600 CFM Range hood and a 4" duct, is it possible or just madness?

Let me try one more time. You cannot use 4″ round duct or fitting. You need to go up with a 6″ diameter round duct, transitioning into a horizontal rectangular duct of adequate cross section so a s to equal or exceed the sq in cross section of the 6″ round. So if you can go directly up, then have fabricated an adapter from 6″ round to 4″X 8″ rectangular, if there is sufficient room to conceal the rectangular duct going across, ending probably with a transition back to 6 round where you exit to the outside. Kapeesh?

brucef

in Brownstoner Renovation 12 years and 5 months ago

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blaqhole | 12 years and 5 months ago

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Hello. I am seeking out advice about a kitchen we are renovating. We would like to get a 36″ 6 burner gas range, 15,000 BTUs for a kitchen we are renovating. Salepeople have all said that at minimum we need a 600CFM range hood for such a beastly stove. For those types of hoods, often the minimum is a 6″ diameter duct. The hood exhaust would run up to the ceiling (which is 9 feet high) and then connect with a duct in the ceiling and travel 4 feet to vent out the building. We would very much like to hide the duct in our ceiling but only have 4″ of clearance between the ceiling and the joists above it so a 4 inch duct is our only possibility if we want to hide the duct. It’s a little too crowded a space (because of piping and electrical stuff that is happnening around there) for a 3 1/4″ x 10″ or a 4″ x 8″ rectangular duct to be used. Now, the one positive is that the stove is around 4 feet from the exterior wall where it will be vented so the exhaust doesn’t have to travel far to exit the building.  So, here is my question: Can we go from a 600CFM hood through a 6″ to 4″ reducer to the 4″ duct (which I found online), given it only needs to travel 4 feet? Will the hood still work properly? What if we added some sort of exhaust fan in the exterior to pull the air out and help the movement of the exhaust?  I asked a bunch of salesfolk but they have never seen it done before so can’t comment if it will work. My general contractor is convinced it will work but I think his past experience is with traditional cooktops that only require 400CFM and thus, 4 inch ducts. Any advice would be much appreciated if you have had experience tackling something like this. Thanks all!

brucef | 12 years and 5 months ago

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Part of what you say does not make sense, but I’ll give it a try. Air flow is dependent on cross section, so 6″ diameter is 28 Sq inches. 4X8″ duct is 32 sq inches. Normally we would have the vent fabricator make up a transition that smoothes the turn, but even a 6″ circle cut in the 8″ side would do. I would be more concerned about noise. Larger vent fans can be quite loud, as they typically reside in restaurant kitchens. I would look for an external fan that would pull, rather than an internal that would push. I am happy for you that DOB is allowing a sideways vent, lately they have been making us go straight up.

Bond | 12 years and 5 months ago

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No, in my experience you cannot do what you are asking.  As Bruce points out, it’s a question of volume, not distance. Going with a smaller duct won’t compress the air into a smaller space.  I can see one of two things happening.  Your fan will move 400 CFM (more or less) of air through the vent, or depending on how the transition is set up,  you could get spill back on the extra air, in all likelyhood creating a downdraft, both scenarios negating the whole point of the 600CFM hood.  Think of it as having a fan set up in a room with a window cracked and you’re not happy with the amount of air that’s going out of the room. .  You can move the fan you currently have closer to the window, or put in a more powerful fan, but you’ll just get swirling air in the room unless you open the window wider, the window being your vent.

blaqhole | 12 years and 5 months ago

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Hi Bond. Thanks so much for your answer. You analogy makes a lot of sense. It sounds like we have no choice but to go with an exposed larger duct. In the end it will probably be better all around. Thanks so much!