13.12. NA7.5.8 Supply Water Temperature Reset Controls Acceptance

NA7.5.8 Supply Water Temperature Reset Controls Acceptance

Use Document NRCA-MCH-09-A

Purpose of the Test

This test ensures that both the chilled water and hot water supply temperatures are automatically reset based on either building loads or outdoor air temperature, as indicated in the control sequences. Many HVAC systems are served by central chilled and heating hot water plants. The supply water operating temperatures must meet peak loads when the system is operating at design conditions. As the loads vary, the supply water temperatures can be adjusted to satisfy the new operating conditions. Typically the chilled water supply temperature can be raised as the cooling load decreases, and heating hot water supply temperature can be lowered as the heating load decreases.

This requirement only applies to chilled and hot water systems that are not designed for variable flow and that have a design capacity greater than or equal to 500 kBtuh (thousand BTU’s per hour) , according to §140.4(k)4.

Instrumentation

Performance of this test will require measuring water temperatures as well as possibly air temperatures. The instrumentation needed to perform the task may include, but is not limited to:

Hand-held temperature probes for ice water or drywell bath. Devices must be calibrated within the last year.

Test Conditions

To perform the test, use the building automation system (BAS) to manipulate system operation to achieve the desired control. BAS programming for the operation of the chillers, boilers, air handling units, and pumps must include but may not be limited to:

Supply water temperature control,

Equipment start-stop control,

Installed and calibrated control sensors, and

Tuned control loops.

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

Chillers, boilers, pumps, air handling units, valves, and piping;

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

Verify all piping is pressure tested, flushed, cleaned, and filled with water. Confirm electric power supply to all equipment. Verify 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. All systems must be returned to normal at the end of the test.

Estimated Time to Complete

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

Functional testing: 1 to 2 hours (depending on familiarity with BAS, method employed to vary operating parameters, and time interval between control command and system response)

Acceptance Criteria

Supply water temperature sensors are field calibrated, to within one percent of calibrated reference sensor, with supporting documentation attached to MECH-09A document.

Sensor performance complies with specifications.

Supply water reset works according to control schedule, and actual water temperature is within 2 percent of control setpoint.

Potential Problems and Cautions

If the hot water temperature reset tests when there is minimal heating load, make sure to test the low end of the reset first (coldest hot water supply temperature).If the hottest supply water temperature is tested first, it may be difficult to dissipate the heat in the hot water loop without artificially creating a heating load. Waiting for a small heating load to dissipate the heat in the loop could add significant time to the test procedure.

Where humidity control is required, chilled water supply water reset is not recommended.

Newly Constructed and Additions/Alterations: Applies to chilled or hot water systems that have a supply temperature reset control strategy programmed into the building automation system.

The most common control variables used to reset supply water temperature setpoint include are provided below:

1. 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. When all heating valves are less than 94 percent open, the hot water supply temperature lowers incrementally until one valve opens to 94 percent and maintains the setpoint. When any valve opens more than 98 percent, the hot water supply temperature incrementally raises and maintains 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 designer determines the chilled and hot water temperature setpoint values, which should be provided in the design narrative, specifications, or control drawings.

2. 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 below 35°F and above 50°F, respectively. The design values should be available from, the design narrative, specifications or control drawings.

3. 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. The designer determines actual chilled water temperature setpoint reset schedules and provides them within the design narrative, specifications or control drawings.

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 sensors and outdoor air temperature sensors (when 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, perform a “through system” calibration, which compares the reference reading to the reading at either 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.

Provide supporting calibration documentation and attach to the MECH-09A document.

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 document. These overrides 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.

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.

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 chilled and/or heating hot water supply temperature setpoint is reset to the appropriate value determined by the designer per the control strategy.

• Verify 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 chilled and/or heating hot water supply set-point is reset to the appropriate value.

• Verify actual supply temperature changes to meet the setpoint. This process 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.