Test Comments
The most common control variables used to reset supply water temperature setpoint include, but are not limited to: coil valve position; outdoor air temperature; and space conditioning parameters like humidity. Examples of each control strategy are provided below.
•Coil valve position. A central energy management system is used to monitor cooling coil and/or heating coil valve positions to determine when the supply water temperature can be reset. The following example highlights a common heating hot water control strategy, in which all heating coil valve positions (central heating and re-heat coils) are monitored to determine current valve position. If all heating valves are less than 94 percent open, then the hot water supply temperature will be incrementally lowered until one valve opens to 94 percent and then the setpoint is maintained. If any valve opens to more than 98 percent open, then the hot water supply temperature will be incrementally raised and maintained until one valve drops back down to 94 percent open. A similar control strategy can be used to reset the chilled water supply temperature. The chilled and hot water temperature setpoint values will be determined by the designer and should be available from, the design narrative, specifications or control drawings.
•Outdoor air temperature. Another very common control strategy is to reset supply water temperature based on outdoor air temperature. Depending on the building type, internal loads and design conditions, the designer may develop a relationship between the chilled and hot water supply temperatures necessary to satisfy building loads at various outdoor air temperatures. For example, hot water temperature may be reset linearly between 90°F and 140°F when the outdoor air temperature is above 50°F and below 35°F, respectively. Actual supply water and outdoor air temperatures will be determined by the designer and should be available from, the design narrative, specifications or control drawings.
•Humidity control. For special applications like hospitals, museums, semiconductor fabrication and laboratories, the cooling coil control may be based on maintaining a constant relative humidity within the space for not only comfort but also indoor air quality and moisture control (i.e. mold issues). Therefore, the temperature of the chilled water delivered to the coil should be sufficient to remove moisture from the supply air stream and the chilled water temperature can be reset upwards as the latent load decreases. Actual chilled water temperature setpoint reset schedule will be determined by the designer and should be available from, the design narrative, specifications or control drawings.
Construction Inspection
Temperature sensors must be either factory calibrated or field calibrated by a Controls contractor, or other appropriate person. Depending on the control strategy used to reset supply water temperature, sensors can include, but are not limited to: supply water temperature sensor; and outdoor air temperature sensor (if used for reset).
Field calibration requires using either a secondary temperature reference or placing the sensor in a known temperature environment (typically either an ice water or a calibrated dry-well bath). When field calibrating temperature sensors, it is recommended that you perform a “through system” calibration that compares the reference reading to the reading at the EMCS front end or inside the controller (e.g. it includes any signal degradation due to wiring and transducer error). Hydronic system temperature sensors must calibrate to within one percent of the calibrated reference sensor, ice water or drywell bath.
Supporting calibration documentation must be provided, attached to the MECH-09A form.
Functional Testing
Step 1. Change reset control variable to its maximum value.
•Manually change the control variable in order to reset supply water temperature. Check the method used to override the control variable on the NRCA-MCH-09-A form. These include:
•For a valve position control strategy, command at least one coil valve to 100 percent open.
•Adjust discharge air temperature or zone temperature setpoints to drive a valve into a 100 percent open condition. For an outdoor air temperature control strategy, override actual outdoor air sensor to exceed maximum water temperature boundary value. For example, if the control strategy calls for 42°F chilled water when outdoor air temperature is above 70°F, command the sensor to read 72°F. For a humidity control sequence, command the humidity setpoint to be 5 percent below actual humidity conditions.
Verify and Document
•Chilled and/or heating hot water supply temperature setpoint is reset to the appropriate value determined by the designer per the control strategy.
•Actual supply water temperature changes to within 2 percent of the control setpoint.
Step 2. Change reset variable to its minimum value.
Manually change the control variable in order to reset supply water temperature. For a valve position control strategy, command all coil valves to only be partially open. Continuing with one of the examples above, if supply water temperature is reset when a valve is less than 94 percent open, command all valves to be 90 percent open. An alternate method would be to adjust discharge air temperature or zone temperature setpoints to drive a valve into a partially open condition. For an outdoor air temperature control strategy, override actual outdoor air sensor to exceed minimum water temperature boundary value. For example, if the control strategy calls for 90°F heating water when outdoor air temperature is above 50°F, command the sensor to read 52°F.
Verify and Document
•Chilled and/or heating hot water supply temperature setpoint is reset to the appropriate value determined by the designer per the control strategy.
•Actual supply water temperature changes to within 2 percent of the control setpoint.
Step 3: Test automatic control of reset control variable to automatic control.
Ensure all schedules, setpoints, operating conditions, and control parameters are placed back to automatic control.
Verify and Document
•Chilled and/or heating hot water supply set-point is reset to the appropriate value.
•Actual supply temperature changes to meet the setpoint. It may take a few minutes for the water temperature to change depending on system conditions and equipment operation.
•Verify that the supply temperature is within 2 percent of the control setpoint.