Advice replacing 3" bathroom fan duct with 4" in a cold climate


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Old 01-10-14, 10:44 PM
J
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Advice replacing 3" bathroom fan duct with 4" in a cold climate

Hello,

I just bought a condo in a cold climate. I decided to splurge and replace the noisy old bathroom fan with a new ultra-quiet Broan XB50, which works with either a 6 or 4 inch duct. When I popped out the old fan, I discovered a few things.
  • Old duct is 3", which is not compatible with my new fan
  • The old duct had actually fallen out of an elbow further back in the ceiling because it was held together with duct tape (sigh), so the fan was just exhausting into airspace above the ceiling
  • No backdraft damper on the duct and fan
  • The 3" duct ends outside with an elbow that points straight down, into a ~5" diameter hole cut in the bottom of an overhang on the exterior of the house.
  • The exhaust hole was not sealed around the duct or insulated at all, so it was effectively a 5" hole leading from outside directly into the airspace above and around the bathroom (which explains why the bathroom was always 10-15 degrees colder than the rest of the house)

So! This is my first ducting project. Your advice is appreciated.

The duct run is only about 4 feet total, since the bathroom happens to be on an exterior wall. I plan to get a section of 4" semi-rigid flexible aluminum duct, and run it from the new fan to right above the exit 'hole'. I then want to attach my backdraft damper in a horizontal orientation, and finally attach an elbow that points straight down that is flush with the 5" hole to outside.

Then use an insulating foam to seal the gap between the elbow and the 5" hole, and put a wire grate to keep critters out.

- Is there any reason why I should go to the trouble of using rigid duct for any of this? The installation of the fan requires flexible ducting at the fan end.

- The fan is about 6" lower than the exhaust hole. Anything to keep in mind regarding keeping the duct as level as possible, at a slight incline or decline, etc?

- What should I use to connect to the elbow and fan piece? Mastic, 1/2" #8 self tapping metal screws, UL181 foil tape, all of the above ... ?

- Should I put insulating duct wrap on the small portion of duct downstream of the backdraft damper? Or on the whole run since it's short? I'm actually not really sure if you can even use duct wrap on semi-rigid.


Thanks for your help!
 
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Old 01-11-14, 04:29 AM
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You use foil tape on the joints.
No screws needed.
Should not be venting out the soffit. Needs to go out the roof or gable wall. Reason being the exhaust can be sucked back into the attic.
soffit venting damage - Bing Images
 
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Old 01-11-14, 10:19 AM
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Great, thanks for the info!
 
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Old 01-11-14, 11:05 AM
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No way would I run out the roof. I'd go with a soffit vent. Install it correctly and there will be no issues
 
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Old 01-11-14, 03:22 PM
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Ya I live in a condo community so it's a paperwork hassle to put holes in the roof. I was planning to keep it simple and just reuse the existing exhaust hole, which I am learning is basically a poor man's soffit vent. Except, seal it from the elements this time around, and put a backdraft flapper inline.
 
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Old 01-11-14, 03:27 PM
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The flapper should be built right into new fan.
Not sure where he's getting his info.
Want to see some more pictures of what happens to soffit mounted vents?
It's just not the right way to do it.
 
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Old 01-11-14, 03:52 PM
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There is a flapper built into the fan, but for the cost of the fan, I was disappointed that it's not really weatherproof. A draft can blow right in through some of the cutouts in the flapper mechanism, and the flapper doesn't seat flat against a seal.

So full disclosure here, I don't think it's actually a soffit vent -- that's part of the roof, right? It's really hard to describe without a picture - the 5" hole is cut in the location indicated by the green arrow in the attached picture, so it's only about 8' off the ground and nowhere near any attic vents.

Name:  3101_Lake_Forest_Rd___219__Tahoe_City__CA_96145_is_For_Sale_-_Zillow-4.jpg
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Hope the picture helps.
 
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Old 01-12-14, 11:23 AM
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FWIW, mine goes out the roof and uses alumninum semi-regid duct. I have been religiously checking during & after rain & snow for leaks. So far so good. With the snow from last week, minimal ice damming, but my attic has very little insulation (less than 6 inches of loose fill FG). I insulated the vertical portion of the duct with one of those duct insulation kits, which I think only yields an R6. For the horizontal run, I just threw some R30 batt over it & adjacent joists.

Prior to doing it, I was also concerned about roof vs soffit vs wall, but I ended up going through the roof, hiring someone who was roofer to do the roof venting.

Note: eventually I will get my whole attic up to at least R38.
 
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