Test Comments
Some control variables used to reset supply water temperature include, but are not limited to: outdoor air wet bulb temperature, difference between condenser water supply temperature and chilled water return temperature, or load signal from the chiller. Examples of each control strategy are provided below.
•Outdoor air wet bulb temperature. A common control strategy is to reset supply water temperature based on outdoor wet bulb temperature. For example, the entering condenser water set point may be reset at a fixed amount (e.g. 7°F) above the outdoor air wet bulb temperature, with high and low limits to meet the limits of the chiller and cooling tower operation. The cooling tower may then meet the set point by increasing or decreasing the amount of water circulating through the tower, staging on or off cooling tower fans, or adjusting tower fan motor speed for VFD-equipped fan motors. 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.
•Condenser water and chilled water temperatures. A cooling tower may operate to maintain a certain temperature difference between the condenser water supply and chilled water return. This is done to maintain chiller lift or pressure across the compressor. For example, the control may cycle tower fans on and off, or modulate fan speed, to maintain a 14°F difference between condenser water supply of 70°F – 78°F and chilled water return of 56°F – 62°F.
•Load signal from chiller. The condenser water temperature may follow a load signal from the chiller. For example, condenser water temperature may follow a “horseshoe” shape, increasing in times of highest and lowest load, and decreasing during low and moderate chiller loading. This strategy enables the chiller to maintain capacity at high load, benefit from increased efficiency during times of moderate load, and maintain adequate lift during times of lowest load.
Construction Inspection
Prior to functional testing, verify and document the following:
•Check if the condenser water supply system and control system are installed per the system design, as documented on the building plans or as-builts.
•Check if condenser water supply temperature control sequence, including condenser water supply high and low limits, are available and documented in the building documents.
•Check if all cooling tower fan motors are operational, and cooling tower fan speed controls are installed, operational, and connected to cooling tower fan motors per OEM start-up manuals and sequence of operation.
•Check if cooling tower fan control sequence, including tower design wet bulb temperature and approach, are available and documented in the building documents.
•Check if the following temperature sensors are installed per plans: outdoor air dry bulb and wet bulb, entering condenser water, and leaving chilled water. Note any discrepancies on the Acceptance Form.
All ambient dry bulb temperature, and relative humidity / wet bulb sensors used by controller must be factory calibrated (with certificate), field calibrated by TAB technician or other technician (with calibration results), or field checked against a calibrated reference standard by test technician (with results). Attach supporting documentation to the Acceptance Form.
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).
Document the following from the control system or using test sensors:
•Current outdoor air dry bulb and wet bulb temperatures
•Current entering condenser water supply temperature
•Current leaving chilled water temperature
Functional Testing
If the control sequence differs significantly from that implied by the tests, and / or has already been tested during the building commissioning process, attach a description of the control sequence, a description of the tests that were done to verify the system operates according to the sequence, the test results, and a plot of any associated trend data.
Document reset control parameter (e.g. outside air wet-bulb temperature) on the Acceptance Form.
Step 1. Adjust the reset control parameter to decrease the condenser water temperature (toward the lower supply temperature limit).
Within the programmed reset strategy, change the reset control variable to its minimum value to decrease condenser water supply temperature downward towards the lower limit. For example, if the control strategy calls for the condenser water supply to reset downwards from 85°F to 70°F with a difference of 10°F above current ambient wet bulb temperature of 75°F, override the sensor reading to read a wet bulb temperature below 70°F.
If the reset control variable input cannot be modified, then change the limit of the variable around the currently occurring value. For example, in the example above, adjust the sequence to a difference of 6°F between the condenser water supply temperature and ambient wet bulb temperature.
Take care not to allow condenser water temperature to drop below the chiller low temperature limit. Allow time for the system to stabilize.
Verify and Document
•Condenser water temperature controls modulate as intended.
•Actual condenser water supply temperature decreases to meet new set point within ± 2°F.
•Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet lower set point.
•Chiller load amps decrease.
Step 2. Adjust the reset control parameter to increase the condenser water temperature (toward the upper supply temperature limit).
Using the desired reset strategy, override reset control variable towards its maximum value to increase the condenser water supply temperature upward to its high limit. If the reset control variable input cannot be modified, then change the limit of the variable around the currently occurring value. Allow time for the system to stabilize.
Verify and Document
•Condenser water temperature controls modulate as intended.
•Actual condenser water supply temperature increases to meet new set point within ± 2°F.
•Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet upper set point.
•Chiller load amps increase.
Step 3: Restore reset control parameter and system to automatic control.
Restore all controls and equipment to original settings, and/or restore the high and low limits of the reset control variable. Remove all system overrides initiated during test.
Verify and Document
•Condenser water temperature controls modulate as intended.
•Actual condenser water supply temperature changes to meet new set point within ± 2°F.
•Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet set point.
•All equipment returns to normal operation.