Air conditioners naturally go through on and off cycles so that the temperature in your home is maintained around your set-point, but sometimes you may notice the system is going through faster and more frequent cycles. This process of shutting off too soon and starting up too quickly is known as short-cycling, and it’s not only indicative of a problem — it’s also further damaging your AC system. The additional strain could even burn out a compressor. And in the best-case-scenario: it’s already costing you money on your energy bills.
There are a few ways you can end up with this problem. And it might have started as early as the initial installation.
Size Does Matter
The root cause for this could be that the HVAC system is too large for the space it’s trying to cool. When systems are sized improperly, the end result is always inefficient cooling and increased wear and tear — in this case, an overpowered AC unit generally lowers the temperature at the thermostat pretty quickly, even if the rest of the house is still cooling down. After a moment of thermal exchange, the unit has to switch on again. The ductwork installed plays into this as well, but the basic principle is the same: if your AC system isn’t appropriate for your home, this is one of the ways it will try to tell you.
On the other hand, you might be getting ice on your evaporator coils. While the cause for this is more often neglect of the air filters, a system that’s too small to cool your space might be showing the signs of trying to work itself to death. What was probably once a run-on AC unit is simply becoming a short-cycling AC unit.
Clogged Air Filters
I know it can be easy to forget, but air filters are cheap, they need to be replaced monthly, and they absolutely deserve their own section in all things related to residential HVAC.
Size mismatches aside, the most common reason for short-cycling and numerous other issues I’ve seen is clogged air filters. If an air filter is clogged, the air conditioner has to work harder to compensate for the reduced airflow. But it can only do so at the expense of its own parts. The evaporator coil will not be able to absorb enough heat, and the system will start to shut down early. Typically, the blower motor and compressor will also be stressed and start to overheat, again leading to short cycles. Components will continue to wear and fail prematurely in this fashion until they are repaired or replaced.
Refrigerants and Compressors
The compressor circulates refrigerant through the system during operation, but the part is only designed to handle gas, and if exposed to liquid, can become damaged and throw off the system. Your short-cycling could be the first symptom you’ve noticed, but the end result of a damaged compressor is a replacement compressor.
Low refrigerant can also cause short-cycling and will tend to stress the compressor in particular. Most compressors are equipped with a low-pressure control to protect the compressor by shutting it off when evaporator pressure falls below the point where it should cut out. When the compressor is turned off, the pressure rises until it is high enough to restart. In this case, the low refrigerant is causing this to occur more frequently. Short-cycling associated with low refrigerant can also be accompanied by unusual amounts of heat (be careful not to try to touch the compressor directly, it may be extremely hot) and a slight burning odor. If you notice anything along the lines of heat or burning odors, shut off the unit and call an HVAC technician immediately.
Thermostat problems are another common reason for short-cycling. A malfunction in the thermostat, potentially from a short in the wiring, can lead to the thermostat sending the wrong signals to the AC system. The thermostat may also be reading the wrong temperatures due to its placement either in direct sunlight or directly under an AC vent. The sensor inside the thermostat may also be entirely defective. More modern thermostats will frequently have a timer to restrict short-cycling. This helps prevent further damage to the system, but makes short-cycling as a symptom a little harder to notice. Still, if the AC seems like it waits exactly five minutes and switches back on, that could be exactly what it’s doing.
The bottom line is that although there are several causes for short-cycling, it’s an important symptom to look out for and pay attention to. If your system gets to the point that you notice it, something’s already wrong, and it will only get worse if it’s not taken care of.
Oh, and when was the last time you changed your air filter?