Why Does My Furnace Keep Turning Off?

In this new blog segment, our HVAC.com experts will be answering common HVAC questions sent in by readers. Today, our question comes from Cynthia in Michigan. She writes:

“Winter is getting colder in Michigan, and my family is having issues with our furnace. We turn it on, set it to 68 degrees, and wait. It blows warm air out of all of the registers for around four to five minutes and then shuts off well before the temperature has reached 68 degrees. It does this repeatedly and never actually heats the house. What can I do at home to fix the problem? Do I need to call an HVAC professional?”

Cynthia appears to be experiencing a problem called short cycling. This term describes issues originating from the heat system that cause the heating cycle to stop and restart prematurely. There are four common reasons why a furnace would short cycle; It has a dirty air filter inhibiting airflow to your blower motor, your thermostat is malfunctioning, you have poor insulation, or your furnace is oversized.

If you don’t address short cycling:

Short cycling, if left alone, can slowly erode the integrity of your HVAC system and cost you more money in the long run. If your system is constantly turning on and off, it is putting unnecessary strain on the components of your furnace. This extra strain can lead to system failure and expensive repairs. Additionally, the inefficiency caused by a system that constantly turns on and off can increase your energy bills, costing you money.

How to address short cycling at home

• Check Your Furnace Filter – The most common cause of short cycling is a dirty or clogged air filter. If your filter is dirty, air cannot pass over the heat exchanger, causing the system to overheat and shut down prematurely. Before calling an HVAC specialist, check your air filter and wash (if it is washable) or change it if it is dirty, clogged, or fits improperly. In general, it is good to check your air filter every month for dirt buildup and wash/replace it if it is dirty or clogged.
• Check Your Thermostat – If your air filter is clean, the next thing you can do at home is test your thermostat. To do this, simply replace your thermostat’s batteries and turn it off and on again. Let your system run for an hour and see if that fixed the problem. If your thermostat is malfunctioning or low on battery, it could be telling your system to shut off prematurely.
• Another thing to check is your thermostat’s location. Your unit should not be located near any heat sources like a space heater, heat register, or in direct sunlight. These may be causing your thermostat to inaccurately read the temperature of your home and shut down your system.
• Check Your Windows – Lastly, if your air filter is clean and your thermostat is not malfunctioning, then the next step to take is to check your windows. A short cycle could be caused by poor insulation in your home. Make sure that you did not leave a window open somewhere in your home and check the seals on your windows. You may have to caulk around the windows to help seal in the warm air your furnace is producing.

When to call a professional

If the problem persists after checking your air filter, thermostat, and windows, then it is time to call a professional. Either your system is too big, causing it to heat up your home quickly, shut off, and start again as soon as it determines it is colder than your target temperature, or there could be something more complicated causing the problem. Locate an HVAC technician in your area to assess the problem and discuss potential solutions.

Here is the bottom line…

If you are experiencing short cycling problems like Cynthia, act fast. Check your air filters to ensure they are clean,your thermostat to make sure it is running properly and not exposed to external heat sources, and lastly check your windows for cracks, holes, or inadequate caulking letting a draft come in. If these steps do not change anything, call a professional.