Snow Blocking Your Furnace Vent Pipes? What to Check and Clear

The first real snow comes in overnight. In the morning the yard is white, the kids are hunting for boots, and somebody notices the house is sitting at 63 when the thermostat says 68. The furnace hums for a few seconds, clicks, and goes quiet. Then it tries again.

Plenty of times, nothing inside that furnace is wrong. The problem is outside, two feet off the ground, under a few inches of snow.

Why Snow Stops a High-Efficiency Furnace

If your furnace is a high-efficiency model, the kind rated 90% AFUE or better, it doesn't vent up a chimney. It vents through white PVC pipe, usually out the side of the house. Most have two pipes — one brings outdoor air in for combustion, the other carries exhaust out. Some use a single fat pipe with one inside the other, called a concentric kit, which does both jobs through one hole in the wall.

Before the burners light, a small fan called the inducer starts up and pulls air through that whole path. A pressure switch watches it. If the inducer can't move air because the intake is packed with snow or the exhaust is plugged with ice, the switch stays open and the furnace won't light. It'll usually retry a few times and then lock itself out for an hour or so.

The furnace is doing its job there. One that can't move combustion air in and exhaust out has no business burning gas, so the fix is usually on the outside of the house, not inside the furnace.

How to Find Your Pipes Before You Need To

Do this on a nice day, not at 6 a.m. in a snowstorm.

Go to the furnace and follow the white pipes. Where they go through the rim joist or the foundation, that's the wall to check outside. Most terminate on a sidewall somewhere between a foot and three feet off the ground. Some go up through the roof instead, especially in houses where the furnace sits in the middle of the basement.

Once you've found them outside, look at what's around them:

• What's above them? A roofline with no gutter, a valley that dumps, or a porch roof that sheds snow in a sheet.
• What's next to them? A deck, a set of steps, a fence, or a spot where you pile snow off the driveway.
• Which way do they face? In Ottawa and Kent County the drifting snow mostly rides a west or northwest wind off the lake. Pipes on that side of the house get buried a lot more often than pipes on the east side.
• How far up are they? If the openings are only twelve or fourteen inches off the grass, a normal lake-effect week will cover them.

Snap a picture with your phone. If anyone else in the house might need to find these pipes while you're at work, show them.

Clearing Snow and Ice the Right Way

When the heat quits after a snowfall, the pipes are one of the first places I'd look. The full overnight no-heat checklist covers the thermostat, breaker, and the rest, but if you've got PVC out the side of the house, start here.

Go out and look at both openings. Not just from the window. Get close with a flashlight if it's dark.

Clear by hand or with a broom. Scoop out a circle at least a foot or so around each opening so the next gust doesn't fill it right back in. Reach into the first few inches of each pipe and pull out packed snow.

Knock off the ice gently. The exhaust gives off warm, wet air, and on a cold still morning that moisture freezes into a crust of frost around the opening, sometimes right across the intake next to it. Tap it loose with your gloved hand or a broom handle.

Don't swing a shovel, an ice chopper, or a hammer at it. PVC gets brittle in the cold, and a cracked fitting on the exhaust side is a much bigger problem than a little snow.

Don't use heat. No hair dryer, no torch, no hot water. Water you pour in there runs back into the pipe and freezes again where you can't reach it.

Then give it a minute. A lot of furnaces will try again on their own. If it doesn't, flip the furnace switch off, count thirty, and turn it back on. Once. Stand there and listen for the inducer, then the click of the igniter, then the burners.

I got a call from a family in Hudsonville one winter where the dad had been out early running the snowblower down the driveway. The chute was aimed at the side yard, and the side yard was where the furnace pipes came out. He'd buried both of them himself — the house was cold by lunch. Five minutes with a broom and the heat was back. He aims the chute the other way now.

When It's More Than Snow

Snow on the outside is easy. Some vent problems look the same but live somewhere you can't reach with a broom.

Frozen condensate inside the pipe. High-efficiency furnaces make water, and some of it runs back down the exhaust pipe to a drain. If a section of pipe was installed with a sag or runs through an unheated garage or crawlspace, water can collect and freeze into a plug. The furnace locks out the same way, but the outside openings look perfectly clear.

Icicles dripping into the intake. A pipe under an eave can get a steady drip that freezes into a cap over the opening, then melts in the afternoon and freezes again at night. That's the furnace that quits every morning and runs fine by suppertime.

Nests and screens. Birds and wasps like these pipes in the warm months. And some people add screens or caps to keep critters out, which is fine in July and a problem in January, because screens frost over fast.

A failing pressure switch or inducer. If the pipes are clear and the furnace still locks out, the switch, the inducer motor, or the little hose between them may be the issue. That's a service call.

A furnace that cycles on and off without ever really heating the house can be a venting problem too, so if you're seeing that pattern, mention it.

Where Carbon Monoxide Fits In

People hear "blocked exhaust" and think carbon monoxide. It's a fair instinct, but the furnace is built to head that off.

A blocked vent on a properly working furnace mostly causes a no-heat problem, not a CO problem, because the pressure switch won't let the burners light. The furnace protects you by shutting off.

The CO risk shows up when something else is wrong at the same time: a vent pipe that's cracked or come apart inside the basement, a pressure switch that's failed closed (or one somebody jumpered to keep the furnace running), or an exhaust that ends up right next to a window or an air intake. That's why I tell people a furnace that keeps locking out on its pressure switch needs somebody to look at it, even if clearing the pipe seems to fix it. And it's why you need working CO detectors on every level. What every West Michigan homeowner should know about carbon monoxide covers the rest.

Fixing a Vent That Gets Buried Every Year

If you're out there with a broom every time it snows, the termination is in the wrong place, and it's fixable.

The usual options:

• Raise it. An extension that runs the pipes up the wall a couple of feet before they turn out. This is the most common fix.
• Move it. Bring the termination out a different wall, away from the drifting side and the roof dump.
• Change the kit. Switch from two separate pipes to a concentric kit, or the other way around, if that fits the wall better.

Any change has to follow the furnace manufacturer's venting instructions and keep the clearances from windows, doors, the gas meter, and the neighbor's house. This isn't a weekend job with pipe and glue from the hardware store. A bad joint on the exhaust side puts flue gas where it shouldn't be. Depending on the house it usually runs from a few hundred dollars to around a thousand.

If you're shopping for a new furnace anyway, it's a good time to talk about where the pipes come out. Getting a high-efficiency furnace installed right includes putting the termination somewhere it won't spend February under a drift. And a fall tune-up should always include a look at the vent run and terminations, which is part of what we check during furnace maintenance.