Why is my furnace blowing cold air?
Four things cause it most often. The fan is set to on instead of auto, the filter is clogged and the furnace is overheating and shutting the burners off, the flame sensor is dirty, or the ignitor has failed. The first is not a fault at all and takes ten seconds to check.
Check the thermostat first
If the fan setting is on rather than auto, the blower runs constantly, including between heating cycles. Between cycles it is pushing room temperature air, which feels cold coming out of a vent.
Set it to auto. If the air is warm during a cycle and the fan simply 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 normalises.
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 briefly, then quits. That is a cleaning job and a common one.
A cracked or failed ignitor means it never lights at all. Both are ordinary repairs on a system that is 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 sounds. Ventilate and get out before you call if it is gas or carbon monoxide.
Repeated short cycling is worth a technician regardless, because the high limit switch is doing its job and something is making it work.
The Work Behind This Answer
Still Want Someone to Look at It?
Send photos and a description and you get a written price with the scope spelled out. Open every day, 7:00 AM to 7:00 PM. Emergency response in 24–48 hours for urgent property damage.