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Why does my AC Compressor Cycle On and Off?

Friday, March 08, 2013

Why does my AC compressor cycle on and off is a common question that every AC contractor hears repeatedly. Cycling is part of the normal process through which your HVAC operates. It’s not the fact that your air conditioning system is cycling that is a problem, but the frequency that indicates problems.

To understand why cycling occurs you must first understand the principle of refrigeration within an air conditioning system. HVAC units use refrigerants repeatedly in order to operate in an economical manner. Chemicals such as R-22 and R-410a, both forms of Freon, move heat from an area, and then cool that location, followed by expulsion of heat to the outside. To accomplish this task, the air conditioning unit goes through a cycle of compression, condensation, expansion and evaporation. In other words, the liquid refrigerant absorbs heat as it changes from liquid to gas, and then gives off heat when it turns from gas back into liquid.

In this process, refrigerant enters the compressor as a low-pressure gas where it is transformed into a high-pressure gas and moved to the condenser. Here, it turns into a liquid where it gives off heat to the outside air. The liquid then moves under high pressure to the expansion valve, which restricts flow and lowers pressure. Next, the low-pressure liquid moves to the evaporator where it absorbs heat from inside air and then changes back into a gas. The hot, low-pressure gas then flows back to the compressor where the entire cycle repeats.

The compressor is the mechanical heart of your air conditioning system. Residential HVAC systems typically use a hermetically sealed unit that combines a compressor with an electric motor. A thermostat that controls run times by sensing indoor air temperature is another essential system component. On a moderate summer day, typically three cooling cycles will occur in an hour when the design heat load is at 50 percent. This means the compressor will run for 10 minutes, then shut down and start up again after 10 minutes. Problems, however, often do not lie within the compressor but in another part of the HVAC system.

Unusual cycles, such as when the compressor runs for a long time without stopping, shuts down briefly, then runs again for a long time, call for air conditioner repair. Short cycling, when the compressor constantly shuts on and off, is one of the most common air conditioning problems. Several conditions cause short cycling, including a faulty or obstructed thermostat, leaking refrigerant, icy coils or an HVAC system that is too big for the building in which it is installed.

Your AC contractor can diagnose and solve the problem. Solutions include replacing or relocating the thermostat, cleaning or replacing fins on the condenser, replacing or cleaning a dirty air filter, adjusting refrigerant charge, detecting and repair refrigerant leaks or replacing an oversized HVAC unit.

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