what's the minimum size for a range hood duct?

olreaderMarch 18, 2014

We have a microwave with a fan which blows the exhaust back into the room and does nothing about the smoke/odors. We want to
1) put in a duct to the outside, either to the side or to the roof
2) get a real hood

But starting with #1, I'm worried that we might put in a duct that is too small or otherwise ineffective. Any advice?

Because of an eave they might want to run the duct back towards the window over the sink, so that would be another bend. It's a 2 story house with a bathroom above and I think venting all the way to the roof would be complicated. I'm not too worried about a metal duct running on top of the cabinets but that's something to think about too. And this is Colorado with a well insulated house, so maybe we would have to open a window for make up air while cooking? Not a big deal even in winter I think.

This post was edited by olreader on Tue, Mar 18, 14 at 16:35

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It depends on the fan and the length of the ducting. See the install guide for the fan you are considering.

There is something called "equivalent feet" in ducting. For example a duct run that is 20 feet long with 3 or 4 elbows or turns in the ductwork will not perform the same as ductwork that is 20 feet straight. Each turn or elbow (depending on its configuration and actual angle) has a mathematical value of its equivalent feet. These figures are all added together to get the ducting total equivalent feet. That is what should be used to make sure the duct is long enough for the fan.

The need for makeup air will be determined by the cfm of your fan and local codes. If you want to avoid the extra expense of adding makeup air, then keep the fan size below where it is required. Check with your local building department for this info.

    Bookmark   March 18, 2014 at 6:58PM
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I just put one in, nearly exactly in your configuration. I ran through the top of the cabinets and out the wall. Mine is 8" round duct, but it is 900cfm on high. I believe many residential hoods are only 6" ducts. Mine was too large to run above the cabinets and still be hidden by crown, but goes through top of 42" cabinets quite nicely, and I cant reach that top shelf anyway, so I didnt mind giving up the cabinet space. I just couldnt use glass front cabinets in that stretch. Also, having it through cabinets may make it come out low enough that your eave isnt an issue.

    Bookmark   March 18, 2014 at 8:48PM
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What Julie said. First choose the vent hood you want. Then look at the specifications for it. It will explain to you exactly what Julie did, only with figures.

A.J. Madison is good at having installation guides for appliances they sell. In fact, they are better than some manufacturers websites.

Here is a link that might be useful: A J Madison

    Bookmark   March 18, 2014 at 9:17PM
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Thanks for the good answers, I found OOTM's experience encouraging. If 6" or 8" is all we need I think we can find a place for that. We were getting some bids for other work last week and asked about getting a duct put in. That was a lot of progress, we have been thinking about a hood since we moved in 4 years ago.

We make decisions very slowly around here....we will probably pass on getting a duct put in now, and maybe eventually we will start looking at what hood to get...

    Bookmark   March 21, 2014 at 11:04PM
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