Short answer
First check that the thermostat fan is set to Auto, not On; On runs the blower even when the burners are off. If it is on Auto and the air is still cold, the burners are not lighting or are shutting off early, most often because of a dirty flame sensor, a failed igniter, a clogged filter tripping the high-limit switch, or a blocked vent pipe.
Quick checks you can do safely
These take a few minutes and solve a surprising number of cold-air calls.
- Thermostat set to Heat, fan on Auto, setpoint above room temperature
- Fresh thermostat batteries
- Furnace service switch and breaker on
- Furnace door fully closed (a door switch stops the furnace if it is open)
- Air filter not clogged
- Outdoor PVC vent pipes clear of snow and ice
Burners light, then go out
The furnace's flame sensor must prove the flame within seconds or the control shuts the gas off. A thin film on the sensor rod is enough to cause this, and the blower may keep running and push unheated air.
This is one of the most common winter repairs and is usually quick for a technician: clean or replace the sensor and check its ground.
Not sure it is safe to keep running? A local technician can check it today.
Call NowBurners never light
If you hear the inducer fan start but nothing lights, the igniter, pressure switch, gas valve or control board may be at fault. Many furnaces blink an error code on the control board; the code chart is usually on the inside of the door.
Overheating and the high-limit switch
A clogged filter, closed supply registers or a failing blower can make the heat exchanger overheat. The high-limit switch then shuts off the burners while the blower keeps running, so you feel cool air in cycles.
A new filter often fixes it. If it keeps happening, have the blower and airflow checked; repeated overheating stresses the heat exchanger.
When to stop and call
Call a local technician if the furnace locks out after one reset, you smell gas, a carbon monoxide alarm sounds, or you see scorch marks, soot or water around the furnace.
Related questions
Can I clean the flame sensor myself?
It is a common repair, but it means opening the burner compartment of a gas appliance. If you are not comfortable working around gas controls, have a technician do it.
Why does my furnace blow cold air at first?
Many furnaces delay the blower until the heat exchanger warms up, but a brief cool puff from ducts that sat in a cold basement is normal. Continuous cold air is not.
Does a dirty filter really stop the heat?
Yes. Restricted airflow can trip the high-limit switch and cut the burners, leaving only the fan running.
Services that fix this
Furnace Repair
Gas and oil furnaces that will not heat, blow cold, short-cycle or make noise.
Furnace repair in MarlboroughFurnace Ignition & Flame Sensor Repair
Burners that will not light or shut off seconds after lighting.
Ignition repair in Marlborough
Emergency Heating Repair
No heat on a cold night, CO alarms and furnaces that will not restart.
Emergency heating repair in MarlboroughThis guide is general information for homeowners, not a diagnosis of your system. For anything involving gas, carbon monoxide, electricity or active leaks, stop and call a qualified professional.
