How much fresh air does your home need?

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This article was published 29/04/2017 (3053 days ago), so information in it may no longer be current.

Question: My concern is with the fresh air intake for my furnace. I was told that I needed a 12-centimetre duct, which they ran through the foundation at the front of the house. This is in addition to the 10-cm duct that the previous furnace had running into the cold air plenum. There is an electric dryer with the standard dryer vent in a nearby room. I have a fireplace in the basement, but I keep the damper and a valve on the fresh air intake closed using an RV trailer waste gate. Isn’t this too much fresh air? I’ve never had a whole house leakage test. What do you suggest?

— Phil Geisel

 

Answer: How much fresh air is the right amount for our homes, for proper ventilation and combustion air for our natural gas heating appliances? There may be some debate on this issue, but your home and its contents should dictate the proper amount required.

Homes in our area vary considerably in air tightness, depending on age and upgrades. The more airtight the home is, the more fresh air may be required to be brought in through dedicated ducts and systems. Also, the type of heating system and other oxygen-burning devices in the home will dictate how this issue is approached.

If you have an older home, with older windows and doors that are not well sealed, that may be enough to provide sufficient fresh air infiltration to replace that lost through the dryer, exhaust fans, furnace vents, chimneys and other areas.

If your home is less than 20 years old — or has had extensive upgrades to windows, attic and basement insulation and heating systems — more fresh air may be required to prevent high relative humidity and more serious health issues.

I will assume from your question that you have a home with a mid-efficient or older furnace. That style of furnace will still require significant combustion air from the area around the unit for safe operation. If the furnace is in a small room, with a well-sealed window, or no window and well-insulated and sealed foundation, combustion air may not be sufficient. If there is a standard gas-fired water heater sharing the space, that further adds to the demand for fresh air. In that situation, bringing in additional air through a duct from outside the basement makes good sense.

If your home has a high-efficiency furnace with a sealed combustion chamber and an electric water heater, there may not be the same need for a dedicated air duct in that area. In those style of homes, there is usually no chimney, or the older chimney flue is sealed.

Newer high-efficiency furnaces typically have a dedicated fresh-air duct supplying exterior combustion air directly to the burner compartment, so no additional combustion air is necessary. It may still make sense to bring in outside air if there is a naturally aspirated gas water heater, or if the home is very airtight.

The requirement of an additional air duct is likely for combustion air, primarily. While it will add some fresh air to the inside environment as well, the requirement is to ensure there is enough air for the appliances to operate in a safe and efficient manner. This is different from the current air duct that runs directly into the return air plenum for the furnace. That air will circulate through the furnace, but will not enter the burner compartment before it is pushed through the heating ducts by the furnace blower. That intake is primarily installed to ensure that there is sufficient fresh air entering the home to compensate for the stale, damp air exhausted through exhaust fans, chimneys and your dryer. That type of duct is a great idea, even if a newer high-efficiency furnace is installed. It also will become more important as energy and air sealing upgrades are done to the home.

Another important thing to remember is, even though you have an additional duct bringing in outside air, it does not mean that your home will become more inefficient. A fundamental principal to know is that air will always flow from an area of high pressure to that of lower pressure. The amount of air entering through the new duct will depend on the air pressure differential between the inside and outside environments.

In simpler terms, air should only be drawn in through the ducts in significant volumes when the air pressure drops inside the home. That normally occurs when appliances or exhaust systems are operating, sucking air from the interior and blowing it outside. That will drop the indoor pressure, allowing replacement fresh air to flow through the ducts and into the basement of your home.

Installing a loop near the bottom of the insulated intake duct should also create a small “trap,” preventing excessive air infiltration. The rationale is that cold air coming into the duct in the heating season will stay inside the trap, until there is sufficient pressure drop to draw it upwards. Since warm air rises and the exterior air entering the duct may be significantly colder that inside the basement, it will remain in the duct longer if it is not warmed up by entering directly into the home. Hopefully, all of these points combined should reassure you that you are not flooding too much cold air into your home by adding a new fresh-air intake duct in your furnace room.

Installing a new, properly designed and insulated fresh-air intake duct in the area near your furnace should only ensure that you have enough air for safe and proper operation of your heating system. Determining whether it is necessary will depend on several factors, which should have been taken into consideration by the individuals recommending this upgrade.

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

trainedeye@iname.com