Furnace Shutting Off in a Blizzard? How to Clear and Protect Your Furnace Vent Pipes

After a big northwest Minnesota snowstorm, one of the first calls we usually get sounds like this: "The furnace was fine last night, and this morning it's not running and the house is cold." More often than you'd think, the furnace isn't broken at all. Its vent pipes are buried in snow.

Why high-efficiency furnaces are vulnerable

A high-efficiency (condensing) furnace doesn't use a chimney. It pulls combustion air in through one plastic pipe and pushes exhaust out through another, usually two white PVC pipes that exit the side of the house a foot or two off the ground, or sometimes through the roof. That design is efficient and safe, but it has one weakness in our climate: if either pipe gets blocked, the furnace's safety system shuts it down.

Around Waubun and across Mahnomen County, pipes get blocked in a few ways:

• Drifting snow piling against the side of the house, especially on the north and west sides, which catch the prevailing winter winds
• Wind-driven snow packing into the pipe openings during a blizzard, even when the drift isn't that deep
• Frost and ice buildup on the exhaust in extreme cold, as moisture in the exhaust freezes at the opening
• Snow sliding off the roof and burying pipes below the roof edge
• Snowblower or plow piles thrown up against the house

Signs your vents are the problem

• The furnace stopped during or right after a storm or a stretch of wind
• The furnace tries to start: you hear the inducer motor (a small fan) run, then it stops without lighting
• The control board light is flashing a pressure switch code (check the chart on the inside of the furnace door)
• You can see snow or ice covering the pipe openings outside

How to safely clear blocked furnace vents

1. Find the pipes. Look where your furnace is located and trace outside to where two white PVC pipes exit the wall or roof. Some homes have a single concentric vent, a larger pipe with a smaller one inside it.
2. Clear the snow around them by hand or with a soft brush or small plastic shovel. Clear a good area around and below the openings, not just the openings themselves.
3. Remove ice gently. If ice has built up on the exhaust opening, break it away carefully by hand. Don't hammer on the pipe, don't use a metal tool that can crack the PVC, and never use an open flame.
4. Reset the furnace. Turn it off at the furnace switch (it looks like a light switch near the unit) for about a minute, then back on. The furnace should go through its startup sequence and light.
5. Watch it for a few minutes. If it lights and keeps running, you've found the problem.

Don't clear roof-mounted vents yourself by climbing on a snowy roof. Call us instead.

If the furnace still won't run

If the vents are clear and the furnace still won't start, or it starts and then shuts off again, something else is going on. The next most likely causes after a storm are a frozen condensate line (water from the furnace backs up when the drain freezes), a pressure switch that's failed or has a cracked hose, or an inducer motor problem. Check our cold-snap troubleshooting guide, then call us.

How to stop it happening every storm

If you're clearing your vents after every blizzard, that's not bad luck. It's a vent installation problem, and it can usually be fixed:

• Raise the termination. Pipes that exit a foot off the ground on an exposed wall will get buried. Extending them higher on the wall (within the manufacturer's allowed venting length and fittings) keeps the openings above typical drift height.
• Move the vents to a sheltered side. If the pipes are on the windward side of the house, relocating them to a more protected wall can make a big difference.
• Get them out from under the roof edge. Pipes directly below a roof edge get buried by sliding snow and drip ice.
• Use the correct termination kit. Manufacturer-approved termination kits and proper spacing between intake and exhaust help prevent exhaust frost from blocking the intake.
• Keep plow and snowblower piles away from that side of the house.

Any changes have to follow the furnace manufacturer's venting instructions and code requirements for clearances from windows, doors, gas meters and property lines. That's a job for a technician, not a DIY project.

Carbon monoxide: why a blocked vent matters

Modern furnaces are designed to shut down when the venting is blocked, which is exactly why yours stopped. That's a safety feature doing its job. But every home with a fuel-burning furnace should have working carbon monoxide alarms. Minnesota law requires them within 10 feet of every sleeping room. Test them monthly and replace them by the date on the alarm. If a CO alarm ever sounds, get everyone out and call 911.

Need help?

If your furnace keeps locking out in storms, or you've cleared the vents and it still won't run, call Waubun HVAC Pros at (218) 227-4357. We answer no-heat calls 24/7 from our shop in Waubun, and permanently fixing vent terminations for northwest Minnesota snow is something we do all winter. See Furnace Repair.