Furnace Vent Blocked by Snow: How to Find the Pipes, Clear Them, and Get the Heat Back

If snow or ice covers your furnace's vent pipes, clear the pipe ends by hand or with a soft brush, then check that the furnace restarts. On a high-efficiency furnace these are the white plastic pipes on an outside wall: one brings in combustion air, the other carries exhaust out. Trane says its furnaces shut off if the flue is blocked and warns that a blocked exhaust pipe can let carbon monoxide build up in the home, so check the pipes after every heavy snow, along with the water heater vent, the dryer vent, and the gas meter.

If the furnace already quit, this page gets it running again. For a furnace that won't start for other reasons, see furnace not turning on.

Why a Buried Vent Matters

Your furnace can't breathe. High-efficiency condensing furnaces “require an external intake pipe for their air supply,” Lennox says, and a blocked intake can cause a dangerous buildup of carbon monoxide. Trane's furnace manual explains why: if combustion air is restricted, partly burned gases can create dangerous amounts of carbon monoxide.

The exhaust has nowhere to go. Trane says a blocked exhaust pipe can let deadly carbon monoxide build up indoors. Nicor Gas and National Grid give the same warning for snow and ice over the outdoor vents and air intakes of gas furnaces and water heaters, and National Grid adds that it can make the equipment malfunction.

Then the heat goes off. Trane says its furnaces have a safety feature that shuts the gas furnace off when the flue is blocked, so a furnace that quits after heavy snow should send you outside to check the pipes. A furnace off in a deep freeze is its own problem: see frozen pipes.

CO gives no warning you can sense. Trane calls it odorless, colorless, and incredibly toxic. The CPSC says early symptoms are like the flu without the fever: headache, fatigue, shortness of breath, nausea, and dizziness. The CDC warns that people who are asleep can die from CO before they have symptoms.

Find the Vent Pipes

Do this on a clear day, before the snow, so you know where to dig.

• High-efficiency furnace. Trane's manual says condensing furnaces, typically over 90% AFUE, are mainly vented through plastic PVC pipe. Trane's storm checklist calls them the two white PVC pipes. Follow them from the furnace to where they leave the house, often through a side wall.
• Older furnace. Trane's manual says a standard-efficiency furnace may vent through a metal vent or masonry chimney that runs mainly upward. If yours does, the side-wall pipes on this page don't apply, but the CDC still warns that a chimney blocked by debris can cause CO to build up.
• Water heater. Nicor Gas flags vent openings and air intakes for gas water heaters as well as furnaces. If yours vents out the side of the house, add it to the list.
• Dryer, stove, and fireplace vents. The U.S. Fire Administration says to keep the vents for the dryer, furnace, stove, and fireplace clear of snow and other debris.
• The gas meter. Not a vent, but it sits on the same walls and gets buried the same way, and the utilities all ask you to clear it (see below).

Most Likely Causes

1. Drifting snow. Wind can pile snow against a low pipe even when the rest of the yard looks clear, so check the pipes themselves, not just how deep the snow is.
2. Snow you moved there. Peoples Gas tells customers not to bury vents or meters with a snow blower or shovel, and Minnesota Energy Resources says never to shovel snow onto the meter.
3. Ice at the pipe opening. Lennox says to make sure the pipes are free of ice as well as snow, and Peoples Gas tells customers to clear ice and snow from vent pipes.
4. Something other than snow. Lennox also lists bird nests and insects. A pipe can be blocked in any season, which is why Trane's manual says to check that the vent and air intake ends aren't obstructed “in any way during all seasons.”

What You Can Fix Yourself

1. Check the CO alarm first. If it's sounding, stop and follow the callout above.
2. Clear a path. Shovel up to the wall, but finish the last stretch around the pipes by hand. Nicor and Minnesota Energy Resources say to use your hands or a broom, not a shovel, around the meter, and the plastic pipes deserve the same care.
3. Clear the pipe ends. Brush or scoop snow out of and away from both openings, intake and exhaust. Clear the water heater vent and dryer vent while you're there (USFA, Nicor).
4. Remove ice at the opening gently. Lift away loose ice by hand. If ice is inside the pipe and won't come out easily, or a pipe is cracked or pulled loose, leave it alone and call; Trane's manual says any obstruction or blockage in the vent must be removed by the technician.
5. Clear the meter. Brush snow off the meter and regulator by hand or with a broom (Nicor, National Grid). Minnesota Energy Resources and Peoples Gas say to remove icicles that may drip water onto it, and National Grid warns that icicles above it can fall and damage the meter or pipes.
6. Restart the furnace by its own instructions. Many furnaces start again on their own once the pipe is clear. If yours doesn't, follow the lighting and operating instructions on the label inside your furnace's door or in its manual; our how to reset a furnace guide walks through it. In Trane's sequence, you turn the gas valve off, wait 5 minutes, and stop if you smell gas; if the burners don't light when you turn the heat back up, call a licensed HVAC technician.
7. Watch it run. Stay nearby through one heating cycle. If it shuts off again, look elsewhere, such as the condensate drain (see our condensate line guide).

What Not to Do

• Don't hack at ice. National Grid says not to chip away at ice on the meter, and Nicor says never to kick or hit the meter or its piping with a hammer or other hard object. Treat plastic vent pipes just as gently.
• Don't use a snow blower near the meter. National Grid says keep snow blowers and plows away from it, and Minnesota Energy Resources says don't bury the meter with a blower or shovel.
• Don't stay inside with a sounding CO alarm to fix the vent. Get out and call 911.
• Don't heat the house with the oven or range, or run a generator anywhere but outdoors, at least 20 feet from the house, while the furnace is down (CPSC, which says never to use a gas range or oven for heating).

When to Call a Pro

• The burners won't light after the pipes are clear (Trane's manual), or the furnace keeps shutting down.
• Ice is inside the pipe, or a vent pipe is cracked, sagging, or disconnected; Trane's manual leaves blockages and corroded or damaged vent pipe to a licensed HVAC technician.
• A CO alarm went off. The CPSC says to keep the appliance off until a technician has serviced it.
• Your meter is damaged or a gas line is exposed. Nicor says get away from the area and call 911 or the utility from a safe location.
• You can't reach or check the pipes yourself; Lennox says a dealer can do it.

If you'll be away while you wait, keep the heat no lower than 55°F, the American Red Cross minimum for preventing frozen pipes, and have someone check the house and the vents after any snowfall.

Prevent It

• Check after every storm. Trane, Nicor, National Grid, Peoples Gas, and Minnesota Energy Resources all say to keep vent pipes clear of snow and ice.
• Shovel and blow snow away from the wall, not toward it. Don't bury vents or meters (Peoples Gas). A good shovel helps; our snow shovel and ice melt guide compares them.
• Watch the roofline. Snow sliding off a roof or icicles above can land on pipes and the meter (National Grid, Minnesota Energy Resources). If the roof is carrying a lot, see how much snow a roof can hold.
• Keep the dryer vent hood clear, too (USFA); our dryer vent guide covers the outside hood.
• Keep CO alarms working. The CPSC says install battery or battery-backup CO alarms on every level of the home and outside sleeping areas. The CDC says check batteries each spring and fall and replace the alarm per its maker or every 5 years. Our smoke and CO alarm guide covers choosing them.
• Book an annual inspection. The CPSC and CDC say to have the heating system and water heater serviced by a qualified technician every year, and Trane's manual says the venting system should be inspected annually by a licensed HVAC technician. If pipes keep getting buried, ask about the termination height then. The winter guide schedules it.

Frequently Asked Questions

Why did my furnace stop working after a snowstorm?

A buried intake or exhaust pipe is a likely cause. Trane says its furnaces shut off when the flue is blocked and to check the pipes after heavy snow. Clear them by hand, then restart the furnace by its instructions.

Can a snow-blocked vent cause carbon monoxide poisoning?

Yes. Trane, Lennox, Nicor, and National Grid all warn that blocked vents can let carbon monoxide build up. That's why a working CO alarm on every level matters, and why you get out and call 911 if it sounds.

Is ice on the furnace exhaust pipe a problem?

It can be. Lennox says to keep the pipes free of ice as well as snow. Lift away ice at the opening gently; if it's inside the pipe or keeps coming back, call a licensed HVAC technician.

Do I need to clear snow from the gas meter too?

Yes. Nicor, National Grid, Peoples Gas, and Minnesota Energy Resources all say to keep it clear, by hand or with a broom, never with a shovel, hammer, or snow blower. Minnesota Energy Resources says icy buildup can interfere with the flow of gas into the house.

The Bottom Line

After every heavy snow, walk around the house and clear the white furnace pipes, the water heater and dryer vents, and the gas meter by hand or with a brush. If the furnace quit, restart it by its own instructions, and call a technician if it won't relight, the pipe is iced inside, or the vent is damaged. Keep CO alarms on every level, and if one sounds, get everyone out to fresh air and call 911.

Sources

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