aircooledJenkins t1_j9r9ai9 wrote on February 24, 2023 at 12:06 AM

HVAC engineer here.

We use 4" duct to move up to 35 cfm or so. If your fan is strong (most restroom fans are not) it could do more but will be loud.

My sizing criteria puts 3" duct at around 15 cfm for normal design conditions.

Your fan probably claims to be able to move something like 65-100 cfm. 4" duct works, but not well for long runs.

This doesn't help solve your problem, but it might illustrate why 3" isn't working.

rayw1983 OP t1_j9sd7zp wrote on February 24, 2023 at 5:18 AM

My fan does claim to move 50-100, it’s a Panasonic one with options for 50-80-100.

If a 4” duct is good for moving up to 35 then should I just set it to 50cfm? Why would it have the option to go all the way to 100? Marketing to make it sound good?

If someone wanted to upgrade the size of the duct in my situation with no attic access, is the only option to open up walls?

aircooledJenkins t1_j9sev7j wrote on February 24, 2023 at 5:35 AM

When I installed exhaust ducting in my parents attic I used 6" so the full 100 cfm wouldn't be restricted.

4" is good to use in some instances where firewalls are present. They don't need special protection because they're so small.

If you don't have an easy path to outside from that bathroom, you may need to open up a wall and investigate running an oval or rectangle duct in it to get full airflow.

Otherwise yeah, run the fan at 50 cfm so it's not trying too hard for what the ducting can allow. You could lower the lifespan of the motor, or you're just wasting energy.