Why is my furnace blowing cold air?

This page is for a furnace that is operating its blower but delivering cold or lukewarm air at the registers. A furnace may blow cool air briefly during startup or after the burner shuts off, but air that stays cold usually points to a failed ignition sequence, dirty flame sensor, blocked filter, open limit switch, or thermostat or control issue. Confirm the thermostat is calling for heat and check the filter. Do not bypass a safety switch or open a sealed combustion compartment. If the blower runs continuously, note whether the burners light at all and tell the technician. If the furnace will not start at all, see the separate furnace-not-heating guide.