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Old 02-03-2014, 12:10 AM   #1
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Need advice sizing bathroom fan


I have a small 360 cu ft upstairs bathroom with a tub/shower and toilet. The fan has never worked well. I recently had a new roof installed and it got worse. Rather than ask the roofers to fix the problem, I decided to investigate myself. I found they had reinstalled the old roof vent, but broke the vinyl vent pipe and just left the bathroom fan vented to the attic. I knew this was a bad idea, so I went to lowes and bought a new aluminum roof vent with a weighted flap and installed it myself.

My current setup looks like this:


My old Miami-Carey FB-5079 70 CFM 6" fan vents vertically into the attic. It then hits a 6" to 4" reducer (I originally thought this was part of the fan - now I think someone goofed a long time ago), then a length of 4" flex tubing and finally the roof vent.

Unfortunately, there is very little room between the reducer and the roof vent. In addition, the guy who set this up originally offset the vent from the fan by an inch or two. All of this has the unfortunate effect of reducing the 4" flex tube to about an inch, which severely hinders air flow.


Now I know why the fan never worked very well.

I can just barely see a pin hole of light from the bathroom when I turn the fan on. The flap on the roof doesn't lift and there is very little air flow.

A few weeks passed, and the fan has started making death noises and my bathroom is super soggy after a shower, so I figure I need to do something about this soon. First, Miami-Carey was apparently bought out by Nutone and Nutone doesn't sell replacement parts for the MC line. I guess I'm buying something new. I did some research and it appears most new fans are the side venting variety.

I've been reading about static pressure and air flow and such. So far, I've come up with two possible solutions:

1.) Go all in. Buy a pack of spare shingles, wait for warm dry weather, mark a day or two off of my weekend calendar, climb up on the roof and relocated the vent to a more appropriate location for a side venting fan, patching and re-shingling as I go. I really don't want to do this. Cutting holes in roofing is one thing. Patching them is something else. Sounds like a pain. And then I still have to spend more time ripping up the bathroom to install the fan. Kind of a big deal... And ... I was just up there... Sigh...

Or

2.) Just install it this way and hope for the best:


I know having three 90 degree elbows in a system is a huge no-no, but I'm wondering if the whisper green panasonic fans can handle it? Maybe if I use 6" elbows and only reduce to 4" at the vent?

What do you think I should do?

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Old 02-03-2014, 12:44 AM   #2
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Need advice sizing bathroom fan


If you use a pipe this there is no need for any elbows.
http://www.homedepot.com/p/Deflect-o...16-A/100199300

Just cut it to the length you need.

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Old 02-03-2014, 07:24 AM   #3
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If you use a pipe this there is no need for any elbows.
http://www.homedepot.com/p/Deflect-o...16-A/100199300

Just cut it to the length you need.
It will still have to be bent into the same shape with the same number of bends. That will increase static pressure and reduce the effectiveness of the fan. Rigid pipe is actually more efficient, though less convenient.
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Old 02-03-2014, 07:35 AM   #4
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Need advice sizing bathroom fan


If you have room to run a straight pipe for about 12" off the fan before the first ell. It will help a little with air flow. As it allows the air to stabilize before trying make a a turn.

Spinning the fan 180 degrees might allow for it. If not. The hard pipe will still be your best bet for air flow.
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Old 02-03-2014, 02:39 PM   #5
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Ok. Here's the plan. I've ordered one of the 130 cfm whispergreen fans with the adjustable cfm control. I plan to build the duct as described in the second diagram with 6" rigid pipe. I'll then increase/decrease the cfm control on the fan until I'm getting the volume of flow I want. If it is maxed out at 130 cfm, I'll try different lengths of pipe before the bends and see if that makes a difference. If I still can't get the flow I want with 130 cfm on such a small bathroom, I'll replace the roof vent too, perhaps with a 6" model, if they make those, and probably relocate it in the process.

I'll report back with what I find. Should be pretty interesting.
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Old 02-03-2014, 06:53 PM   #6
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Cool, keep us updated.
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Old 02-03-2014, 08:54 PM   #7
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Rigid pip and insulate it to reduce condensation. There are 4" and 6" models in fans with various CFM. Usually the fan size has to do more with the size of the room then the length of the exhaust pipe. Reducing a 6" to 4" will make it noisier and burn out the motor faster. Avoid a 90 right at the fan but it's hard to do. Sometimes fans do not work properly because you are not getting enough new air into the room. The door gap in the bathroom should be around an inch high.
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Old 02-07-2014, 02:51 PM   #8
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When I replaced mine I had to run all new pipe all the way to the roof vent - I originally tried reducing it to use the original piping but it made the fan incredibly noisy from the flow restriction.

I foil taped all the joints (after this picture was taken) to reduce any leakage of air.

I also used adjustable hard elbows to reduce any kinks in the flex line.
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Old 02-07-2014, 03:16 PM   #9
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Quote:
Originally Posted by adgjqetuo View Post
When I replaced mine I had to run all new pipe all the way to the roof vent - I originally tried reducing it to use the original piping but it made the fan incredibly noisy from the flow restriction.

I foil taped all the joints (after this picture was taken) to reduce any leakage of air.

I also used adjustable hard elbows to reduce any kinks in the flex line.
You better go back up there and insulate it otherwise you will get condensation in the pipe. It will drip out of the fan or leak into the attic. There is a lot of pipe insulation available to do that.
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Old 02-07-2014, 04:04 PM   #10
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I'm sorry but that's one messed up install.
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Old 02-07-2014, 04:08 PM   #11
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Need advice sizing bathroom fan


That's exactly how the builder had the original piping minus the hard elbow pieces (the flex line was duct taped onto the ends) and I didn't have any issues the past 10 years - why would I now?

I insulated the elbows which are the point I would think it would condensate - but again that's more then was originally there and I never had issues before.

I don't mean to hijack, but how is this a bad install?
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Old 02-07-2014, 04:48 PM   #12
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Just looks really funky.
Could have all been done in one piece.
http://www.homedepot.com/p/Master-Fl...X300/100396934
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Old 02-07-2014, 04:53 PM   #13
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Quote:
Originally Posted by joecaption View Post
Just looks really funky.
Could have all been done in one piece.
http://www.homedepot.com/p/Master-Fl...X300/100396934
The 4" flex is not great for air flow. Hard pipe is much better. Just needs to be wrapped.
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Old 02-07-2014, 06:53 PM   #14
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Sorry - I thought I was good and now you have me concerned.

I wrapped the elbow corners in duct tape and then again with foil tape - is that not enough? What about the existing flex pipe - I thought that would be ok since it's plastic inside. (1st pic)

Should I buy some type of wrap for the hard pipe and leave the rest alone should I redo the entire run with the flexible insulation?

I looked up tonight and there are no signs of moisture - but the run is right over top of the fan so I want to be safe.

If you thought this one was messy - I assume the other one I did you will think is worse (2nd & 3rd pic).

I apologize OP for taking over - wasn't my intended purpose

I ran the shower on hot tonight for a half hour with the fan going and didn't see any condensation - don't know if that's a true test or not.
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Old 02-07-2014, 07:08 PM   #15
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Need advice sizing bathroom fan


I did a more detailed investigation and I did find some moisture.

Keep in mind the bathroom was streamed to the max - more then usual.

The two places I found moisture were at the flat plate that goes outside (1st pic) and the 2nd place was in the insulation itself that was rubbing against the housing where the air comes out (2nd pic) where the "hole" is - this just felt moist though - didn't find any actual water.

The hard pipe, corners and flex hose was dry was can be.

Can I seal the first pic with duct tape and foil tape like the corners? Not sure what to do with the second pic - I taped the plastic port they gave me but it felt like the housing itself was letting some air out.
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