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HVAC School - For Techs, By Techs

Mar 11, 2021

In this podcast episode, Bryan and Trevor Matthews discuss compressor short cycling. They discuss how to diagnose and prevent that issue. Trevor and Bryan primarily refer to the Bulletin AE17-1262 throughout this episode, which you can find HERE.

Compressor misdiagnosis is very common, but we generally encounter two types of compressor failures: electrical failures and lubrication failures. Short cycling causes a loss of oil in the compressor, which may lead to lubrication-related failure. Each time a compressor starts, there is a reduction in suction pressure; the pressure drop then causes the saturation pressure to drop. That can then cause the oil to flash and shoot out of the compressor.

Short cycling has many potential causes, including protectors, thermostats, low and high-pressure controls, oversized condensers, and oversized compressors. In some cases, the controls can also cause operational short cycling to meet customer demands (or failure to match the load). Each manufacturer may have a different acceptable range of starts per hour, but some customers may request more or fewer starts than recommended.

Cycle length and frequency are keys to system longevity. So, we can prevent compressor short cycling by keeping the system operating within the manufacturer's specs. There are also several components that can help manage the factors that cause short cycling, including bleed resistors on capacitors, which manage relay operation. Troubleshooting is also one of the main preventative measures; if you replace the compressor without troubleshooting, your new compressor may short cycle and fail prematurely just like the first one.

Trevor and Bryan also discuss:

  • Oil behavior and losses
  • Customer demands
  • Manufacturer specs and communication
  • Oversized compressor issues
  • Internal low-leak discharge check valves
  • Digital scroll compressors in a tandem set
  • Short cycling's effects on the whole system
  • Airflow and pressure
  • Load matching

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