HVAC Guide
Why is my furnace blowing cold air?
By All Purpose Anytime Services LLC
It's usually one of four things. The fan is set to ON instead of AUTO, a clogged filter is overheating the furnace and shutting the burners off, the flame sensor is dirty, or the ignitor has failed. The first one takes ten seconds to check.
Check the thermostat first
With the fan set to ON, the blower runs the whole time, including between heating cycles. Between cycles it's moving room temperature air, and that feels cold coming out of a vent.
Set it to AUTO. If the air is warm while the burners run and the fan stops between cycles, nothing is wrong.
Then the filter
A clogged filter starves the furnace of airflow, so heat builds up in the cabinet and the high limit switch cuts the burners to protect the heat exchanger. The blower keeps running to shed that heat, which is why you get cold air.
The tell is a furnace that starts warm, runs for a few minutes, then goes cold and repeats. Change the filter and see whether the cycle goes back to normal.
Flame sensor and ignitor
A flame sensor coated in residue stops confirming the flame, so the control board shuts the gas off within seconds as a safety response. The furnace lights, runs for a moment and quits. That's a cleaning job, and a common one.
A cracked or failed ignitor means it never lights at all. Both are ordinary repairs on a furnace that's otherwise fine.
When to stop and call
Shut the system off and call if you smell gas, if the burner flame is yellow and lazy rather than blue, if there is soot around the unit, or if the carbon monoxide alarm goes off. If it's gas or carbon monoxide, get out first and call from outside.
Repeated short cycling is worth a technician either way. The high limit switch is doing its job, and something keeps making it trip.
Still blowing cold on AUTO
If the fan is set to AUTO and the air is still cold, book a furnace visit. We tell you the diagnostic fee before anyone is sent.