Range hood blower dilemma

10 months ago

We're in the middle of a gut-reno of a kitchen in a very old house. The rangetop is a 36" gas bluestar. The hood will be a 42"x30" Prestige high-capacity mounted 42" (maybe a tad less) above the counter. We have the capability of putting a 90 degree elbow on top, and going straight out the side of the house (about 6 feet). This would be limited to a 6" duct, which would limit to a 600cfm internal blower. This seems low to me, but given the front and side overhang, and the fact that there is a bit of containment area in this hood, along with limited and direct ducting, might work.

Going to a larger duct would require an elbow and a round to rectangular transition to duct down the 2x8 studbay, into the basement, another bend to go across the basement ceiling, and out the foundation wall. This would necesstate (or allow, however you look at it) a 10" equivalent duct, an inline or exterior 1200 cfm blower and maybe a duct silencer). It would also, however, require at least two 90 degree bends, two round to rectangular transitions, and more like 20 ft of duct length. With all that complexity, i imagine the actual cfm through the hood may not be that much greater than the simple approach?

Neither the architect, contractor or HVAC guy are thrilled with this, as its more of a crawlspace than a basement (5' tall) and there's other mechanicals that need to go down there. I suppose the question is, would the 600cfm and simple ducting be sufficient?

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Comments (3)

kaseki
10 months ago
Wouldn't it be easier to re-frame to get at least an 8-inch duct into the direct path?

My recommendation for actual flow rate for this size hood is 800 CFM, typically requiring a 1200 CFM rated blower to handle pressure losses including baffle losses (usually but maybe not in your case) dominating duct pressure losses, and that of adequate MUA.

A large enough reservoir volume under the baffles may allow some averaging, but the best you can get away with is to achieve the actual CFM of the plume(s), and even then you may not be able to deal with transient plumes without a very large volume (tall hood).

A single plume starting at one square foot area and rising a 4 ft/s is 240 CFM by itself at the pan, and depending on plume spreading vs. velocity, hood air entrainment, etc., may be higher at the baffles. A 600 CFM blower (with MUA) likely won't achieve more than 400 CFM. So adequacy of the 600 CFM blower is questionable.

It is undesirable to make a U-turn and duct down and out. Grease issues may lead your code enforcement officer to poo-poo the idea.

You need to plan the MUA as it factors into what the hood will achieve.

Wouldn't it be easier to re-frame to get at least an 8-inch duct into the direct path? What is the joist spacing and depth?
nexp
Original Author
10 months ago
It’s a 150 year old house and the kitchen is in the double-wyth brick basement (which is above grade on 3 sides). The joists are 3x6 on 24” centers. Keeping the duct to 6” allows us to only penetrate the rim joist. Any more and we’d have to notch the timber box beam that the balloon framing sits on. It wouldn’t be the end of the world - I just don’t love modifying historical structure of that sort.

And now after a site inspection, that plan may be moot, as there’s a newly-installed nest of waste line connections in the exact spot. And they can’t move because - of tying together existing lines. Guess the plumber should have been at the meeting between the architect and the hvac contractor!

Going down may be the only option - the hood does have a rear outlet, so it would only take an L to go down. Thanks so much for the advice! And I am planning for a powered makeup air system that operates off a differential pressure sensor and modulates to achieve a set pressure.
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All of the above may be true, but are not in themselves totally limiting. In general, there is always a blower that will move a desired air flow volumetric rate through a given duct, but one might not want to operate it, listen to it, or pay for it. But in this case we are on the margin, and feasibility is not prohibited using conventional devices. For example, I have a Wolf/Broan 1500 CFM rated roof-mounted blower operating with a 10-inch duct, and given various estimated pressure losses, expect that it moves 900 - 1000 CFM. A 1000 CFM rated typical hood blower, no matter how large the duct, at a minimum will be significantly restricted by the hood baffles, and at a maximum by any added MUA pressure losses, and would be unlikely to move more than 700 CFM in use with a hood. An 8-inch duct has a sectional area of 64% of that of a 10-inch duct, so the pressure losses operating at 67% through an 8-inch duct of the air flow rate through a 10-inch duct will be similar. In general, one wants to select a blower having a fan curve that supports the desired flow rate at the pressure loss that one estimates is present. Without my looking up the specified hood, let us assume that its entrance aperture is 42 x 27 inches or just under 8 sq. ft. This calls for 8 x 90 CFM/sq. ft. or 720 actual CFM. A typical 1000 CFM blower may achieve this with a 8-inch duct, and one certainly can check the assumptions against blower fan curves (where available) to either ensure adequacy, or at least force the MUA system to be active and not cause significant pressure loss. Note that this doesn't violate your appliance guy's opinion about duct flow rates, although I am sure that his view is based on typical in-hood blowers. If you have the room in some part of the duct path, then including a silencer will help reduce noise back at the hood for only a small added pressure loss. In any case, numerous commercial kitchen ventilation blowers can be found that will pull 720 CFM against any pressure loss likely from a well designed hood/MUA setup. It is quieter to move a given flow rate when the fan blades are large and moving slowly than small and moving quickly, as the latter generates more high frequency noise from turbulence. Induction motors can be controlled by rheostat adjusted phase circuits, and these can be put into hoods. My Wolf hood has such a control, and now the technology allows for it to be performed with better techniques at low cost. These comments are meant to touch on most of your questions, but feel free to delve deeper as needed. kas
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kaseki
10 months ago
Suggest considering rectangular ducts that fill the joist space. The equivalent round duct diameter is approximated by 4 x [sectional area] / [perimeter]. So 20 inch x 6 inch duct is equivalent to 9 inch duct. If you can fit this then you should be able to achieve the desired flow rate, given a blower that can yield the CFM at the estimated pressure losses of the path (plus baffle pressure loss).

Range hood blower dilemma

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kaseki

nexp
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Range hood blower dilemma

Comments (3)

kaseki

nexpOriginal Author

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nexp
Original Author