Bathroom exhaust duct sizing

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Ted T

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Mililani, HI
Hi all, I think this is the correct forum for these questions - if not, let me know and I'll lift and shift.

Question #1: If most of a bathroom exhaust duct run is 3" diameter steel pipe, is there any benefit to having bigger duct (4"-6" round galvanized steel) at either end, and/or a bigger roof cap? I'm guessing that there is a benefit to the average values of the run as a whole, but what would be the point of diminishing returns?

Question #2: How does one calculate air volume, flow, etc., when a run consists of differing diameter duct (i.e. 2' of 4" duct to 18' of 3" duct to 5' of 4" duct? I'm guessing you calculate the values for each section, then average for total length. For EDL, figure for each section then total?

Question #3: Aside from increased noise, what are the major drawbacks to reducing a 4" fan exhaust to a 3" duct? Or a 6" fan exhaust to a 3" duct?

I'm installing a bathroom exhaust fan in my master bathroom and retrofitting two other fans. For the master and hallway bath, both on 2nd flr, I'm installing 6" galvanized round duct and roof jacks. No issues there.

The problem is the guest 3/4 bathroom on the ground floor. I haven't traced out the duct run yet, as access is very limited, but based on house dimensions, room locations and what I can see through the ceiling opening and in the attic, it looks like it has a short length (maybe 2') of 4" vinyl flex attached to 3" steel pipe (real pipe, not thin-walled duct). I estimate the pipe runs about 8' horizontal, then about 10' vertical into the attic, with at least one, probably two, 90 degree elbows, or one 90 and two 45s. In the attic, 4" vinyl flex is attached to the pipe end and runs vertical around 5' to a 4" roof jack.

My first choice would be to replace the current stuff with 4" galvanized duct directly to the nearest exterior wall (~14' straight run) and exhaust through a wall cap, but I think this may not be feasible without tearing out major stretches of walls and ceiling. If I'm stuck with the 3" pipe, I'm exploring what I can do to make the best of it.
The bathroom is small (47 sqft) with no exterior wall. The current fan (Utilitech model #7108-03-L) is 110 cfm and airflow is strong at the roof jack. House framing is steel, and I live in Hawaii so there are no insulation concerns. The fan I'm looking to install is a Panasonic WhisperGreen Select FV-05-11VKSL1, which will run constant at 30-50CFM and kick up to 110CFM with a wall switch. I would actually prefer the 150CFM model due to the length of duct run but I think that may not work well with 3" pipe. The shower in this bathroom doesn't get much use but my cat's litter box lives in there and the toilet gets used a lot, so fan is mostly for odor-control. The bathroom opens on to a juncture of guest room, dining room and kitchen, so bathroom smells are very unwelcome.

Besides answers to my questions at top of post, any ideas or recommendations would be very welcome. Thanks much!


In the trades
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Flex duct that isn't stretched tight adds a LOT of equivalent length. You're better off using hard-piped ells (the longer the radius, the better). If it's a short radius 3" plumbing ell, that's another huge adder. You're better off yarding it all out and replace it with smooth 4" hard piped duct with smooth ells of 5" or greater radius if you think you really need 150cfm.


10 Chemo sessions 12/06/2023 more to come.
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Orlando, Florida
All exhaust fan companies have a chart or equations to calculate the minimum exhaust fan sizing per room size (width x length x height) for cubic feet. Duct sizing is pretty much what the size of the fan exhaust fitting is. For a fan size I like the largest ones available, 150 CFM such as panasonic brand with squirrel cage fans. They are very quiet. You do not have to worry about having too large of a fan causing negative pressure in the home since your in HI where your windows are probably opened most of the time and you most likely do not have gas appliances. Just keep away from the $50 fart fans made by Nutone and others that are sold at the big box stores. They use fans with blades and are very noisy. I just ordered two of these for the two Panasonic exhaust fans installed in my bathroom with a complete remodel nearly completed.
Leviton LTB60-1LZ Decora 1800W Incandescent/20A Resistive-Inductive 1HP Preset 10-20-30-60 Minute Countdown Timer Switch,White/Ivory/Light Almond

From the Nutone web site.
To determine the correct size of fan needed in a bathroom, you would first take the square footage of the bathroom and multiply by 1.07. This will give you the absolute minimum CFM for a room that has an eight foot ceiling. For a ten foot ceiling, multiply the answer by 1.25, 12 foot ceiling, multiply by 1.5 and so on. The optimum CFM for the room would be 1.5 – 2 times your minimum CFM.
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