Renovation & Design


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Geoff Kirbyson:


Renovation & Design

Negative basement pressure can be combatted with duct

Question: I had a new gas water heater installed by a professional. I think I have negative pressure being created because of my high-efficiency furnace and exhaust. I also installed a six-inch insulated duct, dedicated to the water heater, so fresh air is coming inside my basement. It has eliminated any negative pressure or back draft now, but it is cold. Would this combustion air/fresh air create too much moisture inside my basement and possible damage? How will I prevent this? I can’t convert my gas water tank since it’s brand new and I need a new sub panel just for that.

My second question is, is it possible to heat and cool my attached garage by creating a vent from my furnace?

Thanks in advance.

— Donn Agapito

Answer: Installing a fresh-air intake in a basement plagued by negative air pressure or other issues is a common approach that can be quite effective. Reducing the cold air coming in may be possible, but may negate the benefits of doing it in the first place. Heating your garage with your furnace is not an option, due to inherent safety issues with that type of installation.

Drilling a hole near the top of the foundation, or through the wall just above the foundation, may allow a rather simple installation of a duct to bring additional fresh combustion air into a basement. This duct should be insulated, to prevent condensation and frost build-up during cold weather. It should also be installed with a loop near the bottom, followed by a length of vertical duct terminating near the appliances using combustion air from the basement area for operation. This loop may act as a partial "trap" to prevent excessive air intrusion until the pressure in the basement drops due to appliance operation. While this trap may not solve your cold-temperature problem, it may help prevent excessive cold air escaping the duct when not needed.

The reason for bringing a source of fresh air directly into the area where the gas-fired appliances are operating is to replace air drawn into the appliances and flues when they are operating. If there is no source to replenish this combustion air, the appliances can cause the air pressure in the surrounding area to drop significantly when in use. This can create a condition in which the air pressure is much higher outside the basement.

Since air will naturally go from an area of high pressure to one of low pressure, this can draw air in through unwanted openings. These openings may be the flues for the appliances causing the issue in the first place, causing "backventing" into the lower-pressure basement. This can be a major safety hazard, as dangerous products of combustion may re-enter the home, rather than going harmlessly up the chimney. It can also cause moisture issues, as the exhaust often contains significant amounts of water vapour.

To regulate the amount of cold, fresh air entering your basement on a typical winter day, a damper could be installed in the intake duct. This could be a simple disc affair that could be manually adjusted, or a more complex electronic unit. The electronic unit could be connected to the furnace and only open the damper when the thermostat called for heat.

The downside of that set-up is that the air may also be required when the water heater or dryer operate, both of which can create a negative pressure issue on their own. Either way, this damper must be installed and monitored to ensure sufficient fresh air is allowed to enter your basement when needed.

Whether you do anything about excessive cold air from your new duct or not, that concern should not lead to moisture issues, and may have the opposite effect. The air outside your home in the winter months is much drier than the inside air. That is because the colder the air, the less the amount of dissolved water it can hold. When you bring the outside air inside your basement and warm it up, the Relative Humidity (RH) of that air will drop drastically, helping replace damp exhaust air going up the chimney. This will subsequently lower the overall RH in the house air, preventing moisture issues due to condensation. The slight drop in basement temperature from the fresh air intrusion should only be temporary, and may only cause condensation if the duct is improperly insulated near the beginning, where it enters the home. So, there is nothing you will need to do to prevent moisture issues, except ensuring the duct is properly insulated.

While it may seem prudent to use your gas-fired furnace to heat your attached garage, it can be a major life safety hazard. This is because of the possibility of vehicle exhaust or other noxious gasses entering your living space through the ducts that would be needed to heat the garage. These ducts would be directly connected to the furnace, and all the other ducts that circulate heated air through the house. That is the same reason why any openings in the common wall between the garage and the home must be sealed. For that reason, if you want to heat your garage you must have a dedicated heating source in the garage itself.

Bringing in additional air from outside the home to combat negative basement pressure caused by gas-fired appliances should be well worth the slight drop in temperature associated with it. Adding a damper may help, but any thoughts of using the furnace to heat your attached garage can be abandoned, because of the obvious safety hazard involved.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at

Ari Marantz
March 25

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