In-line silencer placement question for range hood

last year

Hello, I’m having a new range hood installed. I’m planning to use an external blower that will be installed on the roof, as it is my understanding that this option will be quieter than an in-line blower. We have a one story house with low ceilings so I also want an in-line silencer installed. So here are my questions:

Should the silencer be installed closer to the blower, or closer to the hood? I would think closer to the blower since that’s the part producing the noise, but I’d like to be sure.

If the silencer is installed next to the blower, that leads to another question:

The external blower has a 10” duct, whereas the hood has 8” ducting. So there will need to be a duct reducer to connect the two. Would it be better to buy a 10” silencer and put the duct reducer below it, or buy an 8” silencer and put the reducer above it?

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Comments (6)

kaseki
last year
With the blower on the roof, the closer the silencer is to the hood the more duct turbulence noise will be reduced, along with the blower noise (mainly from blade tip turbulence; motor rumble and other low frequency vibration will not be affected by such as Fantech silencers).

Is there a particular reason that the hood duct diameter and blower interface diameter are mis-matched?

What is the rated flow rate of the blower? What is the size of the hood?

In general, the duct size should be such that the velocity in the duct at full power (and considering that the rated blower flow rate is not the actual flow rate) falls between 1000 and 2000 ft/min, with the lower value less noisy. This implies that the answer to where the duct changes from 8 inches to 10 inches depends on the answers to my questions above.
Kherve
Original Author
last year
I’m not sure why they’re mismatched. The hood is the Zephyr Vista Island 36” which has an 8” duct. It says it’s compatible with the Zephyr external blower, of which there is only one model CBE-1000 and it has a 10” duct. I just took a look at the In-line blower option listed for this hood (blower model PBN-1000A) and it’s also 10”. Both of those blowers are 1000cfm.
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One thing that seems to be shown here, from your experience and also laranbrian's, is that mesh filters are noisier. We have "pro style" metal baffles. Here's the (obviously biased, but I think informative) quote off the Modernaire site: "Modern-Aire Ventilating believes like most major range manufacturers and restaurant supply houses that the most efficient filtration system is the Baffle Filter. Unlike non-filtered range hoods, a Baffle Filter catches up to 97% of all grease and grease particles before they ever enter the interior of the range hood, making clean up a breeze. Unlike mesh filters, the air passing through a Baffle Filter is not restricted, thus allowing a smoother and quieter transition. Perhaps the most important aspect of the Baffle Filter is that the baffles in the filter are positioned to eliminate direct passage of flame through the filter thus making it one of the safest and most efficient filtration systems available today." I think it's a far superior design for the liner. And it looks cooler too :-)
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In principle, any hood with an internal blower could be gutted of the blower and operated with an in-line or roof mounted blower. (GreenDesigns may have meant that using it while leaving the blower inside would be a poor plan.) I can imagine construction details that might cause non-optimal internal hood flow patterns, but I have no detailed knowledge of the internal construction of myriad hoods to know whether such are prevalent. There may be nanny jurisdictions that would claim that code approval of the hood was violated when it had its blower removed. Any hood available for order without a blower likely is fully compatible with an external blower. This, of course, can be confirmed with the manufacturer. The most important adaptation requirement is that the blower be continuously controllable, and that the hood control be a continuous motor control. A hood with just a few motor speed positions on a switch might require a compatibly-wired motor. While a multi-position hood control could in principle be replaced by a continuous control, par. 2 may apply. When I was doing my kitchen reno, I chose a compatible hood/roof blower combination (supplied by Wolf) because the entire scope of the project (gutting and reconstructing part of a house) made playing with the details of the blower circuit a side project of tertiary importance that I couldn't afford the time to deal with. As it happens, inside my Wolf (Independent) hood is a diac/triac motor control circuit, about the simplest design available that goes back to the '70s. The compatible Wolf (Broan) roof blower uses a typical induction motor that is easily controlled by this type of circuit. The cost versus time value trade-off needs to be considered when dealing with issues like this. kas
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As I've written many times, that style of hood fails to capture correctly due to the plume's ability to slide out the middle. Also the structure holding that minimal screen filter also blocks the plumes from rising directly to where they can be pulled in and contained. Rising cooking plumes have momentum and tend to reflect off of hard surfaces. Either the surfaces have to be angled to reflect toward the filter, or the airflow near the surfaces has to entrain the plume and direct it toward the filter. I don't think 760 CFM can be pulled through that small screen with any blower that fits in that "duct" section (screaming mimi aerospace blowers excluded), not to mention also pushing air through the charcoal filtering that I hope is next in line. Maybe the blower is rated at 760 air flow rate when hanging in free air (zero static pressure), but I'd be surprised if more than 300 CFM is moved through that device. This only matters if serious cooking is intended that generates greasy and/or odoriferous plumes. For light duty (boiling eggs, making rice, even simmering bacon, this hood may suffice. The image one should have is that there is a pan on an induction hob generating steam and/or grease particulates with some upward velocity -- possibly 2 ft/s for induction without grilling. This plume is also expanding as it rises. There may be drafts that can be similar in velocity if people are walking around, or the house heating is blown air, or an a/c is directed toward the hood. All of these conspire to interfere with capture and containment of the plumes. On top of that, hood filtering is normally not designed to filter everything, just the grease particulates that would otherwise condense on the ducts. The rest is normally expelled outside. With recirculating hoods, multiple filtering layers may be needed to keep the greasy fine particulates from returning to the room and coating the walls, ceiling, furniture, drapes, cat, etc. The charcoal may, if deep enough, deal with odor. The Vent-a-Hood ARS reportedly achieves these goals with reasonable effect at significant cost. If your cooking becomes enthusiastic, and this system fails to meet your filtering goals, you may want to start looking for an upgrade. There may also be a Broan recirculating filtered hood, but I haven't looked into it. As for your present situation, I would bias the hood position to optimize collection of all, front, or rear hobs depending on your intended cooking style.
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Kherve
Original Author
last year
Pulled up the spec sheet. Looks like this is by design for some reason.
kaseki
last year
It would seem that the 8-inch diameter is what the chimney cover is intended to fit. Maybe it looks better that way. It also seems that they want the transition to be near the hood (above the ceiling).

My impression from the table notes above and an image I got from a search is that this is a non-conventional hood, i.e., not baffled and lacking a capture volume reservoir. Generally, the farther a hood deviates from commercial design, the less efficient it will be in capture and containment. It looks like it has mesh filters. While a 1000 CFM (in free air) blower might move 650 CFM through a conventional baffled hood, I can hardly imagine it pulling that much past a mesh area. And this mesh area does not appear to overlap the cooking zone.

Recommend review of myriad hood threads here to gain perspective about hood design principles and requirements. Note that in many municipalities, a 1000 CFM rated blower will require a make-up air (MUA) system. You may wish to review MUA-related threads also.
Kherve
Original Author
last year
I looked through many many hood models and wasn’t able to find any that were baffled, other than commercial models that would not have fit our space and low ceiling. I chose this one as it was the highest cfm available for a low profile which I assume will make up for at least some of the issues with mesh and lack of capture area. It at least beats an equivalent style hood with only 600cfm
kaseki
last year
True, but only if the mesh area does not choke the 1000 CFM blower to be barely better than a 600 CFM blower. This will depend on their relative fan curves.

I am only here to aid in achieving the performance that is desirable from hood systems over the full spectrum of potential cooking activities. You are welcome to let aesthetics dominate your trade space.

Most 'pro-style' hoods have baffles. Although the Wolf Design guide lacks imagery of the guts of their hoods, I think most of the higher flow ones have baffles.

My Wolf hood is shown below. It hangs from an 8-ft ceiling via a 6-inch extension. While much longer than you need, and an island hood as well, it illustrates what can be done in an 18-inch height hood. Modernaire, Prestige, Abbaka, and many others surely have baffled hoods with some volume below the baffles that can be fitted to your architecture.

If your ceiling is only 7 feet high, then a lower profile hood may be desirable if you want the hood base above 30 inches. Baffles with some slope should still be possible.

In-line silencer placement question for range hood

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In-line silencer placement question for range hood

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