How to Fix a Furnace Blowing Cold Air

The vents are moving air, the air is cool or room temperature, and the thermostat is set to heat. In most cases the furnace is not broken at all, the blower is running while no burner is lit, and the first fix costs nothing. If there is any smell of gas at any point, leave the building, do not flip switches or use a phone indoors, and call the gas utility or 911 from outside.

Set the thermostat fan to Auto and the mode to Heat

A fan set to On runs the blower continuously, during a burn and through the long stretches between burns, so most of what reaches the vents is room air moving fast enough to feel cold. Move the fan switch to Auto and confirm the system mode reads Heat rather than Cool or Off. Give it ten minutes before judging the result.

Pull the filter and hold it up to a light

A filter packed with dust starves the furnace of return air. The heat exchanger overheats, a limit switch cuts the burner, and the blower keeps running to cool the cabinet, which arrives at the vents as cold air. If no light passes through the media, replace it and see how to change a furnace filter.

Check the furnace service switch and the breaker

Near the unit, on a joist or wall, is a switch that looks like an ordinary light switch. It gets flipped by accident more often than anyone admits. Confirm it is on, then check the panel for a tripped furnace breaker and reset it once.

Read the blink code on the control board

Most furnaces flash a diagnostic code on an LED behind a small window in the door panel, and the legend that translates the pattern is printed on the inside of that panel. Counting the flashes is the fastest way to learn what the furnace thinks is wrong before touching anything.

Look for a condensate backup on a high efficiency unit

If the furnace exhausts through white PVC rather than metal pipe, it drains several gallons of water a day. A blocked or frozen drain fills the trap, trips a float switch, and shuts the burner down while the blower keeps turning.

Clean the flame sensor

Cut power at the service switch first. A burner that lights, holds for four or five seconds, then shuts off and tries again is the signature of a flame sensor coated in oxide. Remove the thin metal rod sitting in the flame path, scuff it lightly with fine emery cloth, and reinstall it.

If the furnace locks out again and again. Repeated lockouts point to a fault the control board is protecting the house from. Stop and call a licensed HVAC technician.

If anything smells burned, acrid, or like gas. Shut the furnace off. A cracked heat exchanger and a failing gas valve are not homeowner repairs.