Running exhaust duct through unconditioned attic
I’m currently air-sealing the ceiling of the house I purchased a few months ago. Among the issues I’ve encountered are two exhaust fans whose (flexible) ducts terminate in the attic. One is the kitchen exhaust fan and one is the bathroom fan.
I’d like to replace the ducts with rigid galvanized duct, insulate the duct, and run them through a gable wall. The nearest gable wall for each of these fans is about 25′ away. Given this, I have a couple of questions:
1. Should I consider installing 6″ duct instead of 4″ due to the relatively long length of duct I have to install? Each duct needs to make 180 degrees of turns total. The price difference isn’t much. I hate to admit it, but my only objection to 6″ is that I feel the size of the wall cap is rather unsightly…
2. Speaking of wall caps, could anyone let me know what the best practice is for detailing the wall/siding penetration? I have vinyl siding that I’m not planning on replacing anytime soon. Remove a couple courses of siding and install a wood or PVC trim block, then flash accordingly? Or is there a nice manufactured solution using vinyl trim elements?
Thanks!
Ben
Columbus, OH
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Replies
Ben,
My vote is to upsize the duct to 6 inches. You won't regret it.
Here is a link to a catalog showing some of the available vinyl siding accessories that you are looking for:
http://bluelinxco.com/Portals/0/docs/LiteratureLibrary/Siding%20and%20Trim/GP%20Vinyl%20Siding/Richwood%20Mounting%20Blocks%20and%20Vents%20Brochure.pdf
Even insulated you're likely to end up with condensation, even ice in a 25' duct in an unconditioned attic during cold snaps, made worse if you go with 6" duct. Even though the impedance of the smaller duct results in lower cfm, it'll run at a higher velocity, and you'll have only 2/3 the condensing surface area and 2/3 the thermal mass to bring up to temp when it's running.
No way to side-vent these near the fans, rather than an up & over long-haul duct? The shorter the duct and fewer turns== higher cfm, it's more effective, and you would have no condensation issues to be concerned about, especially if routed mostly inside of conditioned space.
Martin #1 - Thanks for the vote and the link. I'll look over those products.
Dana #2 - Your concern about velocity crossed my mind, but I wasn't sure how valid it was. I will, of course, run as much of the duct downhill as I can, toward the exterior.
There is no way to side-vent closer to the fans. The house is a simple rectangular ranch with wide eaves, so if I terminate on the closest wall I'd be dumping a fair share of the exhaust right under the soffit vents (not to mention right next to my front door, in the case of the bath fan...). I could run a shorter length up through the roof, of course, but I think the benefits of sidewall termination outweight the cons of duct length.
Might be the best or worst idea ever, but I wonder if maybe a combination of 4" and 6" might be an option? Here is my reasoning: 6" allows for more CFM, but 4" allows higher velocity. As long as I run my duct downhill and support it well I feel that some amount of condensation and icing won't be a major concern. However, take a look at the simplified picture of what my duct run will look like.
If condensation occurs in this "drip danger zone" then I might have dripping through the fan back into the bathroom/kitchen. I'm thinking that I could run a 4" duct through this vertical run and than transition to 6" for the horizontal run. I would still get some of the airflow benefits from a long section of 6" as well as velocity benefits in the "drip danger zone".
Thoughts?