Furnace blowing cold air: the 6 causes, in order of likelihood

A furnace that runs but blows cold usually has a simple explanation. Work down this list before calling anyone.

Thermostat fan setting

If the fan is set to ON instead of AUTO, the blower runs between heat cycles and pushes room-temperature air that feels cold. Set the fan to AUTO. This is the single most common cause and costs nothing.

The furnace just started

Most furnaces run the blower for 30-90 seconds before the burners have warmed the heat exchanger. Cold air for the first minute is normal. Cold air for five straight minutes is not.

Dirty filter tripped the limit switch

A clogged filter chokes airflow, the furnace overheats, and a safety switch shuts the burners off while the fan keeps running to cool things down — endless cold air. Check the filter; if you can't see light through it, replace it, then power-cycle the furnace at the switch.

Flame failure mid-cycle

If the flame sensor is dirty, burners light and then drop out seconds later, leaving the fan blowing unheated air. The pattern: you hear ignition, warmth for under a minute, then cold. This one is a cheap fix for a technician (sensor cleaning) — and the control board usually stores a fault code you can look up on this site.

Pilot light out (older furnaces)

Standing-pilot furnaces from before ~2010: if the pilot is out, no heat, fan may still run. Relighting instructions are on the furnace door panel. If it will not stay lit, the thermocouple is likely failing — technician territory.

Condensate blockage (high-efficiency units)

Condensing furnaces shut burners down when the condensate drain backs up. Look for a small hose/trap near the bottom of the unit and standing water. Clearing the trap often restores heat immediately.

Safety line: If you smell gas at any point, stop — leave the house and call your gas utility from outside. Do not keep restarting a furnace that repeatedly shuts its burners off; that safety is protecting the heat exchanger, and a cracked heat exchanger can put carbon monoxide into your air.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.