How to look after your furnace

To look after a furnace, change the filter at least every three months, have it serviced by a licensed heating contractor before winter, keep the outside vents clear of snow, and test your carbon monoxide alarms every month. You can do the filter and the alarms yourself. Leave the service to a licensed technician, because work inside a gas furnace is theirs to do.

What to do, and how often

iSources: Enbridge Gas, "Appliance safety"; Natural Resources Canada, "Maintenance" (modified 19 March 2025); Government of Ontario, "Carbon monoxide safety" (updated 30 December 2025).

Change the filter

The filter catches dust before air goes through the furnace. A clogged one makes the furnace work harder and can cut the heat in your rooms. Enbridge says to change or clean it at least every three months.

1. Turn the furnace off at its switch or the thermostat.
2. Find the filter. It usually slides into a slot where the big return air duct meets the furnace, near the bottom. Some sit behind a door on the front.
3. Note the size and the arrow. The size is printed on the edge of the filter. The arrow shows which way the air flows, and it should point toward the furnace.
4. Slide in the new filter with the arrow the same way, and close the slot or door.
5. Turn the furnace back on, and write the date on the edge of the filter or in your home's records.

Buy a few filters at once in the right size. If your furnace has a washable filter, clean it as its manual says instead.

Book a service

Enbridge says to have the furnace inspected and maintained every year by a certified heating contractor. Natural Resources Canada says natural gas and propane furnaces should be serviced at least every two years, and oil furnaces cleaned and tuned every year. The province says to have every fuel-burning appliance, chimney and vent cleaned and inspected each year before the cold.

Book early in the fall, before the first cold snap, when contractors get busy. Our HVAC hiring guide covers how to check that a contractor is registered and licensed.

Keep the vents and the space around it clear

• Outside vents: most newer furnaces have two pipes that run out through the wall, one to bring air in and one to push exhaust out. Keep them clear of snow, ice and leaves all winter. A blocked vent can shut the furnace off, or push carbon monoxide back into the house.
• Around the furnace: keep boxes, paint, cleaning products and anything else that burns away from it, and leave room for air to move.
• In your rooms: keep furniture, rugs and curtains off the heating registers and the return air grilles, so air can move through the house.

iSource: Government of Ontario, "Carbon monoxide safety".

Carbon monoxide alarms

Since 1 January 2026, an Ontario home with a fuel-burning appliance (a gas furnace, water heater or stove), a fireplace or an attached garage needs a carbon monoxide (CO) alarm next to each sleeping area and on every storey, including storeys without bedrooms. Owners are responsible in their own homes, and landlords in rental homes.

• Test each alarm once a month with its test button.
• Replace the batteries every year, and the alarm when its maker says.
• If an alarm chirps, check whether the battery is low or the alarm has reached the end of its life.

If a CO alarm sounds, get everyone out of the house right away and call 9-1-1 from outside. Signs of CO poisoning include headaches, nausea, dizziness, confusion and drowsiness.

iSource: Government of Ontario, "Carbon monoxide safety" (updated 30 December 2025).

If you smell gas

Leave the house right away. Don't turn lights or appliances on or off, and don't use a lighter or matches. From outside, at a safe distance such as a neighbour's home, call Enbridge Gas's emergency line at 1-866-763-5427, any time of day, or 9-1-1.

iSource: Enbridge Gas, "Contact us".

When the heat stops

Before you call for service, a few things are safe to check:

• The thermostat is set to heat, and a few degrees above the room temperature. If it runs on batteries, try new ones.
• The furnace's power switch is on. It often looks like a light switch on or near the furnace.
• The breaker for the furnace hasn't tripped.
• The filter isn't clogged.
• The outside vents aren't blocked by snow or ice.

If none of these is the problem, call a licensed heating contractor. Don't take the furnace's panels off yourself.

When it's time to replace it

A furnace that needs repairs every winter, heats some rooms and not others, or is around 15 years old or more is worth a second opinion. Our furnace cost guide covers what a new furnace costs in Ontario and the rebates that apply.