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How do I Fit Range Hood Duct to a Smaller Vent Hole Opening?

14 years ago

My old house currently has a 4" duct opening and I'm planning on buying a new range hood that is 7".

What should I do? Is there anyway to get around it without having to hire a contractor to drill a bigger vent hole on the side of the house?

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Comments (6)

weissman
14 years ago
You can probably get some kind of reducer but that's not a good idea - if the hood requires a 7" duct, using it with a 4" duct is going to drastically reduce it's effectiveness - it might even void the warranty if anything goes wrong.

I would tell you to buy a hood for a 4" duct but I've never come across one - the smallest I've found are hoods that require a 6" duct and even those are few and are between.

My recommendation is to replace the duct. Put in at least an 8" duct - that will provide you with much more flexibility in terms of hoods available to you.
guadalupe
14 years ago
Either make the hole bigger and do it right or by a ductless hood and forget about the 4" hole (cover it up)
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@opaone: your numbers, while still higher than my estimates, are at least within an order of magnitude, which in this business probably is reasonable correlation. I agree that this could rise to a significant concern with 10 people really cranking away in a poorly ventilated room. So we're on the same wavelength. @spurpura: you are correct in that I've not invested in air quality monitoring devices or hired PhD's to test my house. But my calculations are grounded in simple physics. The only assumption I have made is in the estimate of CO2 production during exercise, and that was from an academic sports physiology laboratory, authored by a PhD and published in a peer-reviewed journal. If you have figures showing that CO2 production during exercise is roughly 10 times higher than my estimate (which is what would be needed to support your observations), I'll await the citation. There are really only three possible scenarios: You are measuring local concentrations of CO2, presumably close to where you are exhaling. Your PhD consultants and their equipment are wrong. There's an exogenous unidentified source of CO2 infiltrating your exercise room. Enough of that dead end discussion. But...as to your original question, I think you're on the right track. As noted upthread, our 1200 cfm hood runs through a straight run of 10" duct, about 25' upwards, with an attic-mounted Fantech LD10 silencer and FKD10XL blower. It measures 60db (LCS weighted, ear level, roughly 4' from baffles) on high. So we hit the mark you are shooting for and I expect your setup will too. Good luck!
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friedajune
14 years ago
I agree with Weissman. I had a friend who had no duct at all. She hired an HVAC guy to install ductwork by opening her drywall, installing the 8" duct between the studs (or was it the joists?) (just barely fit, she told me), and cutting a hole to the outside, including the damper. While it sounds like a big deal, she told me that in the end, actually it wasn't. She was lucky to have an empty soffit above her cabinets, and also did not have a long distance to the outside, about 6'. She says she is so glad she did it. The cost (she was surprised it wasn't so much) and any aggravation are long forgotten, but meanwhile she will have years of enjoyment from cooking with her hood and good ductwork. Of course it depends on each home and its setup, but you could at least look into it.
weedmeister
14 years ago
gt: it sounds like in your case the vent goes straight through the wall? if so, this is not a big deal, unless power lines get in the way.
davidro1
14 years ago
There is no way around it: 4" is too small by far. In a kitchen, it is below the lowest permissible size, under any Code out there, as far as I know.
ya_think
14 years ago
I agree it shouldn't be a big deal to open up the whole barring any major obstacles.

I also agree I've never seen a range hood with 4" exhaust. But I wonder why. Kobe has range hoods that put out a claimed 800 CFM through 6" pipe. 4" pipe is roughly .43 times the size of 6", so why shouldn't couldn't someone be able to pass 345 CFM through 4"? That would be fairly respectable airflow for an average kitchen.

Im not suggesting it would be ideal, but to have one available for difficult installs?

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