4" outlet duct for a hood fan.

sergitin39December 3, 2013

We live in a condo: second floor of a three floor building.
The vent stack is a 4" duct, we want the max possible for this diameter, I found it to be around 400 cfm.

The big problem came when the inter-web has not a single place where to look hood fans (wall hood fan) by the outlet diameter. I have been here for nights reading and looking, with very poor results.

I would like to hear opinions on those who have use these kind of fans. Propose some names and models, I will do the research to find which one suits me the best.

The picture here is like: 30" induction cooktop, aprox
25K BTU with all 4 magnets running (none boosting).
Hence I am looking for a 30 inches or even better 36 inches wall hood fan around 400 cfm. With a 4" outlet pipe.
(I do not want to go from 6 to 4 inches), , there will be 2 x 45 degrees elbows smooth duct or some flex pipe transitioning from outside to inside the wall before hitting the stack vertically for about 25 feet to the roof, where there is a roof cap, I think, but will have to investigate further to see whats the solution up there, if there is a flapper or damper, etc.

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Sophie Wheeler

The minimum acceptable hood ducting is 6". 4" is completely inadequate for any range venting. You'll have to replace it with 6".

    Bookmark   December 3, 2013 at 7:42AM
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Vent size replacement might be difficult for a one-floor condo. What you need will be a fan that is designed for high pressure loss and still moves 300 or so cfm at that pressure loss. Of course, we don't know what that pressure loss will be, but it is likely that you would need to put a four-inch vane-axial fan in line with the ducting and assume maybe an inch of water column loss. (Example high flow at high pressure loss fans: Ametek Rotron) Such a fan would probably not be quiet. Aerospace application fans in such sizes scream like a flock of tortured banshees.

Quieter would be using the six-inch to four-inch transition and a high pressure loss rated six-inch fan. Screaming would be more subdued (ref. Dante, Inferno).


    Bookmark   December 3, 2013 at 12:25PM
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OK, i copy by what kas have expressed that there is no common range hood moving 400 cfm through a 4" outlet, and anything I try to put to move this amount of air would need an inline fan to help. You seam to be a knowledgeable person, so I would like to ask you whats the maximum cfm a normal wall range hood can move through a 4 inches duct? and if you know any examples of a fan that accomplish this without the " NASA mutilating a flock of spirits inside our house" noise.

I mean we come from a Samsung microwave/hood fan combo, what would that thing move through that duct I guess was ridiculous and we survived, I was aiming more to 400 cfm at first due to some charts I found online telling that amount of air could pass by a 100 feet 4 inch smooth duct, and the fact that my duct equivalent with the 45's or the flex wont be even 50' going straight up after the transition gave me some hope.

Also by BTU calculations and our kitchen volume our need comes some where under 300 without boosting the any element in the induction cooktop (thermador CIT304KB) I was also aiming high in cfm (as mostly everyone) to be on the safe side and let it idle in low most of the time and still accomplish the right drag.

I would think there should be some one filling the niche for people living in condos built in the 70's with 4" ducts, we can't be the only one, right?

Thanks for your time and dedication to respond our questions.

We certainly wish we could do something about the duct, but there is no way we are going that way. Thanks for the suggestion.

    Bookmark   December 4, 2013 at 12:21AM
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While I don't have time right now for research, one has to be careful with claims vs. the reality of product availability. I'm certain very high CFMs can move through a four-inch duct, depending on duct strength to resist the pressure at the source, but this would be very noisy by itself, and require a positive displacement pump (think Rootes blower on a drag car or pavement breaker sized Ingersol Rand compressor).

Kitchen exhaust fans tend to be designed for relative quiet and low pressure loss (difference in pressure from one side of the fan to the other) and when confronted by a high pressure loss condition, move only low CFMs. My Wolf/Broan roof mounted "1500 CFM" blower flow rate drops to zero at 2 inches of water column pressure difference, and it uses a 10-inch duct. Similar performance is provided by Fantech's 10-inch to 14-inch in-line fans.

So the question that has to wait is, what is the pressure drop through 50 ft of 4-inch duct at 300 CFM?

To be continued...


    Bookmark   December 4, 2013 at 11:15AM
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A quick look shows that even bathroom fans rated higher than about 130 CFM call for duct greater than 4". While you'd get some minimal ventilation, I'd be tempted to go for a recirc with mesh and charcoal filters. Probably won't do a ton of good for odors but at least you'll trap a bit of grease.

    Bookmark   December 4, 2013 at 12:08PM
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The chart at the link indicates that 300 cfm in a four inch duct will have a pressure loss of 5 inches of water per 100 ft. At even 20 ft of duct we are in the realm beyond the capability of hood type fan operating conditions at the four-inch size. (Actually, I doubt that there are many hoods with such a small fan.)

If larger fans are envisioned to work at higher pressures, we would have to include the pressure loss of the transition from fan diameter to duct diameter.

As noted previously, aerospace fans can manage this task, but noise is usually prohibitive. I don't know about all the fans that exist, however, so there might be some design somewhere that would manage the task.


Here is a link that might be useful: The Engineerin Toolbox duct friction loss diagram

    Bookmark   December 4, 2013 at 6:40PM
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OK so today I did some more research around the building, went up stairs measure the distance to the roof, checked the roof cap in our ducts, see picture attached on the next message since seams there is only place to attach one pic by message. Lets say that's the roof cap shared to our 3 ducts, now, I couldn't see inside (forgot screw driver at home), so i am still in doubt if this is just a box around the three four inches pipes or it is a bigger square pipe coming from some where in our neighbors wall? I know mine is 4 inches up to our neighbors floor level.

Doing a EDL calculation using flex pipe to hook the fan to the duct and the roof cap thing, I come give and take to 50'.

I also checked the fan of my neighbor underneath us, and ask her about how she likes it, she said it was OK but not great, she needs to open windows for a lot of cooking at once.

Here is the data I got from her fan data tag:

Broan model: BP100
1.7 amp
180 (H) cfm 1" WG
180 (V) cfm 1" WG
210 (7") cfm 1" WG

She has it with 3 nineties and a square to 4" round duct transition plus at least 8 more feet than my location.

Her stove is around 31390 BTU at max power by my calculations.

I thought I'd give you this comparison point.

Her sister has a similar setup, but a bigger fan and a 7 to 4 inches transition. and her fan expands forward to catch more area. Broan elite is the name, I am trying to find some on it online, haven't found much yet.

I will start doing some calculation about the pressure lost on my EDL and materials. and come back with what i found, thanks a lot to every one pitching in.

Picture attached now with the numbers of an online pressure loss calculator. for 50 feet length EDL.

    Bookmark   December 4, 2013 at 8:35PM
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For what its worth...a 6" round duct has an opening of 28.26 square inches where a 4" duct only has 12.56. A 4" duct can carry less than half the air that a 6" duct can. The recommended duct size for a 600cfm hood is 8" which has 50.24 sq inch opening though I've seen hood specs for 600cfm hoods where a min of 7" is listed which is 38.46 sq in. Surprising how much difference diameter makes.

This post was edited by scole250 on Wed, Dec 4, 13 at 20:41

    Bookmark   December 4, 2013 at 8:38PM
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Here is a picture of what I found on the roof, I am not sure if roof cap with box around our 4 inches pipes (3 of them, one each floor) or if it is a kinda plenum getting the 3 pipes into a big square duct at some point in our upper neighbor's wall or her ceiling... not even sure if there is an extra turn or it goes straight down. I will try to ask for the roof access keys again and open the top of the roof cap and pointing a light down to see if I can find some extra info. A lot of cracking ice and slippery roof, makes me think it twice. ;)

Thanks again

    Bookmark   December 4, 2013 at 8:41PM
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I think all these fans are being choked to various degrees. The condo builder deserves opprobrium for such a deficient duct setup, barely adequate for a 80 cfm bathroom fan.

I hope that there isn't any flexible duct in the path unless it is steel.

To get adequate kitchen ventilation, a duct to an outside wall would seem to be needed. This might be an issue with the condo association.

If you found a fan that could actually move 300 cfm through the duct, the duct pressure would likely back-flow into the other two condos, or hold their dampers (if any) closed against the pressure.


    Bookmark   December 5, 2013 at 10:33AM
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