Non-Ducted Make-Up Air for Kitchen Ventilation System
I see on every article that mentions makeup air for a range hood, the makeup is always shown as ducted to the kitchen, and then mentions you have to be careful where to place it so you aren’t blowing cold air on people. There are even expensive kits that come with a heater and ducts, silencers, etc. Is there a reason for this? What if I just put a motorized damper in the crawlspace and put some grills in the floor under the cabinets, kind of like Lstiburek mentions here: https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure#F01 ? Any downsides with this approach? Also, is there a guide for when I need a blower vs just a damper, and what size damper I should use?
I want to roughly follow this guys build, but his seems really overbuilt and expensive. He ducts the air from the crawlspace to the underside of the cabinets but doesn’t mention why you should bother. https://www.youtube.com/watch?v=bMkvCn1sGvk
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Replies
Best to use powered makeup air to maintain pressure balance.
In a simple hole in the wall passive case, you might use the ASHRAE orifice equation to estimate (some air will enter through other leaks) how much depressurization you will get. More complex makeup air paths will need to be much larger.
Q = 2610 * A * dP^.5
Q is differential air flow (supply/exhaust, units of cfm)
A is net open area in ft2
dP is pressure difference across boundary, in inches w.c.
0.0120559 inch/h2o = 3 pascals
Example: a 287 CFM kitchen fan needs a rather large 12" square passive opening for 3 pascals.
If the makeup air isn't delivered near the exhaust fan, then it creates a large heating/cooling load.
Depending on climate the make up air has to be tempered before it enters the house, otherwise it would be super easy to cut a hole anywhere and have a motorized louver on the same switch as the exhaust fan. That is why many of these systems will have an in-duct heating coil or similar.