The design of energy efficient chilled water systems should take into account the unintended consequences of low-load conditions on equipment life expectancy.

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When selecting a chiller for a building, the primary focus can be the maximum load of the HVAC system. If the chilled water system consists of a single chiller, the chiller must be capable of delivering adequate capacity on a design cooling day when outside air temperatures are the highest. This same chiller must also be capable of steady state operation during periods of low cooling load without frequent stopping and starting. If the chiller cannot operate continuously, it will cycle excessively and shorten the life of the compressor(s).  If this cycling occurs over a significant part of the operating season, a chiller could fail within just a few years. An obvious solution is to select more and smaller chillers to lower the minimum tonnage. However, this is frequently not practical due to first cost, available space, and other factors.

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Regardless of the number of chillers, the engineer needs to verify that the total volume of the chilled water in the system is large enough to prevent excess cycling. If the volume of water is insufficient, a chilled water storage vessel can be used. Most chiller manufacturers publish information on minimum loading and minimum run time. This information, along with system volume, should be used to determine whether the chiller selected can operate properly at low load conditions.  When the minimum load and minimum run time are adhered to, it can help ensure a robust life expectancy of the equipment.

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Another factor in short-cycling equipment can be the temperature controls. Control technicians tend to focus on controlling air and water temperatures within tight tolerances, while also programming schedules to reset chilled water and supply air temperatures to reduce energy use. Unfortunately, this quest for energy savings can lead to frequent cycling of the equipment. Therefore, low-load cooling strategies should be included in the design of the controls. Sufficient dead bands for both supply air and chilled water supply temperatures should be considered and be accessible for adjustment during commissioning.

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Optimizing the control of a system should include both minimizing energy use and monitoring the equipment cycling times.  This should be the goal of all members of the project team. In addition to chillers, these concepts apply to other major system equipment, such as boilers and condensing units.
 

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