Construction Inspection
Instrumentation used to perform test may include calibrated differential pressure gauge, pitot tube, and drill. Note the date of calibration for the differential pressure gauge on the NRCA-MCH-07-A Form; calibration must be within the past year.
Check that the static pressure sensor location, setpoint, and reset control meet the requirements of Standards Section 140.4(c)2, as follows:
•Location: If system is multi-zone and static pressure sensor is located downstream of major duct splits, multiple sensors must be installed in each major branch with fan capacity controlled to satisfy the sensor furthest below its setpoint.
•Setpoint: Setpoint 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 form.
•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.
Verify that the supply fan includes a means to modulate airflow such as a variable speed drive.
Discharge static pressure sensor(s) is 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 (building automation system). If the value measured by the BAS is within 10 percent of the field-measured value, the sensor is considered calibrated. Attach supporting documentation to the NRCA-MCH-07-A form.
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 in AUTO to discern this.
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 in Hertz.
•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.
•If not performing the test in conjunction with NA7.5.1 (form NRCA-MCH-02-A), then check if another method for verifying VFD operation (besides commanding to maximum flow and cooling) was used.
•System operation and supply fan control stabilizes within 5 minutes. The intent is to ensure the PID control loops are tuned properly so that the system does not hunt.
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. This typically occurs when all of the VAV boxes are operating at minimum cooling flow rate. 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 (may be accomplished by a global command or it may have to be done per individual box).
•Set maximum flow setpoint to be the same as minimum flow setpoint (may be accomplished by a global command or it may have to be done per individual box).
•Space temperature setpoint can be raised above current space conditions to force the VAV box into minimum cooling or heating mode (may be accomplished by a global command or it may have to be done per individual box or per zone thermostat).
Again, you cannot simply override the VFD as it would negate the purpose of the test.
Verify and Document
•Supply fan speed decreases to meet flow conditions. For VFD, record fan VFD frequency in Hertz.
•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 so that the system does not hunt.
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.