Symptoms
- Thermostat set to heat but furnace stays silent
- No warm air from vents
- Thermostat display looks normal
Likely causes
- CommonFurnace switch or breaker offThe furnace has its own power switch and breaker separate from the thermostat.
- CommonThermostat set to wrong mode or low temperatureHeat mode must be selected and the set point above room temperature.
- CommonDirty furnace filter tripping a safety switchA clogged filter can overheat the furnace and shut it down automatically.
- PossibleWiring mismatch or loose wire at furnace boardWires can be miswired during a thermostat swap or work loose over time.
- PossibleFailed furnace control board or igniterInternal furnace components can fail independent of the thermostat.
What to do
- Confirm the thermostat is set to Heat and the target temperature is above room temperature.
- Check that the furnace switch (often near the unit, shaped like a light switch) and breaker are on.
- Replace the furnace air filter if it's dirty or hasn't been changed in a few months.
- Open the furnace door panel and check for a blinking error light on the control board.
- Listen for a click at the thermostat when it calls for heat; no click suggests a wiring issue.
- If the filter is clean and switches are on but nothing happens, the furnace itself likely needs service.
Tools and parts
- Flashlight
- Replacement furnace filter
Furnace air filterSearch: “furnace air filter size for your unit”
When to call a pro
Call an HVAC technician if the filter is clean, power is confirmed, and the furnace still won't fire, especially if you smell gas or see an error code.
Safety
Use caution
Don't bypass any safety switch on the furnace door; it's there to prevent fire or injury.