Ducting Systems and Air Ventilation - Bathroom fan: 4 inch fan/3 inch duct
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Hauger
09-18-09, 07:15 AM
Okay, here's the situation.
I'm replacing the fan in my main floor 1/2 bathroom. The existing unit (already removed) was a cheap, builder's grade fan, the motor sounded like there was a handfull of loose nickles in it any time it was turned on, and was just plain done.
The existing exhaust duct is flexible plastic, 3 inches. Almost all new fans have 4 inch exhaust. Now, the bathroom is almost dead center in the house, with no access to the duct from above. Since changing the duct to the preferable insulated 4 inch duct work would require a lot of ceiling damage and repair, it's not going to happen, especially since the bathroom is a half bath with no humidity problem (aka: no shower/bathtub) and the fan exists primarily for "smell removal".
Anyway, I was going to stuff up there a 4 inch exhaust fan with a reducer to the 3 inch pipe, secured with a lot of Tuck Tape. I know, I know, not ideal by far, it'll reduce the fan's efficiency and raise noise levels, but I'm okay with that (remember, smell control only), what I want to know is by how much this will effect the fan. Will it go from being semi-quiet (3 sones) to jet airplane take off noise, and will it kill the efficiency to the point that it'd be better not to bother, or is the increase in noise and reduction in airflow minor for a smell-control-only fan?
I'm replacing the fan in my main floor 1/2 bathroom. The existing unit (already removed) was a cheap, builder's grade fan, the motor sounded like there was a handfull of loose nickles in it any time it was turned on, and was just plain done.
The existing exhaust duct is flexible plastic, 3 inches. Almost all new fans have 4 inch exhaust. Now, the bathroom is almost dead center in the house, with no access to the duct from above. Since changing the duct to the preferable insulated 4 inch duct work would require a lot of ceiling damage and repair, it's not going to happen, especially since the bathroom is a half bath with no humidity problem (aka: no shower/bathtub) and the fan exists primarily for "smell removal".
Anyway, I was going to stuff up there a 4 inch exhaust fan with a reducer to the 3 inch pipe, secured with a lot of Tuck Tape. I know, I know, not ideal by far, it'll reduce the fan's efficiency and raise noise levels, but I'm okay with that (remember, smell control only), what I want to know is by how much this will effect the fan. Will it go from being semi-quiet (3 sones) to jet airplane take off noise, and will it kill the efficiency to the point that it'd be better not to bother, or is the increase in noise and reduction in airflow minor for a smell-control-only fan?
goldstar
09-18-09, 08:45 AM
The one thing you did not mention is the CFM's. If your old fan was a contractor special, it probably was rated in the 50 - 70 CFM range. Your new fan is probably in the 90 - 110 CFM area.
Necking it down from 4" to 3" may cause a little reduction in efficiency, but should not be bad enough to overheat the fan motor or increase the sone level.
Necking it down from 4" to 3" may cause a little reduction in efficiency, but should not be bad enough to overheat the fan motor or increase the sone level.
Hauger
09-18-09, 08:59 AM
Thanks for the reply. I just now finished stuffing it up into the ceiling (but not to the point that I couldn't easily take it back out), and was just getting ready to turn on the power. Your probably right about the old fan being around 50 - 70 CFM, the bathroom is quite small (8H x 4L x 5W). The replacement fan is 100 CFM.
Anyway, I'm about to flip on the power and see what happens.
Anyway, I'm about to flip on the power and see what happens.
Hauger
09-18-09, 09:34 AM
Success. Sure, it's not librarian-approved quiet, but it's much quieter than the old internal-combustion V6 powered 50 CFM with loose valves fan that it replaces, and it has enough suction to happily hold up pieces of toilet paper, so I have to imagine it's moving air down the 3 inch exhaust.
What I'm not looking forward to doing is changing all the exhaust vents to 4 inches when I do the two upstairs bathrooms that have actual humidity to move. Can't see that being fun.
What I'm not looking forward to doing is changing all the exhaust vents to 4 inches when I do the two upstairs bathrooms that have actual humidity to move. Can't see that being fun.
gunrunnerjohn
10-07-09, 02:17 PM
I just did that job in my master bath, it wasn't that bad. The builder had just put a couple of feet of plastic duct on it and exhausted it into the attic! Since I replaced the whole unit and moved it too boot, I put in a proper vent duct. It all depends on the access above it, I have a stand-up attic, so it's not that difficult to work there.