The short answer
Before you call, check the thermostat mode and batteries, the breaker, the furnace power switch, the filter, and the furnace door. Those five account for a lot of no-heat calls. If everything checks out, note the blinking code on the furnace control board. A gas furnace that tries to light and shuts off usually needs its flame sensor or igniter serviced. Any smell of gas: get out first, then call.
Safety first
- Gas smell (rotten eggs or sulfur): leave the house, then call your gas provider or 911 from outside.
- Carbon monoxide alarm: get everyone outside into fresh air and call 911.
- Burning electrical smell or smoke: turn the system off at the thermostat and the breaker.
What you can check yourself
- Thermostat: set to "heat," set a few degrees above the room temperature, fresh batteries.
- Breaker: the furnace or air handler breaker, and for heat pumps the outdoor unit's breaker too.
- Service switch: most furnaces have a light-switch-style switch on or near the unit. It sometimes gets bumped off.
- Door panel: a safety switch keeps the furnace off if the blower door isn't seated.
- Filter: a clogged filter can overheat a furnace and trip its high-limit switch.
- Condensate: high-efficiency furnaces shut off if their drain or condensate pump backs up.
- Gas: if other gas appliances also aren't working, call your gas provider.
Heat pump with no heat
If the outdoor unit isn't running, check its breaker and the disconnect box on the wall next to it. If the air handler runs but the air is cold and "aux heat" never appears, the backup strips may also be out. That's a service call.
When to call
If the checks don't bring the heat back, call Thermal Services at (704) 701-6478 and tell us what the thermostat says and what code the furnace is flashing.
Common questions
Why is my furnace blowing cold air?
Right after the thermostat calls for heat, a gas furnace runs the blower briefly before the burners warm the heat exchanger, so a minute of cool air is normal. Constant cold air usually means the burners aren't lighting, often a dirty flame sensor, a failed igniter, or a gas supply issue. Check that the thermostat fan is on 'auto,' not 'on.'
My furnace lights, then shuts off after a few seconds. What's wrong?
That pattern often points to a dirty flame sensor, which can't prove the flame is lit, so the furnace shuts the gas off for safety. It can also be a venting or pressure switch problem. A technician can clean or test the sensor quickly.
What do the blinking lights on my furnace mean?
Most modern furnaces flash a diagnostic code on the control board, visible through a small window in the lower panel. The code chart is usually printed inside the panel door. Write down the pattern before you reset power, because a reset clears it.
Is it dangerous if I smell gas near my furnace?
Yes. Don't touch light switches or the thermostat. Get everyone out, then call your gas utility or 911 from outside. Don't go back in until the utility says it's safe.