NA7.5.6 Supply Fan Variable Flow Controls

Use Form NRCA-MCH-07-A

Purpose of the Test

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.

Related acceptance tests for these systems include the following:

•      NA7.5.1.1 Variable Air Volume Systems Outdoor Air Acceptance

Instrumentation

The instrumentation needed to perform the task may include, but is not limited to:

•      Differential pressure gauge (must be calibrated within the past year)

•      Pitot tube

•      Drill

Test Conditions

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:

•      Supply fan motor control, either VFD or ECM motor control

•      VAV box control (including zone temperature sensors and maximum/minimum flow rates)

•      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.

•      This test can and should be performed in conjunction with NA7.5.1.1 Variable Air Volume Systems Outdoor Air Acceptance test procedures.

Estimated Time to Complete

Construction inspection: 0.5 to 1.5 hours (depending on sensor calibration and minimum VFD speed verification)

Functional testing: 1 to 2 hours (depending on how total fan power at design airflow is determined and system control stability)

Acceptance Criteria

Static pressure sensor(s) is field calibrated to within 10% of reference sensor, with differential pressure gauge and pitot tube.

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.

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).

At full flow:

•      Supply fan maintains discharge static pressure within ± 10 percent of the current operating control static pressure setpoint

•      Supply fan controls stabilize within 5 minute period.

At minimum flow (at least 30 percent of total design flow):

•      Supply fan controls modulate to decrease capacity.

•      Current operating setpoint has decreased (for systems with DDC to the zone level)

•      Supply fan maintains discharge static pressure within ± 10 percent of the current operating setpoint.

Potential Issues and Cautions

Ensure that all disabled reset sequences are enabled upon completion of this test.

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.