Furnace Not Heating
A furnace that won't heat is often caused by a dirty filter, an ignition problem, a tripped high-limit switch, or a thermostat issue — not a major breakdown. Check these before calling for service.
Step-by-Step Fix
- 1
Check the thermostat
Set the thermostat to HEAT and raise the set temperature 5° above current room temp. Listen for the furnace to kick on within 1–2 minutes. Replace thermostat batteries while you're at it.
- 2
Replace the air filter
A severely clogged filter can cause the furnace to overheat and shut off on the high-limit safety switch. Find the filter, check it, and replace it if it's grey or clogged. Standard 1-inch filters should be replaced every 1–3 months.
- 3
Check the furnace power and pilot
Make sure the furnace power switch (looks like a light switch, usually on the unit or nearby wall) is ON. Check the breaker panel. For older gas furnaces with a standing pilot: look through the sight glass for a blue flame. If out, follow the relight instructions on the furnace door.
- 4
Check for error codes
Modern furnaces flash LED error codes when something is wrong. Count the blinks on the LED inside the furnace cabinet (open the lower door to see it). The key is usually printed on the inside of the door. Note the code before calling for service — it tells the tech exactly what failed.
- 5
Inspect the vents and registers
Make sure supply and return registers throughout the house aren't blocked by furniture or closed off. The furnace needs unrestricted airflow to operate safely.
⚠️ Safety Notes
- If you smell gas, don't attempt to relight the pilot — leave the house and call your gas company from outside.
- Don't bypass or remove the door safety switch to run the furnace with the panel open.
When to Call a Pro
If the furnace cycles on and off repeatedly (short cycling), shows error codes for the inducer or heat exchanger, or you hear clanging or grinding, call an HVAC technician. A cracked heat exchanger is a carbon monoxide risk.
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