# Need advice sizing bathroom fan



## joecaption (Nov 30, 2011)

If you use a pipe this there is no need for any elbows.
http://www.homedepot.com/p/Deflect-o-8-ft-Semi-Rigid-Duct-A038-16-A/100199300

Just cut it to the length you need.


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## trevarthan (Feb 3, 2014)

joecaption said:


> If you use a pipe this there is no need for any elbows.
> http://www.homedepot.com/p/Deflect-o-8-ft-Semi-Rigid-Duct-A038-16-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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## beenthere (Oct 11, 2008)

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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## trevarthan (Feb 3, 2014)

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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## beenthere (Oct 11, 2008)

Cool, keep us updated.


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## TotalHomeworx (Jan 18, 2014)

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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## adgjqetuo (Oct 21, 2011)

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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## TotalHomeworx (Jan 18, 2014)

adgjqetuo said:


> 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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## joecaption (Nov 30, 2011)

I'm sorry but that's one messed up install.


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## adgjqetuo (Oct 21, 2011)

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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## joecaption (Nov 30, 2011)

Just looks really funky.
Could have all been done in one piece.
http://www.homedepot.com/p/Master-F...le-Duct-R6-Silver-Jacket-F6IFD4X300/100396934


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## beenthere (Oct 11, 2008)

joecaption said:


> Just looks really funky.
> Could have all been done in one piece.
> http://www.homedepot.com/p/Master-F...le-Duct-R6-Silver-Jacket-F6IFD4X300/100396934


The 4" flex is not great for air flow. Hard pipe is much better. Just needs to be wrapped.


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## adgjqetuo (Oct 21, 2011)

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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## adgjqetuo (Oct 21, 2011)

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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## trevarthan (Feb 3, 2014)

Condensation isn't a leak, so sealing won't help. It's the same as when you place a glass of ice water outside on a warm day. The water in the air phase changes due to the temperature and becomes a liquid. Insulation keeps this from happening.


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## chronojosh (Feb 11, 2011)

You can buy the insulated flexible ducting thst joecaption linked to and remove the insulation from it. Then take the insulation and slip it over the rigid ducting.This is a bit more expensive an option due to the wasted materials, but this is what I have done with my rigid exhaust ducting in my attic.


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## trevarthan (Feb 3, 2014)

I've been curious about this for a while... is duct insulation available at the big box stores, or does one have to special order it, or scavenge as the previous post mentions?


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## adgjqetuo (Oct 21, 2011)

I found this but it looks like 3" is the max they sell - I didn't see 4".


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## trevarthan (Feb 3, 2014)

I installed the panasonic whispergreen 130 cfm fan this weekend using two 90 degree 6" elbows, a 6" to 4" reducer, and a short run of vinyl 4" duct in a corkscrew. Yeah, I know, it's a terrible configuration.

Surprisingly, this performs a little better than my previous fan install. But just barely. At the roof, with the fan dialed all the way up to 110 CFM, it opens the vent flap about an inch. The old fan wasn't even able to open the vent flap, so that's good.

However, I've decided it would be a waste to continue running this expensive fan at severely reduced efficiency, so I'll be installing a new 6" roof vent further up the roof. This will allow me to use just one 90 degree elbow and 6" pipe the whole length. Should solve the problem permanently.


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## beenthere (Oct 11, 2008)

Yeah, you'll get much better results that way.


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