Bathroom fan without damper leaves owner cold

Hey there, time traveller!
This article was published 30/12/2001 (8844 days ago), so information in it may no longer be current.

QUESTION — We recently bought a 27-year-old small bungalow that presents me with a problem that I am unable to explain. The bathroom fan has no damper. I think someone must have taken it out. This in itself is no problem; I’ll just install a new fan. The pipe through the attic is insulated, but not well enough. This also can be helped. The roof outlet also has no damper, but I think I’ll leave it like this because the prevailing winter winds hit it straight on and pile snow all around it and this would render the damper inoperable or ineffective. As it is right now, I would be happy to have just one functioning damper on this darned thing!

What really bothers me is that I can’t figure out the following: With the furnace blower either on or off, with no kitchen range hood or clothes dryer, and regardless of wind direction, there is, at times, cold air coming out of the bathroom fan. Sometimes it really gushes out and sometimes it is less, but I think there is always some air coming into the house.

I cannot detect any real pattern; it seems to happen at any time. Had I a pattern, I possibly could “attack” this darn problem. There is only one thing that I was able to determine — it happens mainly late or very late at night and regardless of the above-mentioned conditions. I am not really satisfied with just installing a new fan without knowing the cause(s) of the air coming into the house. What is happening, why and how? Your explanation and advice will be greatly appreciated!

Thank you for your consideration and I hope you don’t get many long letters like this one.

— Peter E. via e-mail

Answer — The first thing Peter E. must realize about cold air is that it will normally fall as opposed to warm air, which rises. The cold air he feels coming through the fan and duct is directly related to the location of the vent hood for the fan on the roof of his home. The cold air outside will enter the duct and fan if there is higher air pressure outside the house than inside, or air currents that draw or force the cold air down. This is likely worse at night because the outside air temperatures drop dramatically at night. The insulation on the duct will prevent the warm bathroom air exhausted out from condensing on the duct, but will not prevent cold air from coming down the pipe.

The simple solution to the problem is to install a new vent hood with a proper damper and find a location on the roof that will prevent snow from covering the hood. If the fan is used regularly, the warm air exhausting from the house will often melt a small layer of snow covering the vent hood. A second alternative is to relocate the vent hood to a gable end or exterior wall of the house. This will not be subject to closing due to snow cover, but may be more sensitive to damper movement due to strong winds. The damper in the new fan to be installed will prevent some air movement, but the outside vent hood must have a damper to minimize the outside air entering the duct, at the source.

There are more choices than ever in roof-mounted and wall-mounted vent hoods for various types of exhaust fans, but these should not be confused with ones for fresh-air intakes. The fan hoods all have a damper, of some sort, and the intake hoods may have screens or grates over the openings but don’t have dampers. These are designed for appliances and ducts that draw in fresh air from the outside and always remain open to the exterior.

The above letter received from Peter E. was considerably shortened, by myself, due to the limited nature of the space available for this column. I am never disappointed in receiving long letters from dedicated readers and enjoy reading all the e-mail and mail sent in. As 2002 approaches, I would like to wish everyone a happy and healthy New Year and encourage all readers to submit any questions to the e-mail or regular address below. I strongly believe that there is no such thing as a “dumb question” from a homeowner wanting to expand their knowledge about their house. Thanks for your feedback and loyal following of Ask the Inspector, and keep the questions coming!

Ari Marantz is owner/inspector of Trained Eye Home Inspection and is the PR. rep. for the Canadian Association of Home Inspectors — Manitoba (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com