Check these four things first
They are common causes of a no-heat furnace, and all four are free.
- The thermostat. Set to heat, set above room temperature, with fresh batteries if it takes them.
- The switch on the furnace. It looks like an ordinary light switch, on the furnace or the wall beside it.
- The breaker. Labeled furnace or air handler. Reset a tripped one once. If it trips again, stop and call.
- The filter. If you cannot see light through it, replace it. A blocked filter overheats the furnace and trips it out on a safety.
If all four are fine, the fault is in the furnace and it needs someone with test equipment.
What the failure pattern tells you
Watch what it does when you call for heat. It costs nothing and it narrows the job more than anything else you can do.
| What happens | What it usually means |
|---|---|
| Nothing at all, no sound | Power, the blower door switch, the thermostat, or the control board. See won't turn on |
| The draft fan runs but the burners never light | The pressure switch, the igniter, or the gas supply |
| It clicks and glows, then nothing | The igniter glows but gas is not arriving or not lighting |
| It lights, runs a few seconds, shuts off, repeats | Usually the flame sensor. See igniter and flame sensor |
| It runs but blows cold air | The blower is running without the burners, or the thermostat fan is set to On |
| It runs a while, stops, restarts later | Overheating on a limit switch, usually airflow. See keeps shutting off |
Write down which of those you see, and whether a light is blinking in the little window on the furnace. Many furnaces flash a fault code there; count the blinks.
Why repeated resetting is a bad idea
A furnace that locks itself out has decided something is wrong. Each reset tells it to try again, which on a gas appliance means opening the gas valve and attempting ignition again. Once is reasonable. Three times is not. Leave it off and call.
What a diagnosis involves
A technician watches the furnace try to run. A gas furnace works through its start-up sequence and stops at the point where something is wrong, and that one observation narrows the list a long way. From there it is measurement rather than guesswork: flame sensor current, igniter resistance, pressure switch operation, the limit switches and the gas valve. A sound visit also looks at the heat exchanger, because a cracked one is the fault that cannot wait.
Questions to ask before you approve the work
- Which step of the start-up sequence failed, and what reading confirmed the part?
- If a part is replaced, what made it fail?
- If replacement comes up, why, and what is the repair price next to it?
No heat? Tell us what the furnace is doing.
Gas, electric and high-efficiency furnaces across Caldwell, Nampa, Middleton and Parma. Start with the symptom and your ZIP code.
You'll be asked for your ZIP code first, so your call reaches a technician who covers your area.