13.13. NA7.5.9 Hydronic System Variable Flow Control Acceptance

NA7.5.9 Hydronic System Variable Flow Control Acceptance

Use Document NRCA-MCH-10-A

Purpose of the Test

All hydronic variable flow chilled water and water-loop heat pump systems with total circulating pump power larger than 5 hp shall vary system flow rate by modulating pump speed using either a variable frequency drive (VFD) or equivalent according to §140.4(k)6. Pump speed and flow must be controlled as a function of differential pressure, and pump motor demand must be no more than 30 percent design wattage at 50 percent design flow.

As the loads within the building fluctuate, control valves should modulate the amount of water passing through each coil and add or remove the desired amount of energy from the air stream to satisfy the load. In the case of water-loop heat pumps, each two-way control valve associated with a heat pump closes when not operating The purpose of the test is to ensure that, as each control valve modulates, the pump variable frequency drive (VFD) responds accordingly to meet system water flow requirements.

Note that this is not required on heating hot water systems with variable flow designs or for condensing water serving only water cooled chillers.

The related acceptance tests for this systems is:

NA7.5.7 Valve Leakage Test (if applicable)

Instrumentation

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

Differential pressure gauge (hydronic manometer)

Test Conditions

To perform the test, use the control system to manipulate system operation to achieve the desired control. At a minimum, control system programming for the operation of the central equipment, control valves, and pumps must include, but not be limited to:

Equipment start-stop control,

Installed and calibrated control sensors, and

Tuned control loops.

All systems must be installed and ready for system operation, including:

Heat pumps, cooling towers, boilers, pumps, control valves, piping, etc.

Control sensors (temperature, flow, pressure, etc.)

Verify all piping is pressure tested, flushed, cleaned, and filled with water. Verify electrical power supply to all equipment. Confirm start-up procedures for all pieces of equipment are complete, per manufacturer’s recommendations.

Document the initial conditions before overrides or manipulation of the BAS. Return all systems to their initial condition after test.

Estimated Time to Complete

Construction inspection: 0.5 to 1 hour (depending on availability of construction documentation – i.e. plumbing drawings, material cut sheets, specifications, etc – as well as sensor calibration)

Functional testing: 2 to 4 hours (depending on familiarity with BAS, method employed to vary operating parameters, verification method for system flow and VFD power)

Acceptance Criteria

Differential pressure sensor(s) are field calibrated.

For systems without DDC to individual coils, 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) has no location restriction, but are reset according to the valve requiring the greatest pressure and shall be no less than 80 percent open.

System controls to the setpoint stably.

Potential Problems and Cautions

Difficulties could be encountered with manipulating the control system if not familiar with the programming language. Therefore, a controls contractor should be on-site to assist with adjusting system operation and overriding controls.

Newly Constructed and Additions/Alterations: Applies to any water system that has been designed for variable flow, where the pumps are controlled by variable frequency drives (i.e. chilled and hot water systems, water-loop heat pump and air-conditioning systems).

§140.4(k)6 permits two general variable flow control strategies: (1) supply pressure reset by coil demand for systems with DDC controls to the coil level and (2) fixed pressure setpoint control.

Verify the minimum VFD speed setpoint. When the minimum speed is below 6Hz (10 percent) the pump motor may overheat. However, if the minimum speed is set too high, the system will not adjust down, preventing the full energy savings of the VFD. To achieve the highest energy savings, the minimum speed should be between 6Hz and 10Hz for variable flow systems. Note that this minimum speed may be provided either in the EMCS or at the VSD, but not both as a possible cumulative minimum is not desirable.

The static pressure location, setpoint, and reset control must meet the requirements of the §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) locations are not restricted, but the sensors 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 (when applicable) is 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. All pressure sensors must be within 10 percent of the calibrated reference sensor. Supporting documentation must be attached to the Acceptance Document NRCA-MCH-10-A.

This method is acceptable to verify VFD operation even though the control does has a flow meter. If while at minimum flow, VFD speed decreases and system pressure is not 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 by simply commanding each valve to a specific position or by adjusting temperature setpoints within the existing temperature range.

• Current pump operating speed 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 measured system pressure 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 so that they reach between 90 - 100 percent of design flow. Opening the control valves can be achieved in a variety of ways, such as resetting control setpoints so that the valves respond accordingly, or commanding the valves directly using the DDC control system.

• Verify system pressure increases and is within 5 percent of current operating setpoint, Record the measured system pressure at the control sensor. Record the system pressure setpoint.

• Verify system pressure setpoint is greater than the setpoint recorded in Step 1.