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Heating · Wisconsin

Furnace Blowing Cold Air

A furnace moving cold air through the ducts is pulling heat out of the house instead of adding it, so call before the place drops much further.

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When this becomes an emergency

What a pro does

Cold air at the registers almost always means the fan side is healthy and the fire side is not. The first thing a tech does is watch a full call for heat from the start, because the moment the sequence breaks tells you most of what you'd need to know: no spark at all points one direction, a flame that lights and then dies a few seconds later points somewhere else entirely, usually at a flame sensor coated in enough oxide film that the board never sees proof of fire. Lockouts matter here. Most control boards will keep the blower running for a while after a failed trial, which is exactly why the house fills with cool air rather than simply going quiet. On propane-fed forced air, low tank pressure during a cold snap can starve the burner and produce the same symptom even when there isn't a single failed part in the cabinet.

Frequently asked questions

The board runs the blower to clear residual heat out of the exchanger after a failed or finished cycle. On a lockout it can run a long time, which is what makes the vents feel like air conditioning.
It can. A tank that's low, badly sited, or undersized for the load will not vaporize fast enough in deep cold, and the burner starves even though there's fuel in the tank.
It often resets the lockout and buys another try or two. If the same failure repeats within a cycle or two, the underlying fault is still sitting there and resetting again just wears parts.
Not typically. A crack shows up as combustion problems and safety trips rather than an inability to make heat, so this symptom usually points at ignition or fuel supply first.

Furnace Blowing Cold Air?

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