Furnace Stopped in Snow? Check the Vent Pipes Outside

If your high-efficiency furnace stopped during snow or a freeze, look at the white plastic pipes on the outside wall. When snow or ice covers them, the furnace can’t vent safely, so it shuts itself down, often with a pressure-switch code. Clear the snow gently by hand. Don’t hit the pipes, and if ice is inside them, call.

Find the Pipes

Most high-efficiency furnaces vent through one or two white plastic pipes (PVC or similar) on an outside wall, often near the furnace room. With two pipes, one brings in air and the other carries out exhaust. Some have one pipe or a single round cap with both inside. Older mid-efficiency furnaces use a metal chimney instead and aren’t affected the same way.

Clearing Them Safely

1. Turn the thermostat down so the furnace isn’t trying to start.
2. Brush the snow away from the pipe ends by hand or with a soft brush. Clear a space around them, not just the opening.
3. Remove loose ice gently with a gloved hand.
4. Don’t hit the pipe, chip at it or use a torch or heat gun. Cracked vent pipes can leak exhaust.
5. Turn the thermostat back up and watch one start-up.

If the pipe is frozen solid inside, or the furnace still won’t start, leave it and call. A furnace that has locked out may show a code. See pressure switch problems and furnace error codes.

Why It Happens

• Drifting snow piles up against the wall and buries the intake.
• Ice from the exhaust. The exhaust is warm and wet. In a freeze, that moisture can build ice on the cap, or drift back into the intake pipe.
• Pipes too low for the snow your area gets. This is more common on the North Shore, in the Fraser Valley and on hillside homes than in central Vancouver.
• Screens or caps on the pipe ends that catch ice.

Carbon Monoxide Risk

A blocked exhaust can push combustion gases back into the house. The furnace shutting down is the safety system working, but make sure you have working CO alarms. If one sounds, leave and call 911 or FortisBC from outside. See what to do if a CO alarm goes off.

If It Keeps Happening

Repeated blockages usually mean the pipes are too close to the ground, too close together or pointing into the wind. Moving or raising the termination has to follow the furnace manufacturer’s instructions and the gas code, so it’s a job for a licensed gas fitter. David checks the pipes, the clearances and the drain, and quotes any change before he starts.

Questions About Blocked Furnace Vent Pipes

Can snow make my furnace stop working?

Yes. On a high-efficiency furnace, snow or ice over the plastic vent pipes outside can block the air and exhaust. The furnace senses it and shuts down for safety.

How do I clear snow from my furnace vent?

Turn the thermostat down, brush the snow away by hand or with a soft brush, and remove loose ice gently. Don’t hit the pipe or use heat. If ice is inside the pipe, call a gas fitter.

Why does my furnace vent keep freezing?

Usually the pipes are too low for local snow, too close together so exhaust moisture reaches the intake, or fitted with screens that catch ice. Changing the termination is a job for a licensed gas fitter.

Is a blocked furnace vent dangerous?

A blocked exhaust can push combustion gases back inside. The furnace usually shuts down first, but keep working CO alarms and leave the house if one sounds.