ice build up

What to Do if Ice Builds Up Around Your High-Efficiency Furnace Intake and Exhaust Vent

Calgary homeowners know that the wintertime requires some diligence to avoid costly and inconvenient maintenance issues. If your furnace quits when it’s -25°C outside, it’s not only uncomfortable and inconvenient – it can pose a danger to residents and damage household infrastructure.

One of the most common causes of winter high-efficiency furnace shutdowns is the build-up of ice and snow in-and-around your furnace intake and exhaust vent terminations outside your home. Luckily, with a little know-how, you can handle this situation with ease.

What is furnace intake and exhaust venting?

All high-efficiency furnaces have an exhaust vent pipe that moves the exhaust gases generated by your furnace outside your home (learn exactly how your furnace works in our Furnace 101 blog). Also, some high-efficiency furnaces have a combustion air intake pipe that draws fresh air from outside directly into your furnaces for combustion. If your furnace has a combustion intake in addition to an exhaust vent, they will be located side by each on the exterior of your house.

In most cases, the furnace exhaust and intake terminate on the side of your home, usually on the same side of your home as your furnace room. In some cases, these intake and exhaust pipes vent up through an existing furnace chimney chase and can be seen on the roof.

The intake and or exhaust will be easy to spot on the side of your house. They are 2-3” diameter plastic pipes, in most cases white in colour, and will have a short section (the termination) exposed outside, to direct the furnace exhaust gas away from your home.

During cold weather, you will notice water vapor and exhaust gas coming from the exhaust vent pipe while the furnace is running and there will generally be a large piece of ice directly under the exhaust vent. The intake vent pipe (if your home has one) will generally not have any ice build-up directly under it but on cold and snowy days you may notice snow forming around the vent termination.

The dangers of exhaust and intake vent ice build-up

If the intake or exhaust vent is partially blocked with snow or ice, airflow will be restricted to/ from your furnace. Sensing reduced airflow your high-efficiency furnace has integrated safety devices, called Pressure Switches which will prevent the furnace from starting up (if you notice that your furnace is down and can read the furnace error code as ‘Pressure Switch’ related, the first thing to check is for a blockage in your furnaces intake or exhaust venting).

The impact to you is that your furnace will stop working wither periodically or entirely, depending on the degree of blockage.

The causes of ice build-up and how to prevent them

Condensation is the number one culprit for vent freezing. As warm air and steam exits from the exhaust port, a cone of ice can begin to form under it from the ground up. This is usually harmless and can be quickly fixed by knocking the ice down before it connects with the exhaust port.

When a furnace combustion intake vent is located immediately beside an exhaust vent, a portion of the moist exhaust gas can be drawn back into the furnace through this intake pipe. Over time, this moisture can condense and form faceted snow inside the intake vent pipe, eventually blocking it.

Heavy snowfall or blowing snow accumulations can also build up, and cover the exhaust and intake vent terminations.

In other cases, regular and more serious freezing can occur from improperly installed or compromised exhaust pipes (the pipes that carry the exhaust air from your furnace to your exhaust port). To prevent this, your furnace installer should ensure that intake and exhaust vent pipes are properly sloped with grade back toward the furnace, supported, and installed with as short a distance between your furnace and the intake/ exhaust termination as possible with the fewest number of elbows. The exterior vent placed must be placed at a minimum height of 18” above the ground – allowing adequate space in the event of heavy snowfall. All new furnaces must be installed and vented to meet current Canadian building code requirements, which require System 636 Vent Pipe. Early high-efficiency furnace installation may have older PVC vent pipe and not meet today’s code, and therefore may be more prone to cold-weather venting issues.

Another cause…uninvited guests

Another reason a vent may become blocked is due to animals seeking refuge from the cold. Occasionally birds or squirrels are attracted by the warmth. They’ll build nests or take shelter in your vent during the summer months when your furnace is not running. When the weather changes and your furnace fires up for the first time in the fall, the restricted airflow from animals and nests, which prevents the furnace from operating.

How to handle blocked intake and exhaust terminations

A furnace vent that has been blocked by snow or ice can usually be fixed by clearing the opening. Intake vent pipes are usually not fully frozen, rather just clogged with snow and can be cleared with hands or a flexible rod. Remember, it is unlikely that the blockage will extend further than the point at which the venting penetrates the side of your house.

If ice is frozen in the pipe, you can also try using a hairdryer to heat the intake or exhaust terminations to melt the blockage and clear the obstruction. The best advice is to call a furnace professional. This way you avoid the risk of accidentally damaging the exhaust.

If blockages and full pipe freezes happen regularly, it may be a sign that a vent exhaust was improperly installed or requires replacement. Contact a professional furnace installation company to help.

Make an inspection a regular part of your week

One of the easiest ways to prevent ice build-up and the difficulties that come with it is to make regular checks a part of your weekly winter routine. Put a reminder in your calendar to check your pipes every few days during winter months or make a mental note during cold snaps to check on your exterior furnace vent terminations.


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