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Old 03-22-2015, 05:52 PM   #1
ATX
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Increasing 3" bathroom vent to 4" in stone wall


Help. Moved into a house 6 months ago and the fan in the master bath is woefully inadequate for the amount of steam and heat generated by by better half's hair dryer. The existing fan uses 3" duct and I think I could generate more airflow with a fart. All the newer fans I see have 4" duct which is not that big of a deal except that it exits through the wall which is stone. It looks like they cut a hole in the stone, put the 3" flexible pipe in and then put cement in around the edge of the duct to seal it (with a vent cover of course). So can anyone tell me how to increase the size of the vent hole in the stone wall from 3" to 4"? Or should I just hire a plumber to come and do this? I am comfortable with replacing the fan and the electrical but not sure how to go about enlarging this hole.

Thanks for any advice.
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Old 03-22-2015, 06:16 PM   #2
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Quote:
Originally Posted by ATX View Post
Help. Moved into a house 6 months ago and the fan in the master bath is woefully inadequate for the amount of steam and heat generated by by better half's hair dryer. The existing fan uses 3" duct and I think I could generate more airflow with a fart. All the newer fans I see have 4" duct which is not that big of a deal except that it exits through the wall which is stone. It looks like they cut a hole in the stone, put the 3" flexible pipe in and then put cement in around the edge of the duct to seal it (with a vent cover of course). So can anyone tell me how to increase the size of the vent hole in the stone wall from 3" to 4"? Or should I just hire a plumber to come and do this? I am comfortable with replacing the fan and the electrical but not sure how to go about enlarging this hole.

Thanks for any advice.
First thing, is the pipe longer than recommended or have more 90 ells than the manufacturer recommends?

I would only do a pipe size increase as a last resort and that would be after being certain the 3" is completely free of any unwanted debris, the vent flapper ( if applicable ) is capable of opening fully and the present fan is operating as it should by not being inhibited by an accumulation of moisture laden dust, lint etc. I may even go as far as installing a new fan before changing to 4".

I'll not comment on the stone, mason question but leave that to those experts.
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Old 03-22-2015, 07:02 PM   #3
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I can see both ends of the 3" vent and it appears clean to me. The fan appears to be operation normally and the vent flappers are working. It is almost a straight shot from the fan to the hole in the wall so no big bends. I think the fan is simply the cheapest thing available to the builder when the house was built and it might be ok for the little room with the toilet but for a decent size bathroom it simply cannot move enough air. If you go to HD the only fans available that still use 3" ducts are the ones that cost $9.99 which is what's in there now.
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Old 03-22-2015, 07:15 PM   #4
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Have you compared the cfm rating of the 9.99 fans with those such as Nutone and Broan? And one other thought, the fan needs admittance air to at least equal its discharge capability. A tight door isn't necessarily a good thing in this instance.
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Old 03-22-2015, 07:57 PM   #5
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Good point Senior, crack a window if available and see if performance improves. If your house is tight along with a closed bathroom door, your fan may simply be starved for air. Any other duct work in the bathroom?
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Old 04-26-2015, 08:14 AM   #6
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Couple of things:

1. I would not beat a 1" larger hole in my wall just to increase air flow.

2. I would, however, consider buying a new fan that is much larger than I need, so that when I duct it down to 3 inches, it's still at least 100% larger than recommended.

3. In the last couple of years, I have done just that...

Note: Both of the fans that I have purchased came with adapters to downsize the output duct from 4" to 3".

4. Why buy larger than you need. First off, most, if not all of the fans that you are looking at are rated using a 4 inch duct. Adapting to a three inch duct will reduce you units air flow by around 25 to 30 percent, if not a little more.

5. Here's the second reason that I would, and have, bought a larger than needed fan. If you use some of the internet flow calculators to choose and buy your fan...you'll probably be okay for a few years...or less...depending on how much hairspay and dust you have in your home. Read...hairspray is dust glue...and will stick to your fan blades and cover plate, causing any fuzz that passes by to adhere to it. Now, your fan efficiency is compromised and you'll notice that your fan is no longer able to keep up with the moisture from the shower, hair drying, etc.

6. I purchased 150 CFM fans for my modest 5X7' bathrooms. Necked down to three inches, they move between 90 and 110 CFM's. Internet recommendations using various calculators suggests that I needed 35 CFM.

7. Happy...you bet...moisture in my bathrooms, NO. Not any. Noise...minimal.

8. Tip: If you buy a fan that has a light in it, you'll want to buy one that has the light in the fan base / housing and not in the lens / cover plate. Why? Because, you'll want to clean that cover plate...it will collect dust and hairspray, and it will all stick to the danged thing. With my Panasonic, I can remove the cover plate and since the light assembly remains with the fan housing, wash it in the kitchen sink with no worry about getting water in the electrical socket for the light bulbs.

The one in my hallway bathroom is a Broan...and it has the light assembly in the cover plate...meaning that any time I clean it, I must be careful not to get any moisture in the CFL sockets. Read...pain in the butt.

And, if your wife is using hairspray, the dust on your cover plat will not simply vanish with a simple pass of a dust brush.

Both the Broan and Panasonic units work equally well, and have a similar sound level, but I wouldn't buy a similar Broan again...I would buy a Panasonic or similarly designed fan, if I need one in the future.

I had to order my Panasonic from Amazon.com, because the big box stores don't carry that brand.

And, I didn't discover the differences until I installed my second fan, the Broan.

Hope this helps.

BTW...having an extra light in the ceiling sure make a difference.

Last edited by Ed911; 04-26-2015 at 08:19 AM.
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Old 04-26-2015, 09:15 AM   #7
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Thanks to all of you for your suggestions. I ended up getting a Panasonic 110 CFM fan via Amazon with a 4" duct. I could have used a reducer and used the existing 3" duct, but in the end decided I would go to the trouble of enlarging the existing hole in the wall and use a 4" duct. I had a hammer drill already and used it to drill a ring of small holes around the existing hole and then used a chisel for stone and a small sledge to knock pieces out. It makes you feel like a real man to smash stone like that, although doing it while standing on a ladder is a bit scary. It took less time than I thought. After I got the new vent pipe in I sprayed expanding foam around the pipe inside the wall cavity and then used some easy mix cement patch to finish it off and seal the hole against the pipe on the outside. The fan itself was pretty easy to install - that was the easy part. It so quiet it's ridiculous - you are so used to hearing loud bathroom fans your whole life that you can't believe the thing is really working but is. When you feel how much air is coming out the vent its impressive.
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