Basement heat requires proper air-duct system

Hey there, time traveller!
This article was published 05/09/2004 (7797 days ago), so information in it may no longer be current.

QUESTION — I have recently installed a new mid-efficiency furnace in my 1,000-square-foot bungalow. The ductwork has a couple of sliding vents to allow some of the heat into the basement. The ductwork runs along the main beam and ceiling of the basement. Otherwise, there is only one electric baseboard, in a small bedroom, for heat in the basement.

It has been suggested to me by a co-worker that I might open up a hole in the return-air duct just before the furnace filter. The theory being that the furnace would naturally draw in the coolest air from the basement floor and would naturally force warmer air down from upstairs. The same theory holds that this opening would draw cooler air from the basement during the summer and reduce the air-conditioning load. I would appreciate your comments on this proposal.

As an aside, in the above scenario, would this possibly encourage the movement of radon gas throughout my home?

Thank you for your very informative articles. I seldom miss reading them.

–Bob Haegeman, e-mail

ANSWER — Forced-air heating systems in homes are very complex due to the many factors affecting the air movement within the home. As our homes become more energy-efficient and tighter in relation to air leakage, the situation becomes even more difficult to properly assess. I will attempt to answer your question in a very general manner, but a complete answer would be possible only by evaluation and calculations by a licensed heating contractor.

Most homes built in the last 50 years have fairly simple ductwork installed, with one large duct leaving the furnace for warm air distribution and a second duct entering the fan compartment for return air. Smaller ducts are attached to the main warm-air ducting, which terminates in registers in the floor or walls in the various rooms in the home. Somewhat larger registers are installed near the base of the walls or in the floor in the same rooms for return air. Often the open cavities between the wall studs and floor joists are used as makeshift ducts for the return air. These ducts all attach to the main return-air ducting in the basement, which leads to the blower/fan compartment in the furnace. The heated air is circulated through the home by room-temperature air entering the furnace from the return-air ducts, which is blown past the heat exchanger in the furnace. This newly heated air is blown through the warm-air ducting until it reaches the desired registers in the various rooms in the home. This heated air will naturally rise near the ceilings in the individual rooms, and the cooler air will fall and be collected by the return-air registers and ducts.

For this system to work as designed, we should have roughly the same amount of cooled air entering the return-air ducting as is blown through the warm-air ducts. This rarely occurs, as many other factors such as leaky doors and windows, dryers, chimneys, fresh-air intakes and exhaust fans let air in and out of our homes, but this equality is assumed when the heating system is initially installed. Cutting extra holes in the main ducting in your home may dramatically change the initial design of the system and reduce proper airflow to various rooms in your home. This is one of the reasons for frequent homeowner complaints of cold rooms in various areas while others are too hot.

The first problem in your home is the improper registers installed on your main warm-air ducting in the basement. This air in the warm-air ducting is under pressure. In simple terms, this means that it is being pushed through the ducting by the furnace blower fan. Anything under pressure will naturally go to an area of lower pressure, or the larger spaces in the various rooms in your home. This is supposed to occur through the registers in the various rooms throughout the home, but now will include the entire basement area. The problem with this scenario is that the pressure inside the main ducting will now be significantly reduced due to the large openings in the basement. This will likely result in poor airflow to the other rooms in the home, especially the ones furthest from the furnace. You may get very poor heating in some rooms, while the basement air near the main ducting may be unusually warm.

Cutting extra holes in the return-air ducting may also affect proper airflow, but I don’t know if it will affect radon movement throughout your home.

To properly heat the basement with the forced air from the furnace, proper ducting should be installed, similar to that leading to the main floor, with registers and dampers to regulate airflow to individual rooms. Proper return-air ducting should also be installed in all the basement rooms, which should substantially improve the heat distribution. The “vents” in the main ducts that you describe should be permanently closed or removed and sheeted over to prevent loss of air pressure to the main-floor registers. Contacting a licensed heating contractor for modifications and installation of proper ducts and registers is a must. Attempting modifications yourself will only lead to poorer heat distribution and higher heating bills.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and vice-president of the Canadian Association of Home & Property Inspectors (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P.O. Box 69021, #110-2025 Corydon Ave., Winnipeg, Man. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com