Furnace vent blocked by snow or ice: what to do now

Canada's efficiency rules require new gas furnaces to be at least 90% efficient, which means a condensing furnace, and Natural Resources Canada (NRCan) notes these vent through plastic pipe instead of a chimney. Most pull combustion air in through one plastic pipe and push exhaust out through another, both through a side wall. The Technical Standards and Safety Authority (TSSA) and Enbridge Gas warn that these vents can sit as low as one foot (30 cm) above the ground, so a drift, a snowbank from the driveway or a plow windrow can bury them without anyone noticing.

How do I know the furnace vent is blocked?

The usual pattern after a storm is a furnace that tries to start but never lights. You hear the small inducer fan hum for a while, then it stops, waits and tries again, and the house slowly cools. That is a safety switch doing its job: a pressure switch has to prove the exhaust path is open before the gas valve is allowed to open. After several failed tries, most furnaces lock out and show a blinking code on the control board, often the one the manual lists for a pressure switch fault.

• The heat stopped during or just after snowfall, high winds or a deep cold snap.
• Outside, one or both white pipes near the foundation are buried, packed with snow or capped with ice.
• The furnace's status light blinks a code instead of glowing steadily.
• Another gas appliance that vents through the wall, such as a power-vented water heater, has also stopped.

Which pipes are the furnace vents?

Look on the outside wall nearest the furnace for two white plastic (PVC) pipes, usually 5 to 8 cm (2 to 3 in) across, side by side or one above the other. Some houses have a single round terminal with a pipe inside a pipe instead. One pipe is the air intake and the other is the exhaust; in cold weather the exhaust is the one that puffs out a white plume of water vapour when the furnace runs. Both must be clear. Some condensing furnaces are one-pipe installs that take combustion air from inside the house, so you will only find the exhaust outside; it still has to be clear. A blocked intake starves the burner of air, and a blocked exhaust has nowhere to send it. If your furnace vents up a chimney instead, this article is mostly not about you, but the gas meter and any other wall vents still need clearing.

How do I clear snow and ice from the furnace vent safely?

1. Set the thermostat to off so the furnace stops trying to start while you work.
2. Clear the snow by hand or with a soft brush or broom, the way Enbridge and TSSA advise. Leave open air in front of both pipe ends and a clear space around them.
3. Do not chip ice with a shovel, scraper or anything sharp. PVC cracks easily in the cold, and a cracked exhaust pipe can leak exhaust into the wall or the house.
4. Melt ice on or inside the pipe ends with warm tap water poured slowly. Boiling water can crack cold plastic.
5. Look into each pipe end. Clear any packed snow or chunks of ice you can reach by hand; anything deeper is for a technician.
6. Set the thermostat back to heat. If the furnace stays locked out, turn off its power switch, usually a light switch on or beside the furnace, wait about 30 seconds, and turn it back on.
7. Watch it through one full start: the inducer, the click and whoosh of the burners, then the blower. If it fails again, stop resetting it and call a technician.

Why does ice keep building up on my furnace exhaust?

Because the exhaust of a condensing furnace is warm and full of water vapour, and in deep cold that moisture freezes on the pipe end. A little frost ring is normal. Ice that grows until it closes the opening usually has a reason: the pipe ends are too close to the ground or under a deck, a downspout or a dripping eavestrough feeds water onto them, wind blows the plume back onto the intake, or the pipe sags indoors and holds condensate. TSSA's winter advice includes clearing icicles from eavestroughs so water does not drip and refreeze below. Furnace installation instructions written to the CSA B149.1 gas code typically require the terminals to sit at least 30 cm (12 in) above the ground or the highest snow level you expect, whichever is higher. If yours are buried every winter, they may need raising.

Is a blocked furnace vent dangerous?

It can be. Enbridge and TSSA warn that blocked exhaust vents can let carbon monoxide from fuel-burning appliances build up in a home. A modern furnace's pressure switch usually shuts it down before that happens, but not every appliance has the same protection, and a vent that is cracked, disconnected or only partly blocked may still let some exhaust spill. That is why Ontario requires working CO alarms in homes with fuel-burning appliances. Natural Resources Canada (NRCan) lists headaches, lethargy and nausea as signs of low-level CO poisoning, and says to leave the home, call your gas company and get medical attention if the alarm goes off.

How do I keep the furnace vents clear all winter?

• Check both pipes, the gas meter and any other wall vents after every snowfall, and after the plow or snowblower has been through.
• Never shovel, plow or blow snow toward the vents or the meter. Enbridge also warns against kicking or bumping them to knock off ice.
• Put a tall stake or marker next to low vents so you, a neighbour or a snow crew can find them under a drift.
• Fix the eavestrough or downspout that drips above the vents before the freeze–thaw season starts.
• If you hire snow clearing, show the crew where the vents and meter are. On Doyo, Snow Removal is $86.25 per visit, the full price with Doyo’s fee included, and you can tell the pro in the order chat.
• Test the CO alarms when the clocks change, and replace any alarm past the end-of-life date printed on it.

When should I call a technician?

Call a TSSA-certified gas technician if the furnace still will not run after the vents are clear and you have reset it once or twice, if ice keeps closing the exhaust during every cold snap, or if a pipe is cracked, loose, sagging or disconnected inside or out. If the CO alarm went off at any point, have the furnace and venting checked before you rely on it again. Doyo checks the technician's TSSA certificate before they are offered heating work. Furnace Maintenance is a fixed $201.25, and Furnace Repair is priced by written quote after the technician sees the problem, so you agree to the price before work starts.