In wall bath fan
CarsonZone5B
| Posted in Mechanicals on
Hi gba,
I’m building a timberframe with no ceiling space for mechanicals. I was thinking a through the wall bath fan like the whisperwall would be perfect for this application. A local builder here told me they weren’t allowed and instead used a fan routed through an interior wall citing condensation issues. Is there any validity to this? I could see water condensing on the damper, but wouldn’t the same thing happen with the damper for the one going through the interior wall?
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You want to use a Panasonic FV-0810VSS1 WhisperValue DC. I ran into the same issue in my Timber Frame house. I installed one in each bathroom in the Exterior wall. I also bought the 4" to 3" adapter, a 3" 90 and a vent cover with flapper for the exterior of the house from Supplyhouse.com. The FC-0810 has the continuous run also that is adjustable. I framed 24" OC and had just enough room between studs to fit the assembly.
Zdesign, do you also have any recommendations on erv setups? I'm debating soffits vs through the wall lunos/twinfresh.
The fan above is definitely a good one, there is also a similar Broan one but it is not as quiet.
I've also put fans into interior portion walls. With the step down to 3" duct, you can then do a straight run out through the exterior wall. With the fan on the interior wall, you don't have to make such a big hole in an outside wall, easier to seal up.
thanks for the great advise! So the only issue is the size of the hole in the exterior? I guess if I can mount this right by the exterior wall it should be just as easy to install. The spec sheet says it needs a 4" duct, would a 3" reduce cfm? Is this something easy to DIY? How do I get the duct run through the vertical stud connecting the interior wall to the exterior wall? I cannot run it up, it has to be right out the wall.
I can't find the equivalent length of a 3" wall cap, but a short length of 3" duct won't do much to reduce the air flow. These units have very beefy blowers, if you look at the curve, the flow vs pressure is vertical line, so even with a short length of restrictive ducting you'll get whatever CFM you set the unit to. For getting the oval outlet down to 3", best is order the official panasonic adaptor FV-VS43R but you can squish a standard reducer in a pinch.
The get through the vertical stud, you need a good hole saw and patience. If you have a lot of patience, you can try it with a budget hole saw.
Go with 3 1/8 or 3 1/4 hole saw. With a timber building, getting though the outside wall will probably take longer than the stud though. Provided your partition wall is not load bearing, if the unit is not in the last stud bay, you can notch the studs before it to run the duct.
My concern was cutting a 3+ inch hole in a 3.5” stud. So these should be 6” stud walls just for this?
Carson,
You might also consider a bulkhead at the exterior wall, which could also include some pot-lights. Design it so that the proportions are such that it is an architectural feature, not so it looks like it's there to cover up a duct.
Thats why the through the wall unit sounded so nice and much easier to install. Surely it’s easier to just cut the 6” hole? Or is the issue really with heat loss from the larger hole?
No problem to cut the hole especially for the stud against the outside wall.
For the other studs that need to be notched, keep to 2 3/4 deapth, the duct will squish enough to fit, reinforce with a mending plate after the duct is in.
Thanks Akos, I do have one wall that isn’t loadbearing and this sounds like a good plan. It isn’t near the shower, but I’m doubtful how important that really is.
On a project I designed recently I didn't want a wall-mounted fan--they are a hole in the wall that affects comfort and heat loss. Instead I spec'd vertical ductwork on an interior wall (with a stack head and grill near the ceiling) leading to a remote inline fan below the floor, exhausting through the rim joist. It's extra effort and expense, but I think it's worth it.
Michael,
Can you please explain why the ducted run leads to less heat loss than going right through the wall? Is it because the duct runs down and hot air rises? Unfortunately I don’t think this is an option for me due to snow.
Carson, yes, it's just a theory but I believe that when the fan is not running that the duct will provide some buffering from heat loss, due to the stack effect. In comparison, when it's not running, a wall fan is a dampered duct the thickness of the wall, and dampers are leaky, so the wall around the fan will be cold in winter, potentially resulting in condensation.
To be clear, the main reason we went with this arrangement was due to site conditions--although the bathroom has an exterior wall, there is no place on the wall to place a fan, and there is also a "landlocked" powder room that needed to be vented, with no option of going through the exterior wall. I avoid unnecessary holes in roofs and the roof over these bathrooms would definitely leak if a duct terminated there (low slope, north facing, snow country). If the house was on a slab, the next best option would have been to build a waterproof 6' snorkel on the roof. Fortunately we could use the crawlspace.
that seems to make intuitive sense. So the advise I keep seeing when googling to have an upwards bend to the duct would actually make the heat loss worse because of the stack effect? I think going straight out is more or less my only option, I might be able to bend the duct down a little bit though if it would help.
If you're going straight through the wall, bending the duct slightly won't help with heat loss. But it will allow internal condensation to drain outside, which is good.
sounds good. This is surprisingly complicated to get right, as many of these building issues have turned out to be.
update: for those with the same question I have decided to DIY install a panasonic intellisense 100 CFM ERV instead. Since installing the bath fans through the interior walls was already going to be more difficult than through the wall, I decided I might as well just put that effort into building soffits to run the ERV lines to exhaust the bathrooms. The material cost of the ERV is more, but only by a few hundred.