How to Fix a Furnace That Won't Heat Your Home

Start with the simplest fixes: check your thermostat settings, replace the air filter, and ensure the circuit breaker hasn't tripped before moving to more complex diagnostics.

1. Start With the Thermostat. Set your thermostat to heat mode and raise the temperature 5 degrees above the current room temperature. Make sure it's not set to fan-only mode. If you have a programmable thermostat, check that it's not following a schedule that's keeping the heat off. Replace the batteries if your thermostat uses them.
2. Replace a Clogged Filter. Turn off your furnace and locate the air filter, usually in a slot near the blower unit. Pull it out and hold it up to the light. If you can't see through it clearly, it's blocking airflow and needs replacement. A clogged filter is one of the most common causes of heating problems and can shut down your furnace as a safety measure.
3. Reset the Power Supply. Look for a tripped breaker in your electrical panel labeled for the furnace. Reset it by switching it fully off, then back on. Also check for a power switch near your furnace that might have been accidentally turned off. Some furnaces have multiple switches, including one that looks like a regular light switch.
4. Verify the Ignition. For older furnaces with pilot lights, look through the viewing window to see if the small blue flame is lit. If it's out, follow the lighting instructions on your furnace label. For newer furnaces with electronic ignition, you'll hear clicking sounds when the system tries to start. If there's no clicking, the ignition system may have failed.
5. Confirm Gas Flow. Ensure your gas valve is turned on by confirming the handle is parallel to the gas line. If you smell gas, turn off the gas supply immediately and call your gas company. Also check that other gas appliances in your home are working to rule out a broader gas supply issue.
6. Clear All Airflow Paths. Walk through your home and make sure all heat vents are open and unblocked by furniture, curtains, or debris. Check that cold air return vents are also clear. Go outside and ensure the exhaust vent isn't blocked by snow, leaves, or other obstructions.
7. Verify Blower Operation. Set your thermostat fan to 'on' instead of 'auto.' You should hear the blower start and feel air coming from your vents. If the blower doesn't run, the motor may have failed or there could be an electrical problem requiring professional repair.