Furnace Not Heating? Here's What to Check

When your furnace stops heating in the middle of winter, it feels like an emergency. But many furnace failures have simple causes — a thermostat set wrong, a clogged filter, a tripped safety switch, or an extinguished pilot light. Check these before spending $150-300 on an emergency HVAC call.

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Key takeaway: Check the thermostat batteries first — dead batteries are a surprisingly common cause. Then check the air filter (a clogged filter triggers safety shutdown) and verify the furnace power switch is ON.

What's Happening

Furnaces heat air by burning gas (or using electric elements) and blowing the heated air through ducts. Safety sensors monitor the flame, temperature, and exhaust. If any sensor detects a problem, the furnace shuts down. Modern furnaces have diagnostic LED lights on the control board that flash error codes — check your manual or the label inside the furnace panel for the flash code legend.

What to Check

Check the thermostat. Verify it's set to HEAT, the temperature is above the current room temperature, and the fan is on AUTO. Check the batteries if it's battery-powered — dead batteries are a surprisingly common cause. Try setting the temperature 5 degrees above room temp.
Check the air filter. A severely clogged filter can cause the furnace to overheat and trigger the high-temperature safety switch. Find the filter (usually at the return air vent or furnace air intake), check it, and replace if dirty.
Check the furnace power and gas. Make sure the furnace switch (looks like a light switch, usually on or near the furnace) is ON. Check the breaker. For gas furnaces, verify the gas valve is open (handle parallel to the pipe).
Check the pilot light (older furnaces). If you have a standing pilot light, look through the small window on the furnace. If it's out, follow the relighting instructions on the label. Modern furnaces use electronic ignition — you'll hear clicking when the furnace tries to start.
Reset the furnace. Turn the furnace off at the thermostat and the furnace switch. Wait 5 minutes. Turn both back on. The furnace should go through its startup sequence — you'll hear the inducer fan, then the igniter, then the burner, then the blower. If it starts and stops repeatedly, it's "short cycling" due to a safety issue.

DIY Cost

$0–15

Pro Cost

$150–500

Call HVAC Pro If

You smell gas (leave the house immediately and call your gas company).
The furnace starts and stops repeatedly (short cycling — flame sensor or other safety issue).
The diagnostic LED shows a flashing error code.
No heat after replacing the filter and resetting the furnace.
The furnace makes banging, screeching, or rattling sounds.

Frequently Asked Questions

Why does my furnace keep shutting off?

Short cycling (starts then stops) is usually a dirty flame sensor ($10 cleaning), a clogged filter causing overheating, or a failing igniter. Check the diagnostic LED on the control board for error codes.

How often should I replace my furnace filter?

Every 1-3 months during heating season. A severely clogged filter causes the furnace to overheat and shut down on safety.

Is it normal for a furnace to smell when first turning on?

A brief burning dust smell when the furnace first runs each season is normal. A persistent burning smell, gas smell, or electrical smell is not — shut it off and call HVAC.

Not Sure If It's the Thermostat, Filter, or Something Bigger?

Record a video of your furnace and Toolbox walks through a diagnostic — checking error codes, filter status, and ignition — to narrow down the cause before you call a tech.

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