4" Vent Capacity?

neolitic

Could you run two 80 cfm
exhaust fans into one 4"
vent cap if they might both
be operating at the same time?
Don't want to get into any problems,
but the HO wants to limit roof
penetrations and doesn't care to
see the 6" cap if it's not necessary.

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ChrWright

Evening Neo,

I've run into this issue before as well. The issue I came up against was the risk of back flow from one bath to another--was always worried those cheap dampers might stick open.

If you decide to try it, I'd put the "Y" as close to the roof vent as possible.

Have you ever used the ALDES vent system for multiple baths? I've used two of them and like the system a lot. It's overkill if you're not working with three or more baths but is very quite and very low profile on the inside.

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neolitic

It's a "while you're here" deal.
Re-do one one bath, new fan
part of the plan.
Now want a fan in the other bath
as well, but no more caps on the roof.
In all fairness it will be in the front
of the house, but it would mean @16'
of extra duct to get past the ridge.

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TimelessQuality

How about an under-soffit outlet for the second one?

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neolitic

Funny...that's what my partner
wanted to do in the beginning,
but its continuous vent. (12" soffit)
I believe we'd just be pumping
steam into the attic.

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marchvac

In Ontario the code does not permit it. What you could do is install an inline fan in the 4" close to the the outlet and then the Y branch to the fan boxes minus the motors. Then make the fan come on with the demand for either one. Some of the new inline fans with a backward incline blade are very quiet and powerful. hope this helps.

neolitic

Good thought, but I think this HO
will put up with the extra vent cap
before he will spring for the in-line
option and extra wiring,=.

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neolitic

I'll try again...maybe a bored
HVAC pro out there today.

Since dryers run @180-250 cfm
on 4" vents (I think), would 2
80 cfm fans somehow create
a greater load than that?
Would an 80 cfm fan have a problem
lifting the damper on a 6" cap?

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shaneali

Hi Marchvac,
I saw a post by you in another HVAC forum and had a question about a very similar situation to what you were dealing with re: a Lennox G43UF with red blinking and green on (i.e. limit switch issue). If you are out there, would you mind ideally sending me an email or alterntatively replying to this post? Thanks in advance.

Leo G

In a vacuum system I know that 350 CFM can flow through a 4" duct. How this helps you, IDK. But I know that you can =flow that much in a vacuum, you should be able to flow that much in a pressure system.

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Miller257

In the resi world, when we duct (2) 80 cfm fans through the same penetrations we typically use 5"ø. Run 4" to a 4x4x5" wye. Do it right, do it once. the broan 634 is only 4" wider than the 636, you could probably make the 636 work with a 5" duct even though its listed as 3-4"ø only.


good luck.

bwalley

The Trane Ductilator a 4" round duct has a velocity of 1800 FPM when 160 CFM is run through it.

I did not take into acount any friction loss as I don't know how many elbows are in the duct, this is also assuming it is smooth duct.

I don't think the fart fans will work properly at that velocity.

Going to 6" duct will drop the velocity down to 800 FPM.

I 80 CFM fart fan on a 4" duct will have a velocity of 940 FPM.