Heating problem guide
Furnace Not Heating? What It Usually Means
A furnace that won't warm the house is stressful on a cold night, but the cause is often simpler than it first appears.
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Quick answer
When a furnace won't heat, the usual suspects are a thermostat set wrong or out of batteries, a clogged air filter, a tripped breaker or furnace switch, an ignition or flame sensor fault, or a safety switch that has shut the burners down. A few of these you can check in minutes. Ignition, gas valve and control board problems need a trained technician.
What may be going on
Likely causes
Thermostat settings or power
If the thermostat sits on cool or off, has a setpoint under the indoor reading, or has dying batteries, the furnace never gets the signal to start.
A clogged air filter
A filter packed with dust starves the furnace of air. Many furnaces then overheat and shut the burners off as a safety step.
Power interrupted
A tripped breaker or the light-switch style service switch near the furnace turned off will stop everything, including the blower.
Ignition or flame sensor trouble
Modern furnaces light with a hot surface igniter or spark. If the igniter fails, or a dirty flame sensor can't confirm a flame, the furnace shuts the gas off after a few tries.
A safety lockout
Repeated failed starts, a blocked flue or a pressure switch problem can put the control board into lockout. The furnace then sits idle until the fault is cleared.
Gas supply problems
A closed gas valve or an interruption in service means the burners have nothing to light. Leave gas lines to the utility or a technician.
Safe first checks
- Look at the thermostat: heat mode selected, setpoint raised a couple of degrees past what the room reads, and new batteries if the screen looks faint.
- Slide out the filter. A gray, matted surface you can't see through means it's due for a new one.
- Find the furnace breaker in the panel. Flip a tripped one back a single time, and if it pops again, stop there.
- Make sure the furnace service switch, usually mounted on the wall or a nearby joist, hasn't been flipped off by accident.
- Go room by room opening shut registers, and pull rugs, boxes or couches away from return grilles.
What not to do
- Don't remove furnace panels or reach inside the cabinet.
- Don't try to bypass a safety switch or jumper wires on the control board.
- Don't flip a breaker back on a second or third time after it trips.
- Don't relight a burner in any way beyond the instructions printed on the unit's label.
When to call
When to call a technician
Once the basics look fine and the house is still cold, a technician is the next step. Clicking without ignition, a burner that lights and then goes out, or a blinking light on the control board usually point to parts that need testing with proper tools.
Safety comes first. A gas smell or a sounding carbon monoxide alarm means everyone leaves the home now. Once outside, phone 911 or the gas utility, and skip flipping switches or touching the thermostat on the way out.
- The furnace clicks or hums but never lights.
- Burners light briefly, then shut off again and again.
- A status light on the furnace is flashing an error pattern.
- The furnace breaker won't stay on.
- You notice a burning, electrical or rotten-egg smell.
What a technician may inspect
- Igniter condition and resistance, plus a cleaning of the flame sensor.
- Gas pressure and gas valve operation.
- Pressure switch, inducer motor and venting for blockages.
- Control board error codes and limit switch readings.
- Heat exchanger condition and combustion safety.
- Thermostat wiring and the signal reaching the furnace.
Keeping it from happening again
Swap the air filter on the schedule its packaging suggests, and more often when pets or remodeling add dust. A tune-up each fall, before the first real cold snap in the Ohio River valley, gives a technician the chance to clean the flame sensor, check the igniter and catch small issues while the weather is still mild. See HVAC maintenance for what a visit covers.
Where to go next
Related service and market pages
A furnace that starts, tries to light and then stops is often protecting itself. A dirty flame sensor, a failing igniter or a venting problem can all make the control board shut the gas off. Replacing a clogged filter is worth trying, but repeated short attempts usually call for a technician to test the parts.
If there's no gas smell, no carbon monoxide alarm and the breaker isn't tripping, running the blower for a while usually does no harm. Still, weak heat is a symptom, not a fix. Turn the system off if you hear grinding or smell burning, and have furnace repair arranged soon.
Share what the thermostat shows, whether the blower runs, whether you hear clicking, and any flashing light pattern on the furnace. Those details help the technician who visits arrive ready to test the likely parts.
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Still not sure what's wrong?
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