Why Snow Blocking Furnace and Water Heater Vents Is Dangerous
By Mark Robinson, Content Editor
A heavy snowfall can block exterior furnace and water heater vents, especially in cold climates like Toronto and the GTA. These vents are required for many high-efficiency, sealed combustion systems that bring in intake air and safely release exhaust gases.
When a furnace exhaust vent is blocked by snow, the system may shut off in cold weather as a safety mechanism or, in some cases, may fail to vent exhaust gases safely. This is why checking vents after heavy snow matters.
Not every home is affected in the same way. Many older or standard-efficiency natural gas furnaces vent through a chimney or metal flue rather than side-wall PVC pipes. These systems typically do not have low exterior vents that can be buried by snow, which is why some homeowners will not see any outdoor vents to check.
Below is an explanation of what these vents are, why snowdrifts can obstruct side-wall venting, and what usually happens when they get blocked.
What are “external vents” and why do they matter
Many modern, high-efficiency furnaces and water heaters use side-wall venting rather than a traditional chimney. You will often see PVC intake and exhaust pipes (commonly white PVC pipes outside the house) terminating near an exterior wall, sometimes close to ground level.
These pipes serve two different purposes:
Intake air: brings combustion air into a sealed combustion system.
Exhaust: carries combustion byproducts out of the home.
Because these systems are sealed, they rely on clear airflow. If snowdrifts block the intake or exhaust, the appliance can stop operating or operate improperly. In the worst case, a blocked exhaust can increase carbon monoxide risk if exhaust gases cannot vent safely.
External vents that commonly get blocked due to snow
After heavy snowfall, wind and drifting can pile snow against walls and around vent terminations. Here are the vents to check:
Furnace intake and exhaust vent pipes
Often two white PVC intake and exhaust pipes located near the ground on an exterior wall. If the furnace exhaust vent is blocked by snow, the furnace may shut off as a safety precaution. If the intake or exhaust is obstructed, airflow can be restricted and the system may not vent exhaust gases safely.
Water heater vents
Homes with gas water heaters vent exhaust gases outdoors, which is why snow around those vents can be a problem. Homes with electric-only water heaters typically do not have exterior exhaust vents for the water heater itself.
Dryer vents
A dryer vent blocked by snow can reduce exhaust flow. When the dryer exhaust is obstructed, lint and heat can build up inside the vent.
Here's what usually happens when the vents are blocked
The furnace shuts off in cold weather and may show an error code, or the home stops heating.
A water heater stops producing hot water.
A dryer runs longer than usual or seems to struggle.
High-efficiency systems often have safety shutoff mechanisms that stop the appliance if airflow is restricted. That shutdown is a sign the system is protecting itself and the home.
What most people do
If they are already outside, they take a quick look at their vents and check whether snow has drifted over them.
If the vents are clearly visible and unobstructed, there's nothing else they need to do.
If vents are buried by snow, they clear snow away from the vent openings so air can flow normally.
If you notice a neighbour’s vents are buried, especially if they’re away, clearing them or letting them know can help prevent issues.
A note on safety
Modern furnaces and water heaters are designed so that exhaust gases are not noticeable inside the home. Carbon monoxide is colourless and odourless, which is why carbon monoxide detectors are important, especially during the winter months.
If a carbon monoxide detector sounds, take it seriously. Move everyone outside to fresh air and contact emergency services or your local gas utility for guidance before re-entering the home.
FAQs
What are “external vents”?
External vents are openings in exterior walls or near the foundation that allow air to move in and out of the building. They play a quiet but important role in how a home manages airflow and moisture, helping systems work as intended and preventing problems that can build up over time.
Where are these vents usually found?
External vents are most often located on exterior walls, sometimes close to ground level. Common examples include white PVC pipes used by high-efficiency furnaces, side-wall vents for gas water heaters, and dryer vents near the foundation. Some homes may also have small vent openings near the gas meter or other low-profile exhaust points. Not every home has all of these, and locations vary by house.
Do electric water heaters have exterior vents?
Electric-only water heaters typically do not vent exhaust gases outdoors, because they do not burn fuel. Homes with gas water heaters, however, do vent exhaust outside, which is why snow around those vents can matter in winter. This difference often explains why some households are affected while others are not. Does this apply to all furnaces?
No. This issue mainly applies to high-efficiency furnaces and water heaters that vent through side walls using PVC intake and exhaust pipes. Many older or standard-efficiency natural gas furnaces vent through a chimney or metal flue that exits through the roof. These systems usually do not have low, side-wall PVC vents, and their exhaust is typically warm enough that falling snow does not accumulate in the same way.