1. Check the thermostat
Make sure it’s set to HEAT and the temperature is set a few degrees above the room. If the screen is blank or dim, put in fresh batteries before anything else — a dead thermostat can’t call for heat.
2. Check the power
Furnaces need electricity even to burn gas. Check that the furnace switch (it looks like a light switch near the unit) is on, and look for a tripped breaker in your panel. Flip a tripped breaker fully off, then back on.
3. Check the filter
A badly clogged filter can choke airflow enough to trip the furnace’s safety limit and shut it down. If yours is gray and caked, replace it — then give the furnace a few minutes to reset.
4. Check the gas
Make sure the gas valve to the furnace is open (handle parallel to the pipe) and that other gas appliances work. If you smell gas, leave and call your utility first — don’t troubleshoot.
5. Check the furnace door
Most furnaces won’t run unless the blower door is fully closed — there’s a safety switch behind it. If the panel was pulled for a filter change and not seated right, the furnace won’t fire.
6. Check the condensate drain
High-efficiency furnaces make water, and a clogged condensate drain or full float switch will shut the furnace off to prevent a leak. If you see water around the base, this is a likely cause.
7. Look for a blinking light
Through the little window on the furnace, a blinking LED is the furnace telling you its error code. Count the blinks — the legend is usually printed on the inside of the door. It’s a big head start on the diagnosis.
When to call a pro
If you’ve checked these and still have no heat — or the furnace lights and then shuts off, short-cycles, or smells off — that’s past a quick DIY fix, and it’s worth having a tech look before you’re without heat overnight. A lot of these no-heat calls also trace back to a worn part that a yearly furnace tune-up would have caught early.
Dealing with the AC instead? See AC not cooling — 7 things to check, or get the full before-you-call checklist to save on your phone.
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