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"Upgrade" bathroom exhaust ducting from 3" to 4"?

2770 Views 9 Replies 3 Participants Last post by  NewHomeDIYGuy
The projects seem to never end.. :laughing:

Basically long story short, I replaced a couple bathroom fans on my top floor bathrooms, and they vent to the attic soffits. The fans I changed were the cheap/noisy ones and replaced them with 80cfm quiet fans w/ 4" outlets. However, the original fans only had 3" ducting, that each run approx. 12-15' to the soffits. When I originally installed these fans I just used 4"->3" reducers, but now I'm contemplating whether it's worth just buying new 4" ducting and running new 4" duct for each fan and install new soffit vents. Any thoughts?

Reason why I'm finally thinking about running the new ducting is I'm looking to install soffit insulation baffles and add about 10" of insulation to make things a little more comfy in the house. Now would definitely be the time to replace the ducting if I'm going to do it..

So is it worth upgrading the ducting to 4" or am I just making more work for myself? Thanks!
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I would do the 4"....especially if your going to use solid tube vs the flexible stuff.

Putting a 3" on a fan made for 4" is going to reduce the won't hurt it...but you won't get the CFM you expect. One of the ways you get the CFM along with the low noise is to use larger pipe.

BTW...I have 4".

Aren't those new quiet fans nice? Mine are of them has a built in night light....I use an LED bulb in it....1w I think.

Ya, I'd definitely use the solid tube.. Guess I have the answer, just need to suck it up and get it done. I just hate digging around in the fiberglass insulation and getting to the soffits w/ the pitched roof is a.. :censored: I guess I'll have to finally make myself a small platform that I can lay across the attic joists to make it easier. Any idea/is there a formula how much a 3" pipe actually reduces airflow compared to a 4"? The noise is so minimal I can definitely live w/ the increased noise, but more airflow definitely can't hurt. Thanks!
Before you go tearing anything out, you need to figure out how much air has to be removed from your bathrooms. I have a small bathroom where I just installed a ceiling exhaust fan...4" exhaust tube. The fan that I'm using is 110 CFM...the bathroom requirement is 35 CFM, using calculators that are readily available on the internet.

Necking the exhaust tube down from 4" to 3" only reduces the flow to 90 CFM, according to the manufacture. It's still way more than the suggested minimum. You may have a lot more exhaust capacity than you need...and therefore, a 3 inch tube might be just fine.

Have you calculated your exhaust needs.

Why did I use such a big a fan...because I have family members that can and do steam up the bathrooms...everything is wet, so bigger is better in my book.

Thanks for the helpful insight. When I initially purchased and installed the fans, I looked at these calculators, but figured sizing a fan a little bigger couldn't hurt. The bathrooms are small at roughly 5' x 8' each, so according to the calculator I really only need ~50cfm, but my wife and myself take hot showers, and don't have a problem steaming up the room/getting condensation on the walls. :) I guess the question is whether 4" would necessarily help, maybe not..? One thing I haven't done which I might need to do is clean the grate for the fan and make sure the soffit vents are venting properly. I did notice when I originally installed them that there was a layer of lint/dust lining the inside of the 3" pipe, which probably restricts flow slightly, so either I might want to clean it out, or more realistically just replace it..?
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NewHome....our existing bathroom is almost exactly the same size....5' from the wall to the edge of the tub....then then width of the tub.

Our fan is 100cfm.....and to be looses ground on those hot my the 4" the time you go through the bends and transition out the soffett.....I suspect you won't be getting 100cfm....
NewHome....our existing bathroom is almost exactly the same size....5' from the wall to the edge of the tub....then then width of the tub.

Our fan is 100cfm.....and to be looses ground on those hot my the 4" the time you go through the bends and transition out the soffett.....I suspect you won't be getting 100cfm....
Be sure that you are allowing an adequate supply of fresh air to enter the bathroom while you are showering. If you have closed the bathroom door, and it's a tight fit, you are going to greatly reduce the exhaust fans ability to remove the moist air from the bathroom. If you want your exhaust fan remove 100 CFM, then you have to vent your bathroom to 100 CFM of fresh air. At the very minimum, input has to equal output.

There are a couple of things that you can do to help this out. One, if you have an HVAC vent in the bathroom, is to set the system fan to on, ensuring a fresh air supply for the bathroom fan. Obviously, this will not work if you don't have a forced air system.

Leave the door cracked. Or, undercut the bathroom door about a half inch.

As an example:

In our house, all of our bedroom doors are undercut by at least 3/4 of an inch so that all of the conditioned air forced into the bedrooms, when the doors are close, has an exit back to the HVAC return. When standing in front of a closed bedroom door when the AC is running, it's easy to tell why it was done that way...the air escaping the room under the door feels like a spring morning's cool breeze. Without that undercut, the air becomes trapped in the room...

Just some food for thought.
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Yes, there is plenty of ventilation to the room fan, because the doors have gaps between the finished floor. I finally went up into the attic and am starting to think the best option now might actually be a roof vent. I don't like the idea of cutting a hole in my roof, but getting to the soffits on the ends of the house are a SUPER pita. Also, I was told that roof vents are very efficient.

Also, the current fans don't vent directly out the soffits, but right next to the soffits just past the cardboard baffles.. :( So, that's something that has to be addressed either way, as I don't want the roof sheathing to deteriorate/rot due to the moisture on the underside.. If it's not one thing it's another..

Another issue but related, I'm not sure how I can replace the baffles for the insulation. I'm not sure if the pitch of the roof is very shallow or what, but reaching the edges is a huge pita. The "joists" in the attic are just 2x4's. Laying on the joists I cant even reach the soffits.. Is there some magic to reaching the soffits or do I just need to hire a midget to do the work.. :laughing:
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Yep, I know where you're coming from. The pitch on my roof is low and it's really hard to get to the edges. I'm renovating one of my bathrooms and used a roof vent that I got from Lowe's.

As far as getting ot the soffits, I don't know of any magic fixes. So, Hire a small I've actually considered this for another of my problems...rewiring my house from two wire to three wire. The last installer put a huge air handler right in the middle of my attic...and it blocks access down the center of the you have to crawl around it, between the supports. In order to easily get in and out of those tight spaces, I think the guy needs to be about a hundred

Now back to the roof vent. I had a guy from the construction company, that was working on my next door neighbor's reno and build-out, install it for me. So far so good...we've had some pretty big rain storms and no leaking. He charged me $20. I did cut the hole in the roof from the inside using a 5" hole saw. And, those are expensive, over $60 at the local big box store. I wouldn't worry about cutting a hole in your roof, as long as the vent is installed properly.

Anyways, good luck.
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Thanks Ed.

Ya, at this point I might just wait till spring to do the bathroom exhaust vents and attic insulation as it's getting cold here and I don't feel like climbing on the roof in this weather. I was watching some youtube videos on how you install a roof vent and it *seems* easy enough, just need to make sure the shingles overlap the flanges properly it seems. It'll be much easier than trying to install soffit vents which are hellacious to get to. At this point I guess this project will be postponed till spring, and I'll get back to work on the basement bathroom. :p Thanks.

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