NA7.5.16 Condenser Water Supply Temperature Reset Controls Acceptance

Use Form NRCA-MCH-17-A

Purpose of the Test

The intent of the test is to verify that the condenser water supply (entering condenser water) temperature is automatically reset as indicated in the control sequences, based on building loads, outdoor air wet bulb temperature, or another appropriate control variable. All cooling tower system components (e.g. fans, spray pumps) should operate per the control sequences to maintain the proper condenser water temperature and pressure set points.

Many buildings are served by chilled water plants. Chilled water plants must respond to the varying cooling loads throughout the year.  As the loads vary, the chilled water supply temperatures can be adjusted to satisfy the new operating conditions. Often, water-cooled chilled water plants can decrease the condenser water temperature in times of low cooling load. This can be done by running the cooling tower fans at a higher speed, staging on additional fans, or varying water distribution across the tower fill by closing and opening bypass valves.  Although this causes an energy penalty for the cooling tower, it improves the chiller efficiency and the overall plant efficiency.

This requirement for condenser water reset acceptance only applies to those chilled water systems with a cooling tower that implement some kind of condenser water temperature reset control. 

There is no code requirement that chilled water plants employ this type of control. However, if condenser water temperature reset is implemented, then it must be tested per Title 24. The purpose of this test is not to evaluate whether a particular control sequence is the most appropriate for the facility, but whether the system follows the intended control sequence.

Instrumentation

Performance of this test will require measuring water temperatures, and possibly air temperature, relative humidity, system pressures, and system flow rates. The instrumentation needed to perform the task may include, but is not limited to:

•      Hand-held temperature probe to calibrate or check existing sensors

•      Humidity sensor or wet bulb temperature probe / psychrometer

Installed sensors should be checked for accuracy, and may be used for testing where appropriate. Any instruments used for testing or checking other sensors must be calibrated within the past year, with date of calibration noted on the Acceptance Form.

Test Conditions

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

•      Chilled water and condenser water temperature control

•      Equipment start-stop control

•      All control sensors installed and calibrated

•      Control loops are tuned

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

•      Chillers, cooling towers, pumps, air handling units, valves, piping, etc.

•      All piping pressure tested, balanced, flushed, cleaned, and filled with water

•      Control sensors (temperature, humidity, flow, pressure, valve position, etc.)

•      Electrical power provided to all equipment

•      Safeties, interlocks, and alarms are programmed and function correctly (e.g. high/low water alarms, vibration, back-up system operation)

Start-up procedures for all equipment must be complete, per manufacturer’s recommendations. At a minimum, all components and systems served by the chiller and cooling tower should have completed pre-functional checks and be capable of safe operation.

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: 1 to 3 hours (depending on availability of construction documentation – i.e. plumbing drawings, material cut sheets, specifications, etc. – as well as sensor calibration records.)

Functional testing: 2 to 5 hours (depending on familiarity with BAS, method employed to vary operating parameters, ambient conditions, building loads, and time interval between control command and system response)

Acceptance Criteria

Construction Inspection: All ambient temperature and relative humidity sensors used by controller must be either calibrated (manufacturer calibrated with calibration certificates or field calibrated by TAB technician or other), or field checked against a calibrated sensor by the person performing the test.

Functional Test: System must meet the following criteria during the test:

•      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.

If the control sequence differs significantly from that implied by the tests, and has already been tested during the building commissioning process, this specific functional test may be foregone provided that sufficient documentation is provided proving that the system operates according to the programmed sequence.

Potential Problems and Cautions

Condenser water temperature reset is most effective on a moderately warm day. If testing during cold weather conditions, make sure that freeze protection controls as appropriate are installed and functional to prevent equipment damage. Also ensure the conditioned spaces do not fall below safe temperatures, as this may cause discomfort or unsafe working conditions.

If conducting this test during hot weather conditions, make sure the chiller load amps don’t increase as the condenser water temperature decreases. If so, you will need to conduct this test on a cooler day. Likewise, you need to stop the test if the chiller begins to surge.

This test does not require operation of the plant equipment across all operating stages, so it is not necessary, nor desirable, that the system experience peak load conditions. However, the system cooling load must be sufficiently high to run the test. If necessary, artificially increase the load to perform the functional tests, or wait until a time of stable chiller operation. If necessary, reverse Steps 1 & 2 in the functional test based on atmospheric conditions and building loads.

If the system is designed to employ variable flow simultaneously with temperature reset, allow the system to operate as programmed but take care that the water flow rate stays within the minimum and maximum flow rate limits for the chiller(s) and cooling tower(s).  Minimum flow through a cooling tower is important to provide even water distribution and full wetting of the fill to prevent scaling.

Exemption: There is an important exemption associated with this functional test to provide flexibility given the range of chilled water plant operations, as follows: 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.