How to Fix a Furnace That's Not Heating
A furnace that won't heat is an urgent problem in cold weather. Before calling an HVAC technician, work through these common causes — many can be resolved without a service call. Always check the simplest things first.
Likely Causes
- 1Thermostat set to COOL instead of HEAT, or set too low
- 2Clogged air filter restricting airflow and triggering limit switch
- 3Tripped circuit breaker or blown fuse for the furnace
- 4Pilot light out or failed igniter (gas furnaces)
- 5Failed blower motor or capacitor
In freezing temperatures, a failed furnace can cause pipes to freeze within hours. Treat this as high priority. If you smell gas, leave immediately and call the gas company.
How to Fix It: Step by Step
- 1Check thermostat settings
Set the thermostat to HEAT mode and raise the set temperature at least 5°F above the current room temperature. Check the fan setting — it should be AUTO, not ON.
- 2Replace the air filter
A severely clogged filter blocks airflow and triggers the high-limit switch, shutting the furnace off. Check the filter (usually near the air handler or return vent). If gray or heavily coated, replace it immediately.
- 3Check the power and breaker
Locate the furnace power switch (looks like a light switch, often near the unit or at the top of basement stairs). Make sure it's on. Check the breaker panel for a tripped furnace breaker.
- 4Check the furnace status light
Modern furnaces have an LED status light that flashes a diagnostic code. The code chart is usually on the inside of the furnace door. Common codes indicate igniter failure, pressure switch issues, or limit switch trips.
- 5Relight the pilot or clean the igniter
For older furnaces with a standing pilot, follow the manufacturer's instructions to relight (usually a label on the furnace). Modern furnaces use electronic igniters — if the igniter glows but doesn't light the burner, the igniter or gas valve may have failed.
- 6Clear condensate drain if it's a high-efficiency furnace
High-efficiency (90%+) furnaces produce condensation. A clogged condensate drain line triggers a safety shutoff. Disconnect the drain line and clear any blockage with a wet/dry vac.
Materials & Tools Needed
Warning Signs It's Getting Worse
- ⚠Yellow or orange furnace flame instead of blue (carbon monoxide indicator)
- ⚠Carbon monoxide detector alarming
- ⚠Strange metallic or burning smell from vents
- ⚠Furnace cycles on and off rapidly (short cycling)
When to Call a Professional
Call an HVAC technician if you smell gas (leave first, call the gas company), if the furnace lights but shuts off after a few minutes (could be a heat exchanger crack — a carbon monoxide risk), or if the furnace is more than 15 years old and failing repeatedly.
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