View Full Version : Bathroom Fans: Best way to install ducting?

09-21-2010, 08:55 PM
Hi everyone,
I am remodeling my home in the California Bay Area (mild climate) and must install exhaust fans in all bathrooms, despite the fact that there are windows in each bathroom. I have a call into the local building inspector and I'm waiting to hear back about requirements, but have not had a return call. Maybe someone here can help me figure this out.

1. What kind of ducting is best? I've seen rigid galvanized, rigid aluminum and flex aluminum. I think the rigid is best because it is smooth and there will be better air movement than the flex, which has ridges. At the big box store, galvanized comes in 5' sections and aluminum comes in 2' or 3' sections. My laundry room had a 3" uninsulated aluminum flex for it's exhaust fan. I'll be replacing that too.

2. What is code as far as where to terminate the wall cap from a door or window? I've seen 3' on other forums. The concern is exhausting the bad air outside only to have it return through an open window or door. Is the 3' from the actual opening (vent portion) of the window? Or from any part of the window? My concern is that the 1st floor bathroom is quite small and I may not be able to stay 3' away from the window unless I use a few elbows. Also, the exhaust fan would be 2' - 3' from the exterior wall, where a window happens to be. So, the duct would have to 90 in a few places to avoid being too close the window.

3. The two upstairs bathrooms are back to back and I would like to have just one wall cap rather than two. This would mean running two separate ducts to the gable wall in the attic and then into a wye that would connect to a wall cap. Maybe the wye and wall cap would be larger than the 4" ducts to accommodate the two fans in the event they are running at the same time. Is this a bad idea?

Hope someone can help.

12-14-2010, 11:11 AM
You need to have a separate wall cap for each fan. Otherwise the air from one bathroom will just get pushed into the other bathroom (it's less resistance than pushing the cap open).

12-14-2010, 01:15 PM
1. What kind of ducting is best? I've seen rigid galvanized, rigid aluminum and flex aluminum.
Duct resistance varies with the material and the length. If your final fan/duct setup gives you greater than 8 air changes per hour in each bathroom while all the fans are running you must be doing something right.
Measuring this rate of air removal may prove challenging but some fan manufacturers provide you with rules of thumb for duct lengths, number of elbows used, etc.

12-14-2010, 06:26 PM
Just curious.....where is the area you refer to as the "california bay area"?? There are "bay area" communities in San Diego, Long Beach, LA, San Fran, not to mention other coastal areas!

12-15-2010, 08:54 AM
Thanks for all the replies. I actually installed two fans so far and will be doing the final two in a few months. I used galvanized rigid for the laundry and flex aluminum for the lower bath. The flex was easier to install due to the short distance and two bends.

I'm in the San Francisco Bay Area.