Heating
Furnace Blowing Cold Air? 6 Causes and Fixes
· by Diamond Home Services
Why your furnace runs but blows cold air — thermostat settings, dirty flame sensors, overheating limits and lockouts — plus what you can safely check yourself.
Your furnace is running but the air coming out of the vents is cool. That is a specific failure pattern, and most of the time it is one of six things.
1. The thermostat fan is set to ON
If the fan is set to ON instead of AUTO, the blower runs continuously — including between heating cycles, when it is just moving room-temperature air. Switch to AUTO first. It costs nothing and solves this surprisingly often.
2. A dirty flame sensor
The flame sensor proves the burner lit. When it is coated in oxide, the furnace lights, fails to confirm the flame, and shuts the gas off within seconds while the blower keeps running. Symptom: brief warmth, then cold air, repeating.
3. A clogged filter causing overheat lockout
A restricted filter starves the heat exchanger of airflow. The high-limit switch trips, the burner shuts down, the blower keeps running to cool things off. Change the filter and see if the pattern stops. If the furnace short-cycles again, the ductwork or blower needs looking at.
4. Condensate blockage
High-efficiency furnaces produce acidic condensate. A plugged trap or frozen exterior drain triggers the pressure switch and locks out the burner. Common in Toronto during deep freezes.
5. Failed igniter or gas valve
If the igniter never glows or the valve does not open, the blower may still run on a call for heat. This one needs a technician — do not attempt gas components yourself.
6. Pilot light out on older units
Pre-1990 furnaces with standing pilots go out from drafts or a weak thermocouple. Relight per the label instructions once; if it will not hold, the thermocouple is done.
What you should not do
Do not bypass a limit switch, jump a pressure switch, or run a furnace that smells of gas. Leave the house and call the gas emergency line if you smell gas.
When to call
If the filter and thermostat checks do not fix it in 20 minutes, stop. See furnace repair costs, read the furnace repair page, then call us or request a free quote — same-day appointments across the GTA.