Why Is My Furnace Not Heating?

Common Reasons a Furnace Stops Heating

Thermostat or power

A thermostat left on Cool or Off, dead batteries, or a setpoint below room temperature will keep a furnace idle. So will the switch near the furnace, which looks like a light switch, or a tripped breaker. Rule these out first when the furnace won't start.

Clogged filter and the high-limit switch

A clogged filter restricts airflow until the furnace overheats. The high-limit switch then shuts the burners off to protect the furnace, while the blower keeps pushing unheated air through the vents. That is why a dirty filter can leave a furnace blowing cold air, or make it keep shutting off and starting again.

Ignitor or flame sensor

If the burners never light, the ignitor may have cracked or burned out. If they light and drop out within a few seconds, a dirty or failing flame sensor is the usual cause: the furnace cannot confirm a flame, so it shuts the gas off. Both are replacement parts, and neither is a do-it-yourself job on a gas appliance.

Blocked condensate drain or vent pipe

High-efficiency furnaces drain water and vent through plastic pipes. A clogged condensate line, or snow or ice over the PVC intake and exhaust pipes outside, trips the pressure switch and locks the furnace out. After a heavy snow, check the pipes where they leave the house.

Blower motor or control board

If the burners light but no air moves through the vents, the blower motor may have failed, and the furnace will shut itself down soon after. If nothing responds at all and the power is on, the control board is the likely fault. Both need a technician to test and replace.

Quick Checks You Can Do Yourself

1. Check the thermostat. Set it to Heat and a few degrees above room temperature, and replace the batteries if it uses them.
2. Check the furnace switch and the breaker. Make sure the switch near the furnace is on. If the furnace breaker has tripped, reset it once only.
3. Replace the filter. A fresh filter restores airflow and is the cheapest fix on this list.
4. Check the gas shutoff. The handle on the gas line to the furnace should sit in line with the pipe, not across it.
5. Clear the outdoor pipes and the drain. On a high-efficiency furnace, clear snow, ice or leaves from the intake and exhaust pipes outside, and check the condensate line for a kink.
6. Restart the furnace once. Turn the furnace switch off for 30 seconds and back on, one time only.

If the heat does not come back after these checks, stop there and call for service.

When to Stop and Call

If you smell gas, leave the house and call your gas utility or 911 from outside. Do not relight anything. Also stop and call when a carbon monoxide alarm sounds, the furnace locks out again after one reset, the breaker trips a second time, water is pooling around the furnace, or a burning smell lingers. Do not keep resetting a gas furnace. Our team handles furnace repair across Berks County, often same-day, and emergency HVAC repair when the heat is out.

Schedule Furnace Service

Boiler or Heat Pump Instead?

Radiators or baseboards cold? That is a boiler problem, covered on our boiler repair page. A heat pump blowing cool air or iced over is covered under heat pump repair. In summer, the cooling side of this guide is why is my AC not cooling.