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.

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.

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

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

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

It's been years since anyone commented on this thread but I'm hoping someone might have thoughts and/or knowledge about retrofitting a very loud 52" Vent-a-hood with a roof-mounted upblast fan (perhaps from Greenheck?) with speed control in the kitchen. My husband is very competant handyman though we'll probably hire someone to do the roof work. I am in a hot/temperate climate (rarely freezes). The noise of the fan bugs us so much we don't use it much, which has obvious drawbacks for air quality, grease residue, etc. This thread is the first I've heard of a silencer. Thank you!

I would first consider an inline blower (+ silencer if you can fit it in). That would involve the least amount of work and in particular avoid messing w/ the roof cap. You should evaluate your duct run to make sure it's OK from a material, size and static pressure (bends + transitions + roof cap) standpoint.

I'll second the idea of having a welder mod your VAH carcass to add in rails and drain pan for baffle filters. That'd likely be a lot less expensive than buying a new hood and would likely result in much better performance since you'd have a good chunk of containment volume unlike most consumer hoods.

"...since you'd have a good chunk of containment volume unlike most consumer hoods."

I'm not sure there is all that much volume, at least in the very limited number of older VaH (magic lung) hoods I've seen. I recall structure in there (and lights) that would have to be gutted with the blowers to allow the baffles to have significant space below them. Would S Clark provide a photo looking up into the hood to satisfy my curiosity?

Of course! I really appreciate your input kaseki and opaone! Here are photos. The turbines come out for cleaning. The exterior of the duct that goes up to the roof is about 1' x 2'.

Yeah, that's different than the VAH's I've seen that are rectangular boxes. I still think @kaseki's idea of getting a welder to install rails for baffles could be a good idea if you like the aesthetics of the hood. A new hood is certainly an option.

The baffles could be placed about half way between the bottom and the turbine exit port area. Perhaps an 8-inch high reservoir would be possible. There has to be sufficient space above the baffles for the baffles to dominate pressure loss and not the path above the baffles to the duct.

I don't know if the hood skin is double or not. Welding to the skin would likely leave external 'scars.' However, the upper structure might be used as a mount for dropped baffle assembly frames.