What Is Short-Cycling?

Short-cycling is one of the most damaging behaviors that a furnace can experience, causing all sorts of problems in other parts of the system and increasing the chance of a complete breakdown. It is characterized by the system rapidly turning itself on and off over and over again for a long period of time. Let’s take a look at what the causes of short-cycling are, and how you can prevent it from happening.


Short-cycling is caused by the furnace overheating, which activates a safety device called the “limit switch.” The limit switch shuts down the entire system to prevent it from damaging itself by overheating, but it doesn’t actually address the cause of the overheating. The limit switch is simply a stopgap measure, designed to preserve the health of the furnace as much as possible until the source of the overheating can be fixed. Once the furnace has cooled down sufficiently, it will reactivate in response to the thermostat. It will then overheat again, provoking the limit switch and starting the whole cycle over again. If the source of the overheating isn’t found and fixed right away, this prolonged cycle of turning on and off can cause extreme damage to parts of the furnace. The heat exchanger can develop cracks from the strain, causing a fire risk and releasing combustion gases into the ducts.


The most common cause of short-cycling is actually one of the easiest furnace problems to fix. All furnaces include an air filter in their air return ducts, designed to protect the system from dust and debris that might otherwise damage it. If not cleaned or replaced every couple of months, however, the air filter will become so clogged with dust that it will prevent the flow of air into the furnace. This causes the internal temperature of the furnace to steadily rise, kicking off the whole short-cycling problem. The easiest way to prevent short-cycling, therefore, is to have your furnace air filter changed at least once every other month.

If you think your furnace may be in need of repair, call Intelligent Air Services. We provide heating repair services throughout Lakeway.