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781 Heating
Heating · Massachusetts

Furnace Blowing Cold Air

Cold air where warm air should be usually means the burner side quit while the fan keeps going, so it's worth a call before the house falls further behind.

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When this becomes an emergency

What a pro does

Cold air from the vents almost always means the blower is fine and the heat source isn't. A technician checks whether the burner is actually igniting, whether the ignitor or flame sensor is dirty or worn, and whether the furnace is stuck in a safety lockout that lets the fan run for comfort while blocking the burner from firing again until it's reset. On oil-fired systems the same symptom can trace to a clogged nozzle or a failing fuel pump instead of an electrical fault. None of this is safe to force past a lockout.

Frequently asked questions

Not always. On gas furnaces a dirty flame sensor is the single most common cause, and it reads to the control board exactly like no flame at all, even though ignition happened for a split second.
A clogged filter usually causes weak airflow rather than cold airflow, though restricted airflow can trigger the same safety lockout described above and produce this symptom indirectly.
Running the fan alone is harmless, but leaving the thermostat calling for heat while the burner keeps failing to ignite isn't a great idea over many cycles.
Aging oil furnaces see it often, usually from nozzle wear or a fuel pump losing pressure, both of which tend to be intermittent rather than a constant loss of heat.

Furnace Blowing Cold Air?

Get connected with a local pro now.

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Keep going

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