Furnace blowing cold air

Fan switch left on ON

With the switch on ON instead of AUTO, the blower never stops. Between burner cycles it blows air that was never heated, and on your hand that feels cold. Flip it and wait one cycle before you worry about anything else.

A dirty flame sensor or a cracked ignitor

Either one makes the burners light and then shut off, while the blower keeps pushing air that never got heated.

The furnace is overheating

A clogged filter or a blocked return lets heat pile up inside the cabinet. The limit switch shuts the burners off to protect the furnace, and the fan keeps running to cool it down, so the vents go cold partway through.

Before you pick up the phone

• Thermostat set to HEAT, a few degrees above the room temperature
• Fan switch on AUTO
• Filter isn't caked with dust
• The power switch on or near the furnace is on, and the breaker hasn't tripped
• The furnace door panel is fully closed. Many units won't fire with it off.

If all of that checks out and the vents still blow cold after a few minutes, stop resetting it. Restarting a furnace that keeps locking out won't fix whatever is locking it out. Note whether it lit, how long it ran before the air went cold and whether it clicked, and tell us when you call. That narrows down the part before the technician gets there.

Signs it needs a technician

A gas smell, a carbon monoxide alarm, scorch marks around the burner door or a furnace that clicks over and over without lighting all need a professional. With a gas smell or a CO alarm, get everyone out and call the gas company from outside before you call us.