exterior hood vent fan?

marniemosimanMay 8, 2012

Hi- I have been helped so much by all the postings on this site. I've been lurking for a year, and now that we're getting close to completion on our addition/remodel, I finally have a question that I can't find an already existing answer to-

If there is only a 30" duct run from the back of the hood to the outside wall, can I expect much noise reduction? Or should I just put the fan in the hood? This will be venting a 48" Bluestar which is against a pony wall built at a 45 degree angle across a corner of the kitchen (yes, I know a corner range is odd...) We're planning on using an Abbaka Hy-ex 1400-

Thanks for all the expert advice

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colin3

If you're using the HYEX 1400, it has to go on an outside wall or roof. The alternative would be a blower made to go inside a hood, and Abbaka also makes those.

I have its younger brother, the HYEX 1000, about 40 inches from the hood, enough to fit a muffler in the duct in between. Distance and the muffler certainly help, and the blower itself is relatively quiet for something moving that much air.

    Bookmark   May 8, 2012 at 11:39PM
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marniemosiman

Thanks, colin3...I haven't actually purchased the HYEX 1400 yet, but the GC is chomping at the bit to install whatever we get so he can complete the outside waterproofing. When you say a muffler, do you mean the muffler/silencer from Abbaka? It looks like the 10" oly come in 44" lengths? Can you cut it down to fit if your ducting is shorter?

    Bookmark   May 9, 2012 at 12:42AM
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colin3

I used the fantech muffler: http://residential.fantech.net/residential-products/kitchen/silencers/

("Silencer" is a marketing fib, but it does reduce noise.)

For a 10" duct you'd want the LD10 which is 3 feet long. You could shorten it with the right tools, but it would take a little work and thought.

I'm happy with the Abbaka blower. Expensive, but well built.

    Bookmark   May 9, 2012 at 1:40AM
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kaseki

I think shortening a Fantech muffler would be a lot of work, and the result would be expected to have different acoustic characteristics than the original. A conversation with Fantech might be in order.

kas

    Bookmark   May 9, 2012 at 11:01AM
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billy_g

We have the Hy-ex 1400. It is quiet but you will be surprised how large it is. That's one of the reasons we put ours on the roof rather than a wall.

Is there a possibility to run your duct to the roof? This might give you space for a muffler as well.

A couple of things help with the noise. One is to have at least one elbow between the hood and the fan. The other is a muffler. We have a couple of elbows but no muffler.

I would try to put an elbow in somewhere between the fan and the hood. You have such a short duct run that with a 1400CFM fan it should not be a problem to have some roundabout ducting with your muffler installation -- it does not have to be (and physically cannot be because of the space) a direct duct run. Most of the time you want to avoid extra elbows (they count as about 10 feet of duct run) but in your case it may work well.

Billy

    Bookmark   May 9, 2012 at 11:28AM
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colin3

Interesting re muffler modification - do we think the shape is tuned for some sort of noise cancellation or other acoustic effect? In design the fantech unit appears to be a simple absorption muffler along the lines of a "glasspack" auto muffler illustrated at the bottom of this page: http://auto.howstuffworks.com/muffler5.htm

    Bookmark   May 9, 2012 at 1:36PM
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kaseki

colin3:

My recommendation to call would potentially provide an answer that question, and also allow checking whether Fantech can build a shortened unit.

kas

    Bookmark   May 10, 2012 at 10:42AM
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attofarad

The Fantech LD8 is about 28.5" long. The static pressure drop for 1400CFM through a 3 foot long 8 inch duct is 0.107" H20, compared to 0.033" for a 10 inch duct. Additional static pressure for adapting to/from 8 to 10, and anything else in line (e.g. baffles).

Looking at the HYEX specification of CFM vs. static pressure, the loss of flow with 8" duct would be maybe 6%, depending on what else is in the path.

The LD8 doesn't have as good noise reduction spec as the LD10.

Consider isolating clamps, as mentioned in the HyEx brochure, even if you can't fit a silencer.

Here is a link that might be useful: HyEx spec

    Bookmark   May 10, 2012 at 2:19PM
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colin3

FWIW I would definitely agree on keeping the ducting at 10" and not trying to fit in an LD8, in the context of the OP.

By all means, if anyone is interested in the shortening mod, call Fantech first, and best of luck getting to someone who understands the engineering and wants to talk to you.

I will just note, after looking again at my installed LD8, that the endcaps appear to be held on with two screws each. Pop them off and you have a simple double-walled tube. If you have an angle grinder and a little bit of experience working with metal, shortening would not be much challenge. Obviously you would get less muffling as the thing shortens & the relation is probably nonlinear, but it would still be quieter than bare duct.

    Bookmark   May 10, 2012 at 3:25PM
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attofarad

The online references say that the silencer length needs to be at least 3x the internal diameter to be most effective. I think the LD10 would still be worthwhile, even if shortened a few inches, but I'm willing to spend a bit for modest noise reduction.

    Bookmark   May 10, 2012 at 4:11PM
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