoint has decreased.
•For multi-zone systems, check that supply fan maintains discharge static pressure setpoint within ± 10 percent of the current operating set point. Verification can be accomplished by simply reading the value measured by calibrated pressure sensor and comparing it to setpoint.
•System operation and supply fan control stabilizes within 5 minutes. The intent is to ensure the PID control loops are tuned properly preventing the system from excessive hunting.
Step 3: Return system back to normal operating condition.
Ensure all schedules, setpoints, operating conditions, and control parameters are placed back at their initial conditions.