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Make-up air puzzle

Tyler Keniston | Posted in Mechanicals on

I’m looking for some advice on make-up air for a range hood,

Here is my situation:

1) A 30″ stove, and probably something in the 300 cfm range for a hood

2) A desire to remove the nasties i’ve read so much about

3) A desire not to waste unnecessary energy swapping air.

4) Keep it simple

5) I’m in climate zone 6a (Maine), in an old house but would like to do some extensive air sealing and so want the hood to be ready for that situation

My research has me slightly befuddled, and has me asking a lot of “why can’t I just…” type questions. So…

Why can’t I just install some intake venting/diffusers in some fashion immediately around the stove so that the vast majority of the cold (or hot and humid) air that comes in passes through the cooking zone, taking the cooking fumes with it, and up and out the hood. If this is possible, it would eliminate much of the energy penalty for conditioning the incoming air, and much of the complexity. I would even prefer not to have the make-up air powered, given especially that I am not installing a megatron of a hood.

The problem is I don’t have testing equipment or time to test where these intakes should precisely go… and perhaps there is nowhere they can go to accomplish what I wish?

Has anyone cracked the make-up air problem working with similar parameters?

Intuitively, the best place I can think to put intake would be on both sides of the range top built in under a stove side-overhang. (stove would have to be slightly higher than counter) Given that this may be functionally/aesthetically unacceptable, the next best place I can think would be vertically along the sides of the front of the range (surrounding the oven).

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Replies

  1. Tyler Keniston | | #1

    Perhaps another way to express my befuddlement is: why do so many solutions seem to involve make-up air that is so far away from the stove?

    It would seem to me we should start with an ideal, such as fume hoods that have been specifically designed by industry to remove highly toxic gases for scientists and workers. We would then work backwards until the functionality and aesthetics meets the consumer kitchen needs. That means the ideal is a space that is closed off but for a comparatively small inflow opening opposite the exhaust, with the contaminant source being in between those two points.

    So working backwards, first step is to realize we wont have sides that extend all the way down, but maybe we keep angled partial sides, and we keep a wall behind. Next, since we will have a larger intake zone that will allow for unpredictable turbulence and over-spill, we create flow by introducing air at a point outside the contaminant zone, creating a path from intake > contaminant > exhaust.

    But this is all a thought experiment and perhaps doesn't pan out...?

  2. Geir Gaseidnes | | #2

    I know there are people on here, including builders in Maine, who have had good luck with Broan's makeup air dampers. Considering for our own build (also in ME).

    https://www.broan.com/Accessories/Range-Hood-Accessories/Ducting-Dampers-Adapters-Caps/MD6T

  3. Ethan Foley | | #3

    Here is an excellent resource that I believe answers all your questions. A quick summary, with a small range hood you can use passive intake close to the exhaust source but your ducts are going to be big! And so will your toekick grilles. Range hood design matters a lot for capture efficiency. Angled sidewalls are a big improvement.

  4. Tyler Keniston | | #4

    Geir, that look's like what I'd need! thanks.

    Yupster, Thanks for that document, it had some great info.

    The parts talking about how much air a passive make-up duct can provide based on 2-3 kpa pressure differential between interior and exterior was illuminating.

    It got me thinking more about the dynamics of a vacuum and how suction rates (pressure differential) drops off dramatically as distance increases. The flow rate (velocity) drops off.

    Indeed there is no 'incentive' for the hood to 'pull' the air through all that intake duct (over, say, air from the other side of the kitchen, and eventually another near-by leak or chimney).

    Yet the pressure differential nearer the vent hood is undoubtedly higher than the 2kpa that we're shooting for across the entire house, and so why not take advantage of that localized differential. Why not force air through the source contaminants from a near-by location right into that higher differential, thereby creating a flow stream with increased continuity and velocity (that's what moves things after all).

    The idea of tying intake air in with the central HVAC seems almost ludicrous. Perhaps it works to relieve the pressure differential, but it seems to take no advantage of the velocity stream potential. Hoods depend on CFM not 'water-lift' like a vacuum pump. Additionally all the air would need to be conditioned, whereas a more localized and continuous stream would create (closer to) a closed loop.

  5. John Clark | | #5

    IMO you don't need make up air and typically code only requires it once you reach 600 cfm.

    300 cfm hood doesn't move a lot of air, especially when it's only intermittently used.

    If you insist on makeup air then search the BSC website. IIRC they have designs which show the make-up air vents next to/under the range.

  6. Expert Member
    Michael Maines | | #6

    Tyler, I believe that the best place to bring in makeup air for a range hood is in a toekick next to the range, as we did here: https://www.finehomebuilding.com/2017/05/12/make-air-range-hood. If you put the makeup air closer to the range hood it could bypass the fumes you are trying to get rid of.

    The IRC building codes require makeup air for 400 cfm or higher hoods. Even a 250 cfm hood running for an hour would use all of the air in a 2,000 ft² house, so for tighter homes I think makeup air is important. If your house is large or leaky, or if you don't actually plan to use your hood often, you may find that makeup air is not critical.

  7. Brian Pontolilo | | #7

    Tyler,

    If you are still considering your options, try this link and scroll down to the bottom of the article to get to the most practical advice:

    https://www.greenbuildingadvisor.com/article/makeup-air-for-range-hoods

    And here's a feature article from FHB on the subject:

    https://www.finehomebuilding.com/2012/11/08/how-to-provide-makeup-air-for-range-hoods

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