Short answer
When the heat won't come on at all, it's often power, the thermostat, or a safety switch: a tripped breaker, a furnace power switch turned off, dead thermostat batteries, a door not seated, or a drain float that tripped. Check those first. If you smell gas, leave the house and call your gas utility or 911 from outside.
What's probably happening
Homes around here heat with a heat pump, a gas furnace, or a heat pump backed by a furnace. Whichever you have, a system that does nothing at all usually has one of a few simple causes before anything expensive.
No power
A tripped breaker, or the furnace power switch turned off. That switch looks like a regular light switch on or near the furnace, and it gets flipped by accident more often than you'd think.
The thermostat
Dead batteries, the wrong mode, or a setpoint below the room temperature. If the screen is blank, start with the blank thermostat page.
A safety switch
Furnaces have a door switch that keeps them off if a panel isn't seated. Many systems also have a float switch that shuts everything off when condensate water backs up. Those switches are doing their job. Don't bypass them.
Ignition or flame problems (gas furnace)
A dirty flame sensor or a failing igniter can make a furnace try to light, then shut down and lock out after a few tries.
A failed part
A control board, blower motor, or a heat pump problem outdoors can all stop heat completely.
What you can check yourself in five minutes
- Set the thermostat to heat, raise the setpoint a few degrees above the room temperature, and replace the batteries if it has them.
- Check your breaker panel for the furnace or air handler and for the outdoor unit. Reset a tripped breaker once.
- Find the power switch on or near the furnace or air handler and make sure it's on.
- Make sure the furnace or air handler doors are fully closed and seated.
- Replace a dirty filter. A badly clogged one can make a furnace overheat and shut itself off.
- Look at the condensate drain pan or line. If the pan is full, the float switch may have shut the system down.
- For a gas furnace, make sure the gas valve to the furnace is in the on position, if you know where it is. If you're not sure, leave it.
- Heat pump owners: if the outdoor unit won't run, switching to emergency heat will keep the house warm until it's repaired.
If the heat comes back after one of those, let it run through a full cycle and keep an eye on it the rest of the day. A system that starts once and quits again has a problem the checks didn't find.
When to stop and call
- You smell gas (rotten eggs or sulfur). Don't touch the thermostat or light switches. Get everyone out, and call your gas utility or 911 from outside.
- You smell electrical burning or see smoke. Turn the system off at the thermostat and the breaker.
- A breaker trips again after one reset.
- Water is on or near the furnace or air handler wiring. Shut the power off first.
- The furnace keeps trying to light and shutting down.
- You've done the checks and there's still no heat.
Please don't take panels off or jump a safety switch to get heat going. If you're in Spartanburg or Greenville County, call Alpine Cool at (864) 702-0777.
Common questions
Why won't my furnace turn on after I changed the filter?
Often the blower door or filter door isn't fully closed. Many furnaces have a safety switch behind that door that keeps them off until it's seated. Make sure every door and cover is back on snug, the same way it came off.
My heat pump won't heat but the fan runs. Is that the same problem?
Not quite. If air moves but it's cold, the outdoor unit may not be running, may be stuck in defrost, or may be low on refrigerant. Check the outdoor breaker and look at the unit. If it's silent or iced solid for a long time, call.
Is it safe to use emergency heat if my heat pump stops working?
Yes, as a short-term fix. Emergency heat runs the electric strips only. It will keep the house warm, but it uses a lot more electricity, so switch back once the heat pump is repaired.
Should I keep resetting my furnace if it shuts off?
No. If it keeps locking out, it's usually telling you something is wrong with the flame, venting, or airflow. Repeated resets can hide a real problem. Call instead.