Replace bath exhaust fan details
20 year old builder poorly installed bath fan gave out so must be replaced. The fan is to the 2nd level master bath and the attic above is accessible unconditioned vented attic. I’ve read the GBA article on bath fans.//m.etiketa4.com/blogs/dept/musings/bathroom-exhaust-fans
I’ve sized the fan properly. 400 cf room + 12′ of duct with no elbows (I think; see below) + 30′ to account for wall outlet = 60 cfm fan, perhttps://www.efi.org/factoids/bathroom_sizing.html. So I bought an 80 cfm fan. (Existing appears to be 40 cfm).
Fan I bought has a humidity sensor and can operate manually on full speed mode. It is energy star rated, 0.8 sones, and has 3 year warranty. It has 11 watts input power and 0.1 static pressure, perhttp://www.hvi.org/proddirectory/CPD_Reports/section_1/index.cfm.
I would appreciate some guidance regarding the below:
1. Duct: The existing 3″ flex duct is in there like an octopus. I will replace it with rigid duct. Two questions
a. The GBA article and the instructions with the new fan indicate the duct is to be installed so it is sloped to the exterior. My project cannot include relocating or installing a new exterior vent, and the vent is approx 18″ higher than the fan outlet. I think the best thing to do is install new rigid duct with a slope in a straight line from the new fan outlet to the exterior outlet. Is this correct?
b. New fan has a 4″ outlet. Existing exterior wall cap outlet is 3″. At which point should I transition from 4″ to 3″? at the fan, halfway along the 12′ of new rigid duct, or at the exterior wall outlet?
2. Fan install. GBA article indicates I should install the new fan so it’s lower lip rests on the top (attic) side of the gyp (image 3 of 5). Instructions that come with the fan say the bottom lip should be even with the bottom edge of gyp. Does it matter in a retrofit install?
3. Sealing fan opening in gyp. GBA article indicates sealing between fan and gyp with caulk. Is it ok to seal between them with spray foam? Either way, what specific product is preferred? The yellow expanding foam or the white window/door foam, or other?
Thanks.
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Replies
CL,
Q. "The vent is approx 18 inches higher than the fan outlet. I think the best thing to do is install new rigid duct with a slope in a straight line from the new fan outlet to the exterior outlet. Is this correct?"
A. What is a "vent"? What is a "fan outlet"? Instead of "vent," do you mean the outside termination (the thing that sits on the exterior of your house)? Instead of "fan outlet," do you mean the duct stub on the fan housing?
If you are able to install new ductwork, you want to put an elbow near the fan, aiming up; go straight up until you gain some elevation; install a second elbow; and run a long, straight length of duct from the second elbow (the long duct run should be almost horizontal, but sloping slightly to the exterior) to the duct termination.
Q. "The new fan has a 4-inch outlet. Existing exterior wall cap outlet is 3 inches. At which point should I transition from 4 inch to 3 inch?"
A. At the last possible moment, near the termination.
Q. "GBA article indicates I should install the new fan so its lower lip rests on the top (attic) side of the gyp (image 3 of 5). Instructions that come with the fan say the bottom lip should be even with the bottom edge of gyp. Does it matter in a retrofit install?"
A. Follow the manufacturer's installation instructions.
Q. "Sealing fan opening in gyp. GBA article indicates sealing between fan and gyp with caulk. Is it OK to seal between them with spray foam?"
A. Yes.
Q. "Either way, what specific product is preferred? The yellow expanding foam or the white window/door foam, or other?"
A. Anything you want. Don't overthink it -- just seal the crack.
Appreciate the reply. Yes, fan outlet refers to the duct stub on the fan housing - the pamphlet that came with the fan calls it the fan outlet. By vent I mean the outside termination.
So I should come out of the fan housing duct stub with a 90 going vertical, then another 90 to almost horizontal just slightly above the level of the exterior termination? Or should it be something like two 45's? I thought the more degrees of turns on the duct run, the less effective it is at moving air?
The GBA article indicates I should come out of the fan housing with flex to dampen vibration. Should that just be horizontal out of the fan housing then the 90' to go vertical, or should I make the 90 out of flex, or other?
CL,
There are lots of ways to do it. Yes, flex duct will reduce vibration, but not everyone uses a length of flex duct. You can certainly use flex duct if you want. Two 45s for each 90-degree turn are fine -- and you're right, there is less resistance if you use two 45s.
Would like to reduce the noise of the new fan with the 18" or so section of flexible duct. Specifically, what type of flexible duct is recommended for this?
1. I assume the cheap, uninsulated, foil slinky stuff is not recommended.
2.a "Semi-rigid" flexible duct, all of which seems to be aluminum (and I think is for dryer vents?)
2b If semi rigid is the correct stuff, could I use a 4' section of the semi rigid to take care of the flex coming out of the fan, a 45 degree turn, a straight section that angles up, and another 45 turn (thereby saving on the fittings) then transition from the semi rigid to rigid aluminum duct for the straight run slightly slanted down to the exterior?
3a: Or do I need "traditional" flex with insulation
3b; If traditional flex with insulation is the right stuff, can I run the whole thing with this (since it is sold in 25' lengths):http://www.menards.com/main/heating-cooling/ductwork/flexible-duct/dundas-jafine-4-x-25-black-jacket-r6-flexible-insulated-duct/p-1444429339255-c-9503.htm?tid=-3615831908654072742
If 3b is incorrect, is there a version of it that is correct? ie maybe the straight runs with that and two 45 degree hard duct elbows?
If 3b is incorrect, am I supposed to insulate the bath vent duct? It is in an unconditoned attic, and will come out of the fan then go above the insulation to get is above the level of the exterior termination so it can slope down to that...
CL,
Insulated flex duct is the typical flex duct used for this purpose.
Assuming that you want to use a short section of flex duct to reduce sound transmission, that's the type of duct to use.
For the rest, I stand by my advice in the article you linked to (Bathroom Exhaust Fans). In that article, I advised, "Use galvanized duct or thin-wall PVC pipe. Joints in galvanized duct should be secured with sheet-metal screws and sealed with HVAC tape."
Concerning duct insulation: my article noted, "In a cold climate, attic ducts must be installed in a way that limits condensation. ... Wrapping the ducts with vinyl-faced fiberglass duct insulation reduces condensation."
这duct insulation is essential in Pennsylvania or Vermont -- but optional in Texas or Florida. Where do you live?
Year old post, but relevant to our own project, so I hope it is alright to ask my question here.
Martin, does attic duct insulation limit much condensation in climate zone 3B (Southern California), or does it not get cold enough here for it to be essential? I figure it may not hurt to have the duct insulated anyway, but want to know for either way. Also, I read your article on bathroom exhaust fans and need to make sure I've understood correctly. The installation of exhaust fans, as pictured, is recommended in all climate zones, not just the cold ones, correct? Only the insulation would be determined by climate, not the overall installation?//m.etiketa4.com/sites/default/files/Bathroom%20exhaust%20fan%20illustration%20-%20FHB.jpg
I would like to mention this illustration to the contractor bidding our job since they suggested we exhaust straight upwards through our roof, rather than the exterior gable walls. I suppose the compromise would be to install as depicted but instead terminate through the roof at the same sloped angle, instead of through the exterior gable wall, but I just do not see the benefit to terminating through the roof at all when the exterior gable wall seems to be a much shorter run and simpler way to get the job done?
Natasha,
Your instinct is correct: a gable wall termination is always preferable to a through-the-roof termination, especially if the gable wall is close. So there is no reason you can't direct your builder to install the exhaust duct so that it terminates at the gable wall.
I don't have enough experience building in your climate zone to know whether there are any concerns about condensation in an exhaust duct installed in an attic. It's certainly true that using insulated ductwork can't hurt.