Furnace Exhaust Pipe Frozen or Blocked by Snow: What to Do

How these appliances vent

A high-efficiency, condensing furnace or boiler pulls so much heat out of its exhaust that the water vapor condenses inside the appliance. The exhaust leaving it is cool and very wet, so it cannot use an ordinary masonry chimney. Instead it uses plastic pipe, usually PVC or a similar listed material, run through the side wall or the roof as the appliance instructions require. Many units also have a second pipe that brings in combustion air from outside.

Outside, you will usually see two short pipes or a single concentric fitting near the foundation or on the roof. On a cold day, a plume of steam comes from the exhaust while the heat runs. That is normal.

Why the pipes freeze or get blocked

There are two different winter problems, and they have different causes. The first is snow. Sidewall terminations near ground level can be buried by snowfall, drifting or snow thrown by a plow or snowblower. A buried intake starves the burner of air; a buried exhaust cannot get rid of its gases.

The second is ice. The exhaust carries water vapor, and in hard cold some of it freezes at the outlet, building a ring of ice that narrows the opening. Ice can also form inside the pipe where condensate collects, usually at a sag or a section that slopes the wrong way, or where the pipe runs through an unheated space.

• Terminations too close to the ground for local snow depths
• Snow drifting against a wall that faces the prevailing wind
• Exhaust and intake pipes placed so the exhaust plume frosts the intake
• A sagging or wrongly sloped pipe that traps condensate
• Long runs through unheated attics, garages or crawlspaces

Signs the vent is blocked

Most people notice the heat first. Condensing appliances have a pressure switch that senses whether the vent is clear, and when it is not, the appliance locks out, often showing an error code or a flashing light on the control board. The house cools, the furnace tries to restart and fails, and the cycle repeats.

Outside, you may see the pipes buried in snow, a cap of ice over the exhaust outlet, or no steam plume when the appliance should be running. Gurgling sounds from the pipe or water dripping inside near the appliance can point to condensate trapped in the vent.

Write down any error code before resetting; it tells the heating contractor whether the switch saw a blocked vent, a blocked drain or something else, which saves time on the service call.

• Pipes buried in snow: clear them gently by hand or with a soft brush
• A ring of ice at the exhaust outlet: let it thaw or call for service, and do not chip at it
• Gurgling or water near the furnace: condensate is trapped in the pipe; call for service to check the slope and drain
• Repeated lockouts in cold snaps: the vent routing or termination needs to be corrected

What to do right now

Turn the thermostat down or off while you work. Go outside and clear snow from around both pipes by hand or with a soft brush, leaving open space around and below the terminations. Do not use a shovel blade against the pipes; cold plastic is brittle and cracks easily. Do not chip at ice on the pipe, pour boiling water on it or use any flame or torch.

Once the openings are clear, reset the furnace or boiler as its manual describes, or switch the power off and on at the service switch. If it does not restart, if ice is inside the pipe, or if lockouts keep happening, call your heating contractor.

Keep a soft brush by the door during snow season for this job.

The carbon monoxide question

Because a blocked vent normally triggers the pressure switch, the usual result is no heat rather than exhaust in the house. But safety switches can fail, pipes can crack, and a partly blocked exhaust can still allow some spillage. CO alarms near the furnace and by the bedrooms still matter. Should one go off, or should anyone feel ill, leave the house, and once outdoors call 911 or your utility.

A cracked pipe or a joint pulled apart by ice is a reason to keep the appliance off until it is repaired.

Condensate drains matter too

Condensing appliances produce a steady stream of water, which drains through a trap and a small tube or pump to a floor drain or outside. If that drain line runs through a cold space or exits outdoors, it can freeze too, and a frozen condensate line backs water up into the appliance and trips a different safety switch. The symptom looks the same: no heat and a lockout.

Check where the condensate line ends. If it discharges outside or runs along a cold wall, a heating contractor can reroute it, add heat tape where the instructions allow, or change the discharge point so it stays clear in a freeze.

Keeping it from happening again

Recurring freezing or burial is a sign the vent layout needs attention. Depending on what the appliance instructions allow, the fix might be raising the terminations above the expected snow line, moving them to a less exposed wall, adjusting the spacing between intake and exhaust, correcting the pipe's slope so condensate drains back to the appliance, supporting sagging sections, or insulating runs through unheated spaces. Some installations change to a listed concentric vent kit or a roof termination.

These changes belong to a licensed heating contractor working from the manufacturer's venting instructions, and permits may apply locally. After each storm, a quick look at the pipes and a path cleared around them is good routine.

Where a chimney still matters

When a house switches to a condensing furnace or boiler, the old masonry chimney often stops serving the heating system, but it may still serve a water heater, which can then be left alone in an oversized flue. Some installations run the plastic vent pipes up the old chimney as a chase, where the maker allows it. Either way, the chimney still needs a cap, a sound crown and a yearly look, and any appliance still using it needs a flue sized for it.