A furnace that will not turn on at all usually has a simple cause upstream of the furnace itself: the thermostat is set wrong or has dead batteries, the furnace power switch or breaker is off, the blower door panel is not fully closed, or (on high-efficiency models) the condensate drain has backed up. If none of these apply, the problem is the ignition or control board, and it is time for a technician.
If you smell gas
If you smell rotten eggs or sulfur near the furnace, do not touch switches, do not light anything, leave the house and call your gas utility’s emergency number from outside. Do not try to restart the furnace until it has been cleared.
The thermostat
- Set it to Heat and raise the target
Set the thermostat to Heat and at least five degrees above the room temperature. The fan setting should be Auto.
- Replace the batteries
A blank or faint display usually means dead batteries; see thermostat screen is blank.
- Check for a scheduled setback
A program or hold mode may be keeping the target low.
If the thermostat looks fine but nothing happens, see thermostat not working.
Power switch and breaker
Furnaces have a switch that looks like an ordinary light switch on or near the unit, and it is easy to knock off. Check that it is on, then check the breaker for the furnace in the panel. A breaker that trips again immediately points to an electrical fault — do not keep resetting it. Our breaker tripping guide explains why.
The blower door panel
Most furnaces have a safety switch that cuts power when the access panel is removed or not seated. Make sure the panel is fitted properly and the switch button is pressed in; a panel that is slightly out of place can stop the furnace from starting at all.
A clogged filter
A very dirty filter can cause the furnace to overheat and shut down on its limit switch. Replace it if it is gray and clogged. See when to change your HVAC filter and furnace keeps shutting off.
Condensate drain (high-efficiency furnaces)
High-efficiency furnaces make condensation and drain it through a trap and line. If the line clogs, a float switch shuts the furnace down. Look for water in the drain pan or a clear tube full of water and algae. Clearing a blocked trap is often a simple fix, but follow the manual and keep to what you can reach safely.
Gas supply
Check that the gas shutoff valve near the furnace is in line with the pipe (open). A gas valve turned off, or a pilot light that is out on an older furnace, prevents ignition — see pilot light keeps going out. Relighting a pilot is only safe if you know the procedure and there is no gas smell.
Read the control board light
Most furnaces have a small window on the blower door showing a blinking LED. The number of flashes is a diagnostic code listed on a label inside the door. Note the pattern before you call: it tells the technician whether the cause is the igniter, the flame sensor, the pressure switch or a limit switch. If the furnace starts and then stops, see furnace not blowing hot air.
When to call a technician
If the thermostat, power, panel, filter and drain all check out, the cause is likely the igniter, flame sensor, draft inducer or control board. Winter breakdowns are expensive and slow, so a pre-season service helps; see winterizing your home.
