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Why Your Furnace May Be Short Cycling

Delivering Quality Comfort to the Homes and Businesses of Hartford County and New Haven County

Have you ever noticed your furnace rapidly turning itself on and off throughout the day? If so, you have a problem. That behavior is called “short cycling” and it’s extremely harmful to the furnace. Short cycling is a common problem caused by the furnace’s interaction with a part called the limit switch.

The limit switch is in place to monitor the internal temperature of the furnace. When the furnace temperature gets too high, the limit switch shuts the furnace down to protect it from damage associated with overheating. This is only a temporary measure, however. As soon as the furnace cools down enough, it will start back up again as long as the thermostat is calling for heat. This causes the furnace to become locked in an endless cycle of startups and emergency shutdowns, putting it under much more stress than it’s built to handle and shortening its lifespan. So what causes the limit switch to activate in the first place? Read on to find out.


The first possibility is that your furnace is actually overheating. This can be caused by any number of things. If your furnace’s air filter is dirty, it can restrict airflow into the furnace and trap heat inside. This trapped heat will eventually trip the limit switch. Another possibility is a cracked heat exchanger. This is a dangerous problem in and of itself, partially because it can suck oxygen through the crack and down to the burners. This can cause the burners to flare up more than they’re supposed to, producing extra heat and causing a fire risk.

Oversized Furnace

The other possibility is that your furnace is actually too large for your home. When an oversized furnace starts up, the increased heat output causes it to very quickly reach an internal temperature high enough to trip the limit switch. The furnace is actually fine, as it’s designed to run hotter. The limit switch doesn’t know that, though, so the furnace still short cycles and suffers the same damage.

If your furnace is short-cycling, call Dynamic Mechanical Heating & Air Conditioning and schedule a service appointment with us. We provide professional heating services in the Hartford area.

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