Troubleshooting Short Cycling Air Conditioning Compressors
Most people assume that if their home is cool during the summer, everything is working properly and no action is needed. On the contrary, you air conditioning system may be working overtime to compensate for system deficiencies that will cause the compressor to fail prematurely. By being aware of your air conditioning run times you may be able to prevent a catastrophic failure of your compressor that may cost up to $1000 to fix.
A residential thermostat controls the operation of the air handler and condenser unit by sensing the indoor air temperature. This corresponds to about 3 cycles per hour at 50% system load. For example, during a moderate summer day in which heat load is 50% of the design heat load, a residential air conditioning system is on about 50% of time, and off about 50% of time. If cycle duration is 20 minutes, then it is turned off after about 10 min, stays off 10 minutes and is turned on again. For a different summer day, the average capacity and run times may be longer or shorter.
Compressors are more efficient the longer they run. The figure below illustrates that if the on-time of an air conditioner is only 5 minutes the efficiency (EER) is 6.0. If the air conditioner ran for 9 minutes to provide the same amount of cooling the efficiency would rise to 7.0. This represents a savings of 17%.
Most of the cooling season the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Improper unit sizing is one of the main reasons for short cycling.
Along with compressor damage, shorter run times also limit the amount of moisture the evaporator coil can remove from the air. The coil needs to be cold for at least 5 minutes to allow the condensate to begin running down the coil and into the condensate drain. If the compressor shuts off prematurely, the condensate will remain on the coil and evaporate back into the air stream. This would reduce the comfort level of the home and may promote mold growth inside the ductwork and the home.
Some digital thermostats have a built in time delay to prevent the unit from operating on cycles shorter than 5 minutes. Heat pumps often have time delays built in. If you notice that your unit stays off for 5 minutes every time it may be waiting for this delay to expire and then comes on immediately. If the unit only runs for 5 to 7 minutes and then shuts off, it is short cycling.
Your problem may be with the thermostat, such as its location is too close to a supply register, there may be a draft coming through the hole beneath the thermostat where the wire comes out of the wall, or the sensor inside the thermostat is bad.
Most compressors are equipped with a low pressure control for compressor protection. This will shut off the compressor when the evaporator pressure falls below the cutout setpoint. When the compressor is off the pressure will rise if the case temperature is warm and restart the compressor. The compressor then runs and drops the pressure down below the setpoint and shuts it off again. This is often a indicator of a low refrigerant charge, but can also be caused by a clogged filter. If it is the filter that is clogged, there will be a temperature drop across the filter. If the temperature drops more than 2 degrees F from the filter inlet and outlet this indicates a restricted filter-drier.
See my related article on locating refrigerant leaks
If your unit has a sight glass installed in the liquid line, which is not typical, you would see a continuous stream of bubbles if the refrigerant charge is low. When the charge is correct, the sight glass will look clear.
You may notice that the compressor is very hot. You should not touch the compressor, but if you place your hand close to it of one of the refrigerant lines, you may be able to feel the heat coming off of the unit. In extreme cases you may be able to smell the paint on the compressor or a slight burning odor. Shut the unit off immediately and call an HVAC technician for service.
It is unfortunate that the a common cause for equipment failure is due to improper sizing when the unit is installed. Larger air conditioning systems are not better, they have shorter run times, reduced moisture removal, and have higher initial costs. They will also cost more to operate over the life on the unit. It would be better to slightly undersize the unit and have it run twice as long than to oversize the unit and have it have 10-minute cycles.
Being able to notice slight changes in the operation of your air conditioner can potentially save you hundreds of dollars in repair or replacement costs and make your home more comfortable during the heat of the summer months.
For more information, look at the Energy Star publication.