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Range Hood Exhaust: How Duct Size and Elbow Affect Air Flow

Trevor Lambert | Posted in General Questions on

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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Replies

  1. Expert Member
    Akos | | #1

    At 300CFM, the elbow and reducer won't add much. The elbow is going to be around ~0.15" wg, the 6"->8" transition is pretty much zero.

    At 700CFM, it is a different story though. At that flow rate, you want to go straight with 8" and no 6" duct or fittings.

    You can always use a motorized damper, these tend to seal much better than a most one way dampers. I don't know how well those hold up with grease buildup in the winter, even my budget factory one way with lots of clearance around it takes a while to open when cold.

    1. Trevor Lambert | | #2

      Can you link me to one of those motorized dampers? The ones I've seen look like they're just meant to limit the air flow, not cut it off, as such there's a huge gap all the way around the damper. No positive closure whatsoever. And the motor is only used to open the damper. The closure is just a spring, which really makes it seem completely pointless compared to a passive spring return model. I've seen at least one passive model that has a sealing gasket around the edges.

      1. Expert Member
        Akos | | #3

        The hvac ones usually have a foam seal the damper seats against when closed, at least both Honeywell and Aprilair do. Not sure how well these would hold up to grease as the foam will probably come loose.

        You would probably have better luck with flue dampers. These are powered open, powered close. I think there is also a version that has a rubber gasket on the damper instead of the stainless steel flex tabs.

        https://www.fieldcontrols.com/gas-vent-dampers/

        P.S. Found the rubber seal version:

        https://www.fieldcontrols.com/wp-content/uploads/2019/04/4367_TightSeal_SalesSheet.pdf

        1. Trevor Lambert | | #4

          The datasheet for the rubber seal one states it's power open, spring close. The other is probably the same, but I'd rule it out either way based on poor sealing.

          1. Expert Member
            Zephyr7 | | #5

            The power actuators use a much beefier spring than the ones that are operated solely by air pressure. The power actuators will seal much better as a result. The reason is that the spring in the air pressure operated dampers can't be very strong since it can't overpower the slight air pressure that needs to be able to open the damper. A power actuated damper's spring can be as strong as the actuator is capable of driving, and those actuators can be surprisingly powerful.

            Bill

          2. Expert Member
            Akos | | #6

            The ones I have are power close, same for the standard version. But as Bill says, the spring close do have a much much beefier spring than a butterfly check, I wouldn't worry about. Find one with rubber seals right on the damper gate, it will seal better than any spring one way.

            https://www.fieldcontrols.com/wp-content/uploads/2019/05/46352700_GVD-4-12_Inst_Sheet_Rev_H_10-15.pdf

            Here is their fresh air damper, which looks like a good fit for this, also way cheaper.

            https://www.fieldcontrols.com/wp-content/uploads/2019/07/780500200-FAD-Submittal-Sheet-Rev-C-09-18.pdf

          3. Trevor Lambert | | #7

            I admit that fresh air damper looks to be better than the best spring loaded ones I've seen. I'm just not sure it's better enough to justify $250 extra. What would the ROI on that be? Also need to use a step down transformer for the control, a minor annoyance. Burying it in the wall might not be the smartest idea either, and there isn't really anywhere else it would fit.

  2. Expert Member
    Akos | | #8

    Unfortunately like a lot of green ideas, the ROI on this is pretty much never.

    I do like the idea of a positive seal plus the lack of damper flapping in the wind.

    I would definitely mount it inside to be serviceable. If you are going with the 6" out the top, you can get a 6" damper and mount it right at the elbow. If tight on space you can usually trim a fair bit off the damper body as well as the elbow fitting to get it more compact. Since the unit is $90US, I'm sure it or a comparable unit can be found locally for a more reasonable cost.

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