Furnace Won't Turn On? Fix It Tonight
- 4 parts and tools
A furnace that will not turn on on a cold night is stressful, but many no-heat calls come down to the thermostat, a tripped breaker, a dirty filter, or a switch that got bumped. Those are all checks you can do tonight in a few minutes. Work through them in order, and keep the house safe while you do. If you smell gas at any point, stop everything, do not touch electrical switches, leave the house, and call the gas company from outside.
What's probably wrong
The thermostat is the most common cause. A thermostat set to cool instead of heat, a temperature set below the room temperature, dead batteries, or a thermostat that lost power will all leave the furnace silent. Thermostats get bumped, and batteries die quietly, so this is always worth checking first.
The power supply is the second suspect. Furnaces have a breaker in the electrical panel and often a power switch on or near the unit that looks like a regular light switch. Either one can be off, and the switch is easy to bump when storing things near the furnace. A tripped breaker or a switched off furnace switch means no power at all.
A dirty filter is the third suspect. A filter clogged with dust restricts airflow, and many furnaces shut themselves down on a safety limit when they overheat from poor airflow. If the furnace runs briefly and then stops, or if it has been months since the filter changed, this is very likely.
A tripped safety switch is the fourth suspect. Furnaces have several safety switches, including a limit switch that trips on overheating and a blower door switch that cuts power when the access panel is not fully closed. A panel that was removed for a filter change and not seated properly will keep the furnace off.
An ignition or flame sensing problem is the serious cause. Modern furnaces light with a hot surface ignitor or a spark, and a flame sensor confirms the flame is present. A failed ignitor or a dirty flame sensor will make the furnace try to start, fail, and lock out. Some of this is pro work, and anything involving the gas valve or gas lines is always pro work.
Fix it tonight
Check the thermostat first. Make sure it is set to heat, not cool or off, and set the temperature at least five degrees above the current room temperature. If the thermostat uses batteries, replace them. If the thermostat screen is blank, it has no power, and that alone explains the silence.
Check the breaker. Find the furnace breaker in the electrical panel and make sure it is fully on. Push it to off and then to on to reset it firmly. If it trips again immediately, leave it off and call a pro. Do not keep resetting a breaker that trips instantly.
Check the furnace power switch. Look on or near the furnace for a switch that looks like a light switch, often on the wall next to the unit or on the furnace itself. Make sure it is on. This switch gets bumped more often than anyone admits, especially in basements and utility closets used for storage.
Check the filter. Turn the furnace off at the switch, remove the filter, and hold it up to a light. If you cannot see light through it, it is clogged. Replace it with the correct size, with the airflow arrow pointing in the right direction, which is usually toward the furnace. Turn the furnace back on and wait several minutes.
Check the blower door panel. Make sure the access panels on the furnace are fully closed and latched. The blower door switch cuts power when a panel is loose, and a panel that looks closed may not be fully seated. Press each panel firmly and listen for a click.
Look at the furnace and listen. When the thermostat calls for heat, you should hear a sequence: a small inducer fan starts, then a click or glow as ignition happens, then the main burners light, then the blower starts a minute or so later. Note where the sequence stops, because that tells a pro exactly what failed. If the furnace tries three times and then locks out, turn it off at the switch, wait a few minutes, and turn it back on once to reset the lockout.
Check the condensate drain on a high efficiency furnace. These furnaces produce water, and a blocked condensate drain or a full trap can shut the furnace down on a safety switch. Look for a small plastic drain line and a trap near the furnace. If the trap is full of water or the line is visibly blocked, clearing it may restore operation. If you are unsure, leave it for a pro.
Check the vents outside. For high efficiency furnaces, the intake and exhaust pipes go outside, usually through a side wall. Make sure they are not blocked by snow, leaves, ice, or debris. A blocked vent will shut the furnace down on safety, and clearing it is a simple outdoor fix. Do not reach into a vent pipe.
Keep the house warm while you work. Close off unused rooms, hang a blanket over a drafty door if needed, and run a space heater in the room you are sleeping in, keeping it away from bedding and curtains. Space heaters are a stopgap, not a solution, and they should never run unattended or while you sleep without proper clearance.
Parts and tools
When to call a pro
Call a pro if you smell gas, if the breaker trips immediately when reset, or if the furnace locks out repeatedly after a reset. Call a pro if the ignitor glows but the burners never light, if you hear repeated clicking with no flame, or if the furnace runs but blows cold air. Never touch gas lines, the gas valve, or gas controls yourself. Never bypass a safety switch to force the furnace to run. Safety switches exist because the failure they guard against can be dangerous, and bypassing one turns a repair bill into a hazard.
What a pro typically costs
These are broad typical ranges, not quotes. A diagnostic visit usually runs about 100 to 200 dollars. An ignitor replacement usually runs about 150 to 300 dollars including parts and labor. A flame sensor cleaning or replacement usually runs about 150 to 250 dollars. A blower motor replacement is a bigger job and usually runs about 400 to 800 dollars. A control board replacement usually runs about 400 to 700 dollars.
FAQ
How long should I wait after changing the filter before judging?
Give the furnace several minutes after you turn it back on. The startup sequence takes time, and the blower does not start immediately. If nothing happens after about ten minutes with the thermostat calling for heat, move on to the next check.
Why does my furnace try to start three times and then stop?
That is the lockout behavior. The furnace attempts ignition a set number of times, and if it cannot confirm a flame, it locks out for safety. Resetting once by turning the power off and on is reasonable. Repeated lockouts mean a failed ignition part or a gas supply problem, and that is a pro call.
Can I run space heaters all night instead of fixing the furnace?
Space heaters are fine as a short stopgap with proper clearance from anything flammable, and they should never run unattended. They are not a substitute for a working furnace, especially in very cold weather, because pipes can freeze and the house can suffer damage. Get the furnace fixed promptly.