How quiet are external blowers?

lwernerJune 9, 2012

I'm doing some research on what kind of range hood and blower I want in the house I'm remodeling. I'm going to have a 36" gas cooktop or rangetop. I haven't yet decided whether to open burners (probably BlueStar) or sealed (probably Wolf).

Here's a drawing of what the kitchen will probably look like. I have a straight shot of about 5-6' up from the hood to the roof -- the ceiling will be at 10' and then there's another 2' of joists etc. below a flat roof. (In the drawing I think I have the ceiling 1' too high.)

I haven't decided yet on what to do above the hood: chimney style, under-cabinet with upper cabinets or a soffit to hide the ducts, or whatever. Some of that depends on what kind of ductwork I need to keep it quiet. The kitchen is one end of a big great room, so I want to keep the noise level as low as possible.

What I think I need for a quiet system (based on reading posts here) is something like this:

- A 36" or 42" hood. 36" if possible so I have more upper cabinet space, but I can go to 42" if necessary.

- An external "updraft" blower on the flat roof. Something like what Greenheck makes. Their 095 or 099 models might even be small enough.

- Some ducting in between. Probably 10" diameter, and probably just straight up to the roof to minimize obstructions.

Does that sound like a reasonable plan? Do any of you have a feel for how noisy a system like this would be? I know external blowers are quieter, but with such a short duct run I'm worried that the blower noise will transmit down the duct fairly easily. What do you think?

If I need a silencer, does anyone have suggestions on where to put it based on the picture up above? I can't think of a way to do it with a chimney-style hood, since the diameter of the chimneys isn't big enough to hide one. And the 2' of space between the ceiling and roof isn't enough to hold the Fantech 10" silencers, which are around 35" long.

A couple ideas I had were to build a curb up on the roof that adds an extra foot below the fan, so the curb + ceiling area have room for the 3' silencer. Another option would be to put a soffit above the cabinets and hide the bottom end of the silencer in there. Do those plans sound sane?

If you have any suggestions or answers, or if you just think I'm insane and should start over, please let me know!

Thanks!

Laura

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regbob

External blowers are not quieter that internal blowers. Most of the internal blower motors make hardly any noise. The noise that you hear is the sound of the air moving through the filters. Depending on where you live I would install an internal blower. If you are in a colder climate you run the risk of the motor freezing up on cold days after running the blower and having all of the hot moist air going through it. If any service is needed it is harder to do plus the blowers themselves are big and not that good looking of a piece that I would not want on my roof.

    Bookmark   June 9, 2012 at 10:58PM
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barjohng

Have you looked at Vent-a-Hood? No filters and very quiet. The motors are in the hood and the grease trap is easy to remove and clean. They are hard to beat form what I have seen.

    Bookmark   June 9, 2012 at 11:31PM
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kaseki

This is the first I've heard of a roof blower freezing up. Mine never have, and I used to keep my house very humid in the winter (here in NH). An upblast configuration would be particularly resistant, I would think.

There is plenty of potential noise from a fan that is independent of air moving through the filters. This is primarily blade tip turbulence noise and any rumble from fan unbalance. However, the shorter the ducting, particularly without a silencer, the less difference remote mounting will make.

Clinresga has documented here the relative noise of his preferred fan system and a Vent-a-hood he has in a second house. The Vent-a-hood was significantly noisier.

For serious quieting the silencer will be needed, and it may be useful, depending on what vibration passes along the ducting, to insulate it and/or dampen it with acoustic pads. This would seem to lead one to put the duct in a chase so that all this ugliness is hidden.

I'll leave analysis of the aesthetics of roof blowers to others.

kas

    Bookmark   June 10, 2012 at 12:04AM
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lwerner

Thanks, Kas. I was hoping you'd answer. You seem to be the resident expert on all this.

Thanks for confirming my suspicion that a silencer is likely needed. As you said, I'll probably have to move to a design with a chase to hide the duct and silencer. Or maybe just a soffit above all the cabinets along that wall. That would save me from having to worry about what else to put above them. :-) I do like the look of the chimney hoods -- I'm going for a fairly simple, contemporary, but not uber-modern look -- but I don't see how I can make one work.

Another question: does anyone know if inline blowers have to remain accessible for maintenance? If they do, then I have to use an internal or rooftop blower because I don't have an attic to put the inline blower in. I'm guessing the answer here is "yes", because from what I remember from the NEC, junction boxes and motors have to remain accessible. Even if it's not required by code, I don't think I'd want to rip drywall out to service a bad blower motor.

regbob: I realize there's noise from air moving over the baffles, but there's also a fair bit of noise from the fan blades themselves. (At least on all the fans I have experience with.) So I'm pretty sure I want to go with an external fan, unless I can find an internal one that's extremely quiet.

barjohng: The Vent-a-hood marketing says that their baffle-free design reduces noise, but after seeing the measurements some folks on the forums did I'm not sure I believe them any more. I also tried taking one of them apart at the appliance store to see how hard they'd be to clean. It was pretty easy, but not something I want to do when there's grease in there. The hoods look really nice, though.

Finally, on the aesthetics: I don't think that will be an issue in my case. My duct will exit on a flat, 12' high roof in a spot that's about 20' away from the edge of the roof that faces my back yard. If I'm doing my math right, I won't be able to see it until I get ~50' away from the house. My neighbors on that side might be able to see it, but fortunately there's a big tree in the way. Even with an internal or inline blower there's going to have to be something up there where the duct exits, and an external blower will probably only add another foot or so of height.

As for freezing, I live in the SF bay area. We get maybe 1 or 2 light frosts a year, and occasionally about 5 minutes of small hail, but that's about it.

Laura

    Bookmark   June 10, 2012 at 1:44AM
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kaseki

Yes, all junction boxes need to be accessible. In any case, one might need to repair the fan someday. On the plus side, axial fans (as the inline fans are usually made) should be very efficient and low maintenance.

For belt and suspenders chase construction, use 5/8 firecode sheetrock on the interior of the chase, and whatever is desirable on the exterior. You can include an access port to get to the motor, and then hide it with something (painting, mirror, the mother of all knick-knack collections, whatever.

Consider planning for how the motor/fan assembly can be lifted to its mounting site, and how it can be removed.

kas

    Bookmark   June 10, 2012 at 12:16PM
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