Furnace not heating? Start with these safe checks

This page is for a furnace that will not start or is not producing heat at all. Check the thermostat mode, setpoint, and batteries first. Then look at the furnace switch and breaker, but do not reset a breaker repeatedly. A clogged filter can restrict airflow enough to trip a limit switch, so replace it with the correct size if it is visibly loaded. If the furnace still will not start, the likely causes move into ignition, flame sensing, pressure proving, controls, or fuel supply. Those checks need a professional. For a system that is running but sending cold air from the vents, see the separate cold-air troubleshooting guide. Leave immediately if you smell gas, see smoke, or a carbon-monoxide alarm sounds.