A furnace that quits on a cold night is stressful, but many no-heat problems have simple causes you can check safely in a few minutes. This checklist goes through them in order. If none of them solves it, or if anything feels unsafe, stop and call a certified technician.
Safety first
If you smell gas, the rotten-egg smell added to natural gas, do not touch switches, the thermostat or the furnace. Leave the house with everyone, and call Enbridge Gas at 1-866-763-5427 or 911 from outside. If a carbon monoxide alarm sounds, get everyone outside and call 911. Never remove the burner compartment cover or bypass a safety switch to make a furnace run.
The checklist
1. The thermostat
Make sure it is set to Heat, not Cool or Off, and set a few degrees above the room temperature. Replace the batteries if it has them; a weak battery can blank the screen or stop it calling for heat. On a smart thermostat, check it has not lost its schedule or Wi-Fi after an update. If the fan is set to On rather than Auto, it will blow room-temperature air between heating cycles, which can feel like cold air.
2. The power
The furnace has a service switch, usually a light switch on the wall near it or on the side of the unit, and people switch it off by mistake. Make sure it is on. Check the breaker panel for a tripped breaker marked Furnace. Reset it once; if it trips again, leave it off and call.
3. The furnace door
Most furnaces have a safety switch that stops them running if the blower door is not seated properly. If someone changed the filter recently, make sure the door is pushed fully into place.
4. The filter
A badly clogged filter can make the furnace overheat and shut itself off on a safety limit. Pull it out. If it is grey and caked, replace it. Our guide on how often to change a furnace filter explains which to buy.
5. The vents outside
High-efficiency furnaces vent through two white plastic pipes on an outside wall or the roof. Heavy snow, drifting or ice can block them, and the furnace shuts down. Clear them gently by hand. Do not chip ice off with a tool that could crack the pipe.
6. The condensate drain
High-efficiency furnaces make water. It drains through a small trap and a tube to a floor drain or a condensate pump. If the trap is clogged or the pump has failed, a safety switch stops the furnace. Look for water around the furnace or a full pump reservoir.
7. The gas supply
Make sure the gas valve on the pipe beside the furnace is open: the handle runs in line with the pipe. If other gas appliances, such as the stove or water heater, also are not working, the problem may be the gas supply, and Enbridge can tell you whether there is an outage.
8. The blinking light
Most modern furnaces have a small window on the blower door showing an LED. A steady or slow blink usually means normal. A pattern of flashes is a fault code, and a label inside the door explains it: common codes point to a pressure switch, a flame sensing problem, an ignition failure or a limit switch. Note the code before you call; it saves the technician time.
9. Registers and returns
If the furnace runs but some rooms are cold, check that supply registers are open and that furniture, rugs or boxes are not blocking return grilles.
10. One reset, not five
Switching the furnace off at the service switch for 30 seconds and back on resets the control board. If it starts, runs and then fails again, do not keep resetting it. Repeated attempts can flood the combustion area with unburned gas on some models. Call for service.
Propane and oil furnaces
On rural properties in Haldimand, Norfolk and Flamborough, check the fuel first. Look at the propane tank gauge or the oil tank level: running out is a common cause of no heat, and after a propane run-out the supplier must check the system for leaks before relighting. Oil furnaces have a reset button on the burner; press it once only, because repeated resets can flood the combustion chamber with oil.
Common symptoms and likely causes
- Inducer runs, burners never light: often a failed igniter.
- Burners light, then shut off in seconds: usually a dirty flame sensor.
- Burners run, no air from vents: a blower motor or capacitor problem.
- Blows cold air: the fan is set to On, the burners are not lighting, or it is a heat pump in defrost.
- Short cycles on and off: a clogged filter, an overheating limit, or an oversized furnace.
The furnace repair page describes these parts and typical repair costs.
While you wait for a technician
Close curtains, shut doors to unused rooms and gather everyone in one room with a safe electric space heater, kept well away from bedding and curtains. If the house will stay below freezing for long, open the cabinet doors under sinks on outside walls and let a tap drip to protect pipes. Never use a gas oven, barbecue or camping heater indoors for heat.
Get help
Send one request and mark it as an emergency. We pass it to a licensed contractor in your area who takes no-heat calls, and they call you back to arrange a visit. Mention the fault code and the furnace’s age if you know them. If the furnace is old and failing often, read how long a furnace lasts.
Questions people ask
Why is my furnace blowing cold air?
Most often the thermostat fan is set to On instead of Auto, or the burners are not lighting because of an igniter, flame sensor, filter or vent problem. A heat pump in defrost can also blow cool air briefly.
Why does my furnace keep shutting off?
Common causes are a clogged filter, a dirty flame sensor, a blocked vent or condensate drain, or an oversized furnace. If the filter and vents are clear, call a technician.
Is it safe to reset my furnace?
Once, by turning the service switch off for 30 seconds and on again. If it fails again, stop resetting and call a certified technician.
What should I do if I smell gas?
Leave the house immediately without using switches or phones, and call Enbridge Gas at 1-866-763-5427 or 911 from outside.