Tips for troubleshooting a furnace that runs constantly

Is your furnace running non-stop, turning your home into a sauna or driving up those energy bills? A furnace that won’t quit isn’t just annoying—it could be signaling some issues that need attention. Let’s take a look at some common reasons this might be happening and what you can do about it.

The info here might not fit every furnace out there. We aim to share helpful tips for all kinds of DIY appliance fixes, but different models can have their quirks—like unique troubleshooting steps, error codes, or parts. For spot-on guidance, it’s best to check your manufacturer’s documentation or reach out to us if you’ve got questions.

Check the air filter

A clogged air filter is often the main culprit when your furnace runs constantly. When it gets blocked, airflow is restricted, causing the furnace to overheat and run longer to maintain the temperature you’ve set.

How to replace the air filter:

1. Turn off your furnace at the thermostat or breaker.
2. Locate the air filter compartment, usually where the return duct meets the furnace.
3. Remove the old filter (and try not to make a mess!).
4. Check the size and type printed on the edge of the filter.
5. Replace it with a new one of the same size. If you’re unsure, our guide can help you find your appliance’s model number.
6. Insert the new filter with the airflow arrows pointing toward the furnace.
7. Close everything up and turn the furnace back on.

Tip: Change your filter every month to keep things running smoothly.

Check the thermostat settings

Your thermostat could be throwing things off. If it’s set incorrectly, your furnace might be overworking itself.

Adjusting your thermostat:

1. Check the temperature it’s set to.
2. If it’s set above 78°F, try lowering it a bit.
3. Make sure the thermostat is set to “Heat” and the fan is on “Auto” (not “On”). “On” means the fan runs all the time, while “Auto” only runs it when heating.
4. If you haven’t already, swap out the thermostat batteries.
5. If you have a programmable thermostat, make sure the schedule is correct.
6. Wipe down your thermostat with a soft cloth to remove any dust.

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Check the gas supply

If the gas valve isn’t open all the way, your furnace may be working overtime without heating properly.

How to check the gas valve:

1. Locate the gas valve on your furnace (usually at the bottom).
2. Ensure the handle is parallel to the gas line—this means it’s open. If it’s perpendicular, turn it so it’s parallel.
3. Inspect the gas line for any damage or leaks. If you smell gas, leave the house immediately and call your gas company.
4. After adjusting the valve, you may need to relight the pilot light. Follow your furnace’s instructions.

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Check the pilot light

If your pilot light is malfunctioning, it could cause your furnace to run non-stop without properly heating the home.

How to check the pilot light:

1. Find the pilot light assembly on your furnace.
2. Ensure the flame is steady and blue. If it’s yellow or flickering, that’s a problem.
3. Clean out the pilot orifice with a needle if it’s clogged.
4. Relight the pilot light according to your furnace’s instructions.

Adjust the fan limit switch

The fan limit switch controls when the blower fan turns on and off. If it’s not set correctly, it could cause the fan to run all the time.

How to adjust the fan limit switch:

1. Turn off the power to the furnace.
2. Locate the fan limit switch near the blower.
3. If the switch is pressed in, gently pull it out to set it to “Auto.”
4. Clean it using compressed air.
5. Turn the power back on and check the furnace’s behavior.

Clean the blower 

If the blower is full of dust, it can restrict airflow and force the furnace to run longer to heat your home.

How to clean the blower:

1. Turn off the furnace power at the breaker.
2. Remove the access panel to reach the blower assembly.
3. Disconnect any wires, taking photos if needed for reference.
4. Remove the blower and use a vacuum with a brush attachment to clean the blades.
5. Apply a few drops of electric motor oil to the appropriate spots.
6. Reassemble everything and turn the power back on.

Tip: Regularly cleaning the blower helps your furnace run efficiently.

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Additional information

Thanks for reading! If you need replacement parts, grab your model number and head to AppliancePartsPros.com. We offer over two million parts and most orders arrive in two business days. If you need help finding the right part or placing an order, you can contact our team at 1 (877) 477-7278.

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