OTR Hood/Microwave Vent Details for Exterior Walls
Have noticed an issue on a few homes where ice has built up in the back of over-the-range microwaves that are vented to the exterior and are installed on the venting wall. We install a sealed metal box on the wall behind the microwave, and place insulation between the exterior and interior faces of the box. It’s dampered and back-drafting hasn’t been an issue – it’s simply a hole in the insulation envelope that the cold gets through, to the extreme of freezing condensation inside the MW. I could up the insulation, but then the vent won’t be sized right.
I’ve looked for a better detail for this and haven’t come up with anything that seems to tackle it. My expectation is that it can’t be prevented entirely in the worst cases, but I’m seeking something that reduces the odds.
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Replies
The standard hood went hole is way oversized for the flow coming out of an OTR micro. I've had no issue with going down to a 5" round duct. I make an adapter from 3.25x10 endcap and ovalize the 5" duct to fit. A foam gasket around the perimiter to seal it up against the micro. Never had an issue with freezing in Toronto.
Most OTRs have some strange air bypass around the blower, basically they take some of the air behind the micro and pull it through the motor for cooling, be careful with what you plug up behind the OTR.