Furnace Short Cycling: What It Is and How to Stop It

Do you have a short cycling furnace? Maybe, maybe not. The actual meaning of short cycle in the HVAC industry means that the furnace turns on and off in a very short time, before reaching the set temperature. In general, a furnace cycle will be at least 7 minutes long.

 The trick is to determine if a furnace heating cycle is normal or abnormal. For instance, on a very cold night, a gas furnace with frequent cycling could be normal as the house is losing heat on an accelerated level. The cycling on and off can be more noticeable and frequent with a single-stage or two-stage furnace, whereas a modulating variable-speed furnace might run longer at quieter, lower speeds.

If your furnace is short cycling day and night, even when the temperatures aren’t extremely low, that is abnormal. When your furnace keeps short cycling, it can lead to:

• Decreased comfort
• Reduced energy efficiency and increased gas bills
• Increased wear and tear on furnace components
• Potential damage to the furnace heat exchanger

Furnace short cycling causes cover a range of territory. Some of the most common causes include:

• Thermostat issues
• Restricted airflow 
• Faulty limit switch
• Cracked heat exchanger
• Oversized furnace

We wouldn’t say it’s dangerous, but it can cost you money in higher energy bills and furnace repair costs. If your furnace is short cycling, you want to address the root cause as soon as possible.

The solution depends on the cause. We listed common causes above but will expand on the causes and their solutions here.

Start with the thermostat. A malfunctioning / improperly installed thermostat can send a signal to your furnace to heat at random times. Faulty wiring, dying batteries, and calibration issues can all be an issue. You may need to replace a thermostat if troubleshooting doesn’t resolve the issue. 

A thermostat positioned on a wall that gets direct sunlight can turn off the heat, regardless of what the temperature is in the rest of the house. Your thermostat should be installed in a central location, away from air vents and heat-generating appliances, and out of direct sunlight. Sometimes moving an improperly placed thermostat can resolve short cycling.

Your gas furnace needs good airflow to function. Clogged or dirty air filters can disrupt airflow, causing the furnace to overheat and shut down before a heating cycle is done. Change your furnace filters regularly - every 1 to 3 months, depending on the filter. 

In addition, ensure the return air vents are not blocked by furniture or drapes. 

Finally, ensure the exhaust flue and/or vent pipe are not blocked. If either one is clogged with a bird’s nest, rodent, or somehow a child’s toy got wedged in there, the flue limiter will shut down the furnace to prevent carbon monoxide poisoning.

Furnaces have a safety feature called a limit switch. If it senses the furnace is overheating, it will cause the furnace to shut down. If the limit switch is faulty, it can think the furnace is overheating and shut it down prematurely, causing short cycling.

If it fails, this can cause repeated overheating. A trained HVAC professional can determine if the limit switch is the problem, and replace it if needed. 

A faulty or cracked heat exchanger can cause the furnace to overheat, which will lead to a shutdown. Once the furnace has cooled off, it may try to come on again, quickly overheat, and shut down again. 

An HVAC professional will need to assess the problem. Heat exchangers are designed to last the life of a furnace, so if your furnace is older, you need to be prepared for the repair or replacement conversation. A replacement can cost several thousand dollars.

Learn more in our Repair or Replace Guide. 

This is the most common cause of furnace short cycling. If the furnace has been short-cycling since it was first installed, it may have been improperly sized for your home. Talk with your HVAC service provider to see if the wrong furnace size is a possibility.

A note about oversized systems - while your system may have been sized correctly when it was installed, if you have recently completed any improvements in or on your home to insulate it better, it could reduce the HVAC load in the house. That could now cause the unit to be oversized where it was initially sized correctly. 

The best way to keep your furnace running at peak efficiency is to take care of it. Avoid a clogged air filter and change the filter regularly, every 1-3 months depending on the filter. If you have a smart thermostat, it may display a reminder when it’s time to change the filter. If you don’t set a schedule on your phone. A dirty filter can cause a lot of unnecessary problems.

Schedule an annual furnace tuneup with your favored HVAC company to keep your system working properly. Annual maintenance can catch small problems before they become big ones.

Opt into Trane Diagnostics for peace of mind. You can give your dealer remote monitoring capabilities through the Trane Home App. Your Trane smart thermostat sends data in real-time to your dealer, alerting them to schedule maintenance or of a system malfunction, sometimes before you even realize there’s a problem. 

They will analyze the data and run furnace troubleshooting tests to pinpoint the problem. In some cases, they can solve the problem remotely. In this situation, you would grant them one-time dealer remote access (DRA). 

If they need to come to your home for a service call, the HVAC technician will arrive with the knowledge gained from remote access. This enables shorter service calls and ensures the technician has the proper replacement parts.

Anne Fonda, Content Writer at Trane Technologies

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by John Kim, Senior Product Manager, Furnaces

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