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Discussion Starter · #1 ·
Hi All:

First time poster trying figure out what duct size I should use for an inline bathroom exhaust vent.

I did the math: I have a 512 cubic ft bathroom, actually two of them. and with the ACH of 8 I need a 68 CFM exhaust fan.

An Inline is what I want to use for the low sound.

I'm having difficulties trying to match the needed CFM's with the required duct diameter. The flex duct charts online show that a 68CFM duct should be well really 5.5 inches at SP of .05, since there aren't any ducts of that size the size jumps to 6 inches (75CFM).

I can't seem to find any inline exhaust fans with a rating near 68 CFM and a duct size of 6 inches.

Air King rates their units at a Static Pressure of .2. I think its because of the elbows and vertical lift to the roof exhaust vent. I'd will be venting under the eves with an eves vent.

Now, it just seems strange to me that a 190 CFM exhaust fan at .2 SP would require a 6.5 inch duct diameter but is fashioned with a 6 inch diameter opening.

In my configuration I don't think my SP will be .2 , more likely it will be lower as I will only have a couple of bends and 1 elbow pointing down towards my eves. Probably no longer then 12 ft of flex.

So I'm left confused, is 190 CFM at .2 SP the same as 100CFM at .06 SP?

What I am afraid of is oversizing the exhaust fan and drawing fumes from the waste pipe. That happened when an installer screwed up the return size on our AC system. That was bad, they placed an undersized return right next to the washing machine and it started pulling fumes right from the drain of the washing machine. -nightmare, do not wish to do that in as I want to place the grill directly over the shower.


Hope this is enough info for someone to help me out.

Thanks in advance
-pauli
 

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There is no static pressure chart that shows how much pressure a duct will be under when X amount of air is moving through it. The charts are friction rate charts, and hey only show how much pressure it will lose.

Probably best to run a 6" duct, with a damper in it, and adjust the damper to regulate the amount of air it draws out of the bathroom.
 

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Have to agree with beenthere, with additional comment, you should not use an eave vent as a bathroom vent. The problem arises when you exhaust moist air out the eave, that air travels under the eave and right back into your attic causing possible mold issues later on down the road.

Either run it out a gable end or out of the roof, not the eave.

Mark
 

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Discussion Starter · #6 ·
Either run it out a gable end or out of the roof, not the eave.

Mark
Thanks for your comment, I did want to do this, since I have a 3 story house with a 15/12 pitched roof, no one will get on it to install a roof vent.

So the easiest thing to do was run the exhaust right to the gable vent which starts about 8 feet from the Joices and is about 3 feet high.

A vent installation guy came over and said running that would be a problem because the cold air would rush in, because cold air drops. I said to install a dampener, he said it will still get cold. Then I said what about just making a hole at the joice level and installing the exhaust there, he said same problem and explained that if the hole is under the eves, the cold air wont rush in.

Made sense, but you do as well, so again the best solution eludes me.

-pauli
 

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I wouldn't be looking at the existing gable vent, make a new openning ensure the exhaust point has a back draft damper, and raise the discharge line above the discharge point (this will trap the cold air from travelling back).

Mark
 

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Discussion Starter · #8 ·
I wouldn't be looking at the existing gable vent, make a new openning ensure the exhaust point has a back draft damper, and raise the discharge line above the discharge point (this will trap the cold air from travelling back).

Mark
Genius, that's a great Idea...
 
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