Test Comments
§140.4(k)6 permits two general variable flow control strategies: supply pressure reset by coil demand for systems with DDC controls to the coil level and fixed pressure setpoint control for all others.
It is recommended that minimum VFD speed setpoint be verified. If the minimum speed is below 6Hz (10 percent) the pump motor might overheat. However, if the minimum speed is too high, the system will not be allowed to turn down and the full energy savings of the VFD will not be achieved. To achieve the highest energy savings the minimum speed should be between 6Hz and 10Hz for variable flow systems. It is important to note that this minimum speed can be provided in the EMCS or at the VSD. It should be provided at only one or the other as providing it on both sometimes causes a cumulative minimum that is much larger than the one intended.
Construction Inspection
The static pressure location, setpoint, and reset control must meet the requirements of the Standards Section 140.4(k)6B:
•For systems without DDC, pressure setpoint control is fixed and pressure sensor(s) are located at or near the most remote HX or control valve, or the HX requiring the greatest differential pressure.
•For systems with DDC to individual coils, the pressure sensor(s) may be located anywhere, but are reset according to the valve requiring the greatest pressure and shall be no less than 80 percent open.
•For heating hot water systems or condenser water systems, variable flow is not required, and an Acceptance Test is not required.
The differential pressure sensor (if applicable) is either factory or field calibrated by a Controls contractor or other qualified person. Field calibration requires measuring system pressure, or differential 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 building automation system (BAS). All pressure sensors must be within 10% of the calibrated reference sensor. Supporting documentation must be attached to the Acceptance Form NRCA-MCH-10-A.
Functional Testing
It is acceptable to use this method to verify VFD operation even if the control does have a flow meter. This method compares VFD speed and pressure at full and minimum flow. If at minimum flow, VFD speed is decreased and system pressure is no greater than at full flow, the system is compliant.
Step 1. Modulate control valves to reduce water flow to 50 percent of the design flow or less, but not lower than the pump minimum flow.
Modulating control valves can be accomplished by simply commanding each valve to a specific position or by adjusting temperature setpoints to be within the existing temperature range.
Verify and Document
•Current pump operating speed has decreased (for systems with DDC to the zone level).
•Current operating setpoint has not increased (for all other systems that are not DDC).
•System pressure is within 5 percent of current operating setpoint. Record the system pressure as measured at the control sensor. Record the system pressure setpoint.
•System operation stabilizes within 5 minutes after test procedures are initiated.
Step 2. Open control valves to increase water flow to a minimum of 90 percent design flow.
Open control valves to reach between 90 and 100 percent of design flow. Opening the control valves can be achieved in a variety of ways, such as: resetting control setpoints so that valves respond accordingly, or commanding the valves directly using the DDC control system (i.e., building automation system).
Verify and Document
•Pump speed increases to 100%.
•System pressure increases and is within 5 percent of current operating setpoint, Record the system pressure as measured at the control sensor. Record the system pressure setpoint.
•System pressure setpoint is greater than the setpoint recorded in Step 1.
•System operation stabilizes within 5 minutes after test procedures are initiated.
Step 3. Restore system to initial operating conditions.
•Restore all setpoints, valve commands, etc.