A furnace that won't heat almost always comes down to one of a handful of causes. Some you can check in two minutes; others involve gas and combustion and should be left to a licensed technician. Here's how to narrow it down quickly and safely.
Start with the thermostat
It sounds obvious, but the thermostat is the most common culprit. Confirm it's set to Heat, the target temperature is above the current room temperature, and the fan is set to Auto rather than On (On runs the blower even when there's no heat call, which can feel like cold air). If your thermostat takes batteries, a dying set will cause intermittent or no operation — swap them.
Check the air filter
A clogged filter is the single most common cause of a furnace that short-cycles, overheats, or shuts down for safety. If you can't remember the last time you changed it, that's your answer. A dirty filter restricts airflow, the heat exchanger overheats, and a safety limit switch shuts the burner off. A clean filter is cheap insurance.
Make sure it's actually getting power and gas
- The furnace switch: there's usually a standard light switch near or on the unit. It gets flipped off by accident more often than you'd think.
- The breaker: a tripped breaker for the furnace circuit will stop it cold.
- The gas supply: confirm the gas valve at the furnace is open and that other gas appliances (stove, water heater) are working. If nothing gas-powered is working, contact your gas utility.
Signs it's time to call a technician
If the basics check out and you still have no heat, the issue is usually inside the burner sequence — a failed ignitor or flame sensor, a faulty inducer motor, a control-board fault, or a tripped safety limit. These involve gas and combustion, so they're not DIY territory.
Call right away — don't keep resetting it — if you smell gas, see soot or scorch marks, hear repeated clicking with no ignition, or the furnace lights briefly and then shuts off (a classic dirty or failing flame sensor). On the coldest nights, a no-heat call is a same-day priority.
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Questions, answered
Why does my furnace blow cold air?
Most often the thermostat fan is set to On instead of Auto, so the blower runs between heat cycles. If the fan is on Auto and you're still getting cold air, the burner isn't igniting — commonly a dirty flame sensor, a failed ignitor, or a safety lockout from a clogged filter.
Is it safe to reset my furnace myself?
Resetting once (at the breaker or power switch) is fine. If it shuts down again, stop — repeated resets on a unit that's locking out for a safety reason can be dangerous. Have it looked at, especially if you smell gas.
