Furnace runs then shuts off and starts again

HVAC Logo IconBy HVAC.comJuly 31, 2023

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HVAC technician repairing a furnace

If your furnace runs then shuts off and starts again, you’re experiencing “short cycling.” Short cycling is a common symptom of a few different furnace issues.

Read on to learn why your furnace is short cycling and how to fix it.

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What causes furnace to start then shut off?

On average, a furnace will run three to eight heating cycles per hour. The typical heating cycle lasts 10-15 minutes. If your furnace shuts off after a few minutes and starts more than eight times an hour, it may require repair.

Common causes of furnace short cycling include restricted airflow, a malfunctioning flame sensor, or a poorly placed thermostat.

Restricted airflow

Blocked vents or a clogged air filter may cause your furnace to run and then shut off too early. It’s easy to fix these issues on your own.

You should change your furnace filter at least once every three months. Follow our guide to do it yourself. If you’re not sure which type of filter to purchase, we’ve got you covered here.

If a new filter doesn’t stop the short cycling, check your vents. Your vent louvers should be in the open position. Remove furniture, piles of clothing, or other household clutter covering them.

Poor thermostat placement

The location of your thermostat might attribute to your furnace’s short cycling. If it’s not in a site that reflects the temperature in the rest of your home, it may cause problems.

Your thermostat should be in a central location where your family spends time. Avoid putting it in a hallway, near a heat source (like the fireplace), near windows and doors, or on an external wall.

If you need to move your thermostat, follow the installation instructions it came with. And check out our guide to thermostat wiring. Don’t feel up to DIY? Contact an HVAC technician for help moving your thermostat.

Overheating

If your furnace is overheating, it will turn itself off automatically. You may also notice a burning smell and a humming sound coming from the system.

A mechanical failure may cause your furnace to overheat. This is a safety issue. It could cause a fire or a carbon monoxide leak.

If you suspect your furnace is overheating, turn it off at the breaker box. Call a heating and cooling professional for help.

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Broken or dirty flame sensor

A flame sensor is a safety component that detects your burner’s flame. If it doesn’t sense a flame, the furnace will shut down.

Cleaning the flame sensor may resolve this issue. If you’re up for a DIY project, give these steps a try:

  • Turn off your furnace at the breaker box.
  • Open your furnace’s outer panel. You may need a screwdriver to do this.
  • Unscrew the flame sensor with a screwdriver. It’s usually located near the panel opening or behind a burner.
  • Using a microfiber cloth, scrub the metal rod until most of the carbon buildup comes off.
  • When you see bare metal, switch to wire wool and delicately scrub the rest.
  • With your microfiber rag, wipe dust off the sensor and sensor mount.
  • Screw the flame sensor back in, re-cover the outer panel, and turn your furnace back on.

If those steps don’t work, you’ll have to call an HVAC professional. A faulty flame sensor can become a safety concern if it goes unchecked.

Improper furnace size

HVAC installers typically use industry-standard Manual J calculations to determine the furnace size you need. Your system may be too powerful for your home if your furnace runs then shuts off and starts again.

Your furnace might be too big if you’re experiencing short cycling and any of the following symptoms:

  • Atypically high utility bills
  • Loud noises when the furnace runs
  • Uneven temperatures throughout your home
  • Excessively high or low indoor humidity

If you suspect you have the wrong size furnace, call a heating and cooling expert. They can discuss the options for more effectively warming your home.

Book an appointment with an HVAC pro now.

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How to fix furnace stopping and starting

You can attempt to fix your furnace’s short cycling by completing a few troubleshooting steps:

  • Change your air filters
  • Unblock your vents
  • Clean your flame sensor

If you suspect thermostat placement, furnace overheating, or furnace size are the cause of your furnace running then shutting off too early, contact an HVAC technician for help.


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