22102Examples of Good and Bad System Controllability

Example.

• BAD. A single step controller on a 50-ton HVAC package unit that has one compressor attempting to maintain a constant 55 degree discharge temperature. Excessive equipment cycling will result, regardless of controls, and the consequence of poor control and likely premature equipment failure.

• GOOD. A four-step controller on a 50-ton HVAC package unit that has four compressors, where 12.5 tons represents a typical part load day for the building. Cut-in and cut-out set wider than the temperature change of each stage, to prevent short-cycling. Minimum on-off times further safeguard equipment from short-cycle damage.

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