Furnace Blowing Cold Air

When this becomes an emergency

• The registers push air that feels cooler than the room instead of warm.
• The blower keeps running long after the burner has quit.
• You hear the ignition sequence start, then everything drops back to just the fan.
• The furnace goes through several attempts and then stays silent for a long stretch.

What a pro does

When the air is cool, the fan is generally doing its job and combustion isn't. A technician watches a full call for heat from the start: inducer spin-up, pressure switch proving, ignitor glow, then whether the flame sensor holds the burner in past the trial period. Dirty sensors are boringly common. On the older units heating additions that were tacked onto farmhouses, the return duct is often undersized, and the resulting heat buildup trips a high limit that leaves the blower running to purge while the burner stays locked out, which reads at the register as cold air. Propane and oil systems add nozzle and pump questions on top of that. A lockout that keeps repeating shouldn't be reset over and over, because each cycle sends unburned fuel into the chamber.

Frequently asked questions

Furnace Blowing Cold Air?

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