Furnace blowing cold air?

What usually causes this

These are the common possibilities, not a diagnosis. A page cannot tell which one it is — that takes a technician testing the system.

Fan set to On

With the thermostat fan set to On, the blower runs between heating cycles and moves unheated air. Setting it to Auto usually fixes this.

Warm-up delay

Many furnaces start the blower a moment after the burners light. A few seconds of cool air at start-up is normal.

Burners shut off by a safety limit

A clogged filter or blocked return makes the furnace overheat. The limit switch turns the burners off while the blower keeps running, so air turns cold.

Ignition or flame-sensor fault

If the burners fail to light or drop out, the blower may still run and push cold air.

Heat pump defrost or low charge

Heat pumps briefly blow cooler air during defrost. Air that stays lukewarm can mean a low refrigerant charge or a reversing valve problem.

Leaking ducts

Ducts leaking in a cold attic or crawlspace can deliver air noticeably cooler than it left the furnace.

What you can safely check yourself

• Set the thermostat fan to Auto and the mode to Heat.
• Replace a dirty filter.
• Check that return grilles aren't blocked.
• For a heat pump, watch the outdoor unit: steam and a few minutes of cooler air during defrost are normal.

What we check when we get there

1. 1The technician confirms the thermostat call, then watches a full heating cycle to see whether the burners light and stay lit, or whether a limit or flame-sensing fault is shutting them down.
2. 2For heat pumps, pressures, reversing valve operation and defrost behavior are checked.

What it is likely to cost

A thermostat setting costs nothing. Flame sensors and igniters are moderate repairs; blower motors and reversing valves cost more. You get the price before work starts.

Heating diagnostic

$89

Flame sensor

From $149

Ignitor

From $199

Blower motor

From $499

Reversing valve

From $699

Prices are starting points for standard work in the Savannah area. Final price depends on the system and site, and is confirmed before work begins. Permits and code-required upgrades may be additional.

Questions people ask about this

Is it normal for a heat pump to blow cool air in winter?

Briefly, yes. During defrost the system reverses for a few minutes to clear frost off the outdoor coil. Air that stays lukewarm for long periods isn't normal.

The furnace blows warm air, then cold, then warm again.

That pattern often means the furnace is overheating and tripping its limit switch — frequently because of a clogged filter or restricted airflow. Replace the filter; if it continues, have it checked.

Where to go next