Bath Fans: The One Piece of Equipment You Really Want to Suck
By Paul Eldrenkamp
Your bath fan sucks, but probably not as much as you think. That's what we've been finding, anyway, as we measure the performance of bath fans in the houses we're working on.
In a lot of old houses, bath fans are pretty important because they provide the main method of mechanical ventilation. Old houses often have a lot of "non-mechanical" ventilation - i.e., holes in the house through which air can leak in or out. The problem with relying on holes in your house for your fresh air is that you're never sure if the holes are working the way you need them to - how much fresh air are they letting in? Is it enough? How much indoor air are they letting out? Is it too much?
Theoretically, anyway, you know how much stale air a bath fan is exhausting and, by extension, how much fresh air it's bringing in. For the most part, a house stays at equilibrium with the outdoors - if a fan exhausts one cubic foot of air to the outdoors, one cubic foot of air will come in from the outdoors to make up the difference.
It's not unusual to install a fan rated at 80 cubic feet a minute (cfm) for an average bathroom, or maybe one rated at 120 cubic feet a minute if it's a larger bathroom. "Cubic feet a minute" refers to how long it takes the fan to exhaust a set volume of air. A bathroom that measures 5' wide by 8' long and whose ceiling is 8' high contains 320 cubic feet of air. On paper, a bath fan rated at 80 cfm would be able to completely change the air in that bathroom in 4 minutes. That seems more than adequate for moisture and odor control. And a bath fan only needs to be able to exhaust 50 cfm to meet code and fill its important role in helping you maintain good indoor air quality.
The problem is that none of the bath fans we test turn out to move the rated amount of air. It's not unusual for us to find that an 80 cfm-rated fan is moving only about 40 cfm of air - half the rated capacity and less than code. There can be all sorts of variables at play: the duct that connects the fan to the outdoors is too long, or has too many elbows, or has birds nesting in it, and so on.
If you're finding that your bath fan just doesn't seem to do its job, drop us a line and we can come by and test it for you. This is the one piece of equipment in your house that you want to really suck.