Renovation & Design

ASK THE INSPECTOR: Furnace new, but outside-air intake useful

Question: I have a question about cold-air intakes on gas furnaces. I recently had my old 80 per cent efficient gas furnace replaced with a high-efficiency unit that obviously has its own cold-air intake and exhaust pipes. Do I still require my old six-inch outside air intake on the return duct that was used for the old furnace? To me, I shouldn't, because of the new setup on the 95 per cent furnace. As I see it, this is now just an entry for cold air. Could you advise? Thank you.

G. Taylor

Answer: Fresh-air intakes for basement and natural gas heating appliances can be a confusing subject, partially because the requirements are constantly changing with upgrades to heating equipment and our homes. Many homeowners are unsure of the true function of these systems and often block or close these ducts, to the detriment of the indoor air quality in the home. Having a fresh-air intake duct in your basement is rarely a bad thing, but often it can cause a cool draft or frost issues that need to be dealt with.

There are two main functions for most fresh-air intakes in homes. The first is to bring in sufficient combustion air for natural gas-burning appliances such as furnaces and water heaters. This is a more critical item if the units are enclosed in a small area with limited air available. Often, these appliances can be cut off from sufficient combustion air by the walls and doors in finished basements. These items and insulation around the furnace and water heater can severely restrict the amount of air available. If this is the situation with your old furnace, bringing in a source of exterior combustion air was necessary for proper and safe operation of the furnace.

A modern combustion-air intake will normally be installed with pre-insulated flexible plastic ducting and run horizontally under the main floor joists to the area near the heating appliances. Once it is near the desired termination location, the duct is dropped down until it nearly touches the floor, then returns up to eye level. This configuration will create a loop at the bottom that acts as a "trap" to prevent excessive cold air from pouring into the home when not needed. Once the burners of the heating appliances come on and a draft is started up the vent, cold fresh air is automatically drawn in through the duct due to the reduction in air pressure in that area. This fresh air is then available to provide adequate supply for the heating devices. Starving these units of air can cause incomplete combustion, backdrafting or other issues that may cause safety issues in the home.

Unfortunately, the fresh-air intake you have described is the second type and does not provide direct combustion air to the furnace room, as it is directly connected to the return-air plenum for the furnace. This fresh air will enter the air circulation system of the heating system, which is separate from the combustion chamber on your furnace. Fortunately, you are correct that your new high-efficiency furnace should have an integral combustion-air intake duct that supplies air directly to the combustion chamber. Regardless, you should not be blocking or disconnecting this old duct just because you have upgraded your furnace. In fact, you should be doing the exact opposite.

Bringing in fresh air for the entire house through the return-air ducting on the furnace may help prevent a common problem when people upgrade their old heating appliances to high-efficiency units. Higher indoor relative humidity and window condensation often increase after upgrades to furnaces and installation of new windows. I get numerous calls and emails every year with this complaint. Tightening up the building envelope by replacing old, leaky windows and closing an old chimney flue after installation of a direct-vent furnace will trap warm air that used to leak out these two areas. This warm air can hold a significant amount of moisture, which will now remain in the home. Bringing in cold, dry, fresh air from outside is one way to prevent this issue from occurring. For that reason, leaving your old intake ducting in place is a very good idea.

You may find some modifications to the old duct are necessary to prevent a problem. Because of the changes in your home and the more powerful fan on your furnace, there is a chance that the fresh-air intake ducting may become frosted up, especially if it is poorly insulated. If that occurs, insulating the duct farther away from the entry through the foundation may solve this problem. Otherwise, replacement of the old metal ducting with pre-insulated flexible plastic ducting should also work. The flexible, corrugated ducting will also reduce the intake volume of air somewhat, which may satisfy your concern about a high volume of air intake reducing the efficiency of the overall heating system.

While not absolutely necessary, a properly insulated fresh-air intake duct attached to the return-air ducting for the heating system is a good idea to maintain healthy indoor air quality in your home. If you find too much cold air coming in or the return-air ducting frosting up, installation of a mechanical damper on the intake duct connected to the furnace fan control will help limit the air intake until it is needed.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and 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


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