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Why is my furnace not turning on?
Quick answer
A furnace that will not start in winter is most often stopped by the thermostat, a power switch, a tripped breaker, or a clogged filter tripping a safety limit, not by a dead furnace. Work through those homeowner checks once, in order. If you smell gas at any point, skip every other step, leave the house, and call your gas utility or 911 from outside.
Causes, most likely first
Thermostat not calling for heat
Beginner. Dead batteries, a mode left on cool, or a set point below the room temperature all look exactly like a broken furnace. Set the thermostat to heat, raise the target five degrees above the displayed room temperature, and replace the batteries if it uses them. Wait five minutes, since many systems delay between cycles.
Power switch or breaker off
Beginner. Most furnaces have a wall switch near the unit that looks like a light switch, and it gets flipped by accident during storage cleanouts. Confirm that switch is on, then check your electrical panel for a tripped breaker labeled furnace or HVAC. Reset a tripped breaker once only. If it trips again, stop and call a pro.
Clogged filter and overheating shutdown
Beginner. A furnace with a blocked filter overheats its heat exchanger, and a safety limit switch shuts the burner down to protect it. The blower may run while no heat comes out. Replace a dirty filter, let the unit cool for thirty minutes, and try again. Repeated shutdowns after a clean filter mean the cause is deeper.
Blocked condensate drain on high-efficiency furnaces
Intermediate. Condensing furnaces make water as they run, and if the drain tube or trap clogs, a float switch stops the furnace to prevent overflow. Look for standing water in the drain pan or a kinked, algae-filled tube. Clear only the accessible tubing you can reach. Do not open sealed burner compartments.
Dirty flame sensor or failed igniter
Pro only. If the burners light for a few seconds and then cut out, the flame sensor may not be proving flame, and a cracked hot-surface igniter can stop startup entirely. Both parts sit inside the burner area with gas and high heat involved. Cleaning or replacing them is technician work, not a homeowner repair.
Gas supply or control board problem
Pro only. A closed gas valve, a failed gas valve, or a control board fault prevents ignition even when the thermostat and blower work normally. Blinking error lights on the board are diagnostic codes for a technician. Never open the gas line, test valves with a flame, or bypass any safety switch to force heat.
When to call a pro
- Any gas smell or hissing near the furnace: leave first, call from outside
- Breaker trips a second time, or you see scorched wiring
- Burners light and shut off repeatedly within seconds
- Error codes keep returning after a filter change
- No heat in freezing weather with pipes at risk of bursting
- Carbon monoxide alarm sounds, or anyone feels dizzy or nauseous
What the repair typically costs
Directional national ranges for US homeowners. Your city and system age will move these.
| Repair | Typical cost |
|---|---|
| Service call and diagnosis, often credited toward repair | $75 to $200 |
| Thermostat replacement | $150 to $350 |
| Flame sensor cleaning or replacement | $150 to $350 |
| Hot-surface igniter replacement | $200 to $450 |
| Limit or pressure switch replacement | $200 to $500 |
| Control board replacement | $500 to $1,200 |
| Full furnace replacement | See our HVAC replacement cost guide |
Questions homeowners ask
My blower runs but there is no heat. What does that mean?
Air is moving, so power and the fan work. The burner side is not firing or is shutting down on a safety limit. Check the filter first, then call a technician if the pattern continues.
Is it safe to reset the furnace again and again?
No. One reset after fixing an obvious cause, like a filter or switch, is reasonable. Repeated resets can flood a burner with unburned fuel and hide a fault that a safety switch is correctly reporting.
How do I keep pipes safe while I wait for repair?
Open cabinet doors under sinks, let faucets drip slowly, and use space heaters carefully away from bedding and curtains. Never use an oven or outdoor grill to heat the home.