One fan, two vents
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One attic-mounted fan for two bathrooms
One in-line centrifugal fan can be mounted in the attic to exhaust the moisture from two bathrooms.
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Figure A: Two bathrooms, one roof vent
Both bathrooms are vented by a single in-line fan that has
one exhaust vent running through the roof.
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Figure B: Two bathrooms, two roof vents
Each bathroom has its own exhaust fan; each fan vents
separately out the roof.
If you have two bathrooms that are close together and one has an exhaust fan and the other doesn't, you might be wondering if you can tie a new exhaust duct into the existing one. Well, you can't! You'd often blow air from one bathroom into the other, and local building inspectors wouldn't approve it.
But while you can't have two fans with
one vent, you can make one fan and one
vent serve two bathrooms. This setup
requires an in-line centrifugal fan mounted
in the attic drawing air simultaneously
from both bathrooms (see photo). A
grille in each bathroom attaches to ducts,
which then fasten to a “Y” connector at
the fan. A single exhaust exits through the
roof (Figure A). You mount a switch in
each bathroom. This system is quiet, too.
Because the fan is in the attic, you'll hardly
hear it. Look for the special fans (starting
at $160) at heating-cooling equipment
dealers. We used a system by Continental
Fan Manufacturing (continentalfan.com)
Fantech makes a similar fan (fantech).
It's less expensive to simply add a fan in
the second bath and vent it separately
(Figure B), although that will entail cutting
another hole in the siding or roof. If
you go this route, consider a fan with a
built-in humidity sensor. It detects a rapid
increase in humidity, like during a shower,
and automatically turns on the fan. The
sensor switches off the fan when the
humidity drops. The fans with this
upgrade aren't cheap, but they eliminate the need for a
timer switch. Two companies that make
the fans are Broan (broan.com) and Panasonic