Range hood blower dilemma

3 years 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
3 years 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
3 years 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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kaseki
3 years 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).