Range Hood Exhaust: How Duct Size and Elbow Affect Air Flow
I am figuring out how to convert my recirculating range hood to a proper range hood. I am going to use an external fan to reduce noise. I already have a fan that I’m not using, which is similar to this one, but 8 inches: https://www.amazon.ca/gp/product/B07FJMFDF1/ref=ppx_yo_dt_b_asin_title_o05_s01?ie=UTF8&th=1
The range hood I’m converting has a 6″ port on the top. The most obvious solution is to attach duct and an elbow to this to go out through the wall, increasing the size to 8″ at some point. Another option would be to cap off the top, and cut an 8″ hole in the back and duct it directly out the wall, saving the elbow and the reduction to 6″ duct. Is this worth it? I’m only planning on running this at about 300cfm, and while the fan can theoretically deliver over 750cfm at max, obviously the higher it has to work, the louder it’s going to be. The question I have: is this extra work worth it? I don’t know exactly how much more work it will be.
I was planning on mounting the fan outside and building a rain shroud for it. Rigid metal duct with two spring loaded back-flow flaps inside the wall, which is about 24″ thick. I figure that makes the fan the most accessible for service and cleaning. Open to suggestions or better ideas. I’d kind of like to have the duct made of plastic to reduce thermal bridging, but that’s probably not significant enough to worry about.
Here’s a crazy idea that just popped into my head. What about an air tight duct cover on the inside, connected to a microswitch that turns the fan on when you open it? Have to figure out some kind of remote opening mechanism, so that’s probably not worth it.
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