Range hood installation and dampers

Post date: 2019-11-15 05:41:51

Do I need an internal and external flapper on the ductwork for my range hood?

I am installing a new range hood to replace an old over-the-stove microwave/range hood combo unit. The duct vents straight out the back to the outside through a brick wall with a short wall-thick run (15" or so) of 3.5"x10" duct. There is a very good quality steel vent cap like this one on the outside wall that has a flapper built into it. The old microwave has been removed for a week now, and there is no air leakage with just the outside flapper, although there is some condensation in the duct as it is -15°C outside already.

Here is my question... the new range hood came with a really cheap and flimsy damper that attaches to the back of it, and because the old vent hole is in a just slightly different spot (0.25" too high) so the new internal flapper binds up. It occurred to me that I could just remove the internal flapper and be done with it, it really is a loose fit and not likely to do much to stop drafts anyways. My other option is to get out the metal snips and trim the flapper so it has an even looser fit, but at least won't bind. Cutting a new hole for new ductwork in the brick wall or replacing all the kitchen cabinets so there is an extra 0.25" of space are not options I will even consider. I've already replaced the over-the-stove microwave twice now (which is why we are going for a range-hood only now), and despite all claims of standard sizes, the vent holes on the back of the damn things are always slightly higher of lower than each other. Last time the difference wasn't enough to cause any binding though.

Do I need two dampers for such a short run of ductwork, or can I just chuck the cheap and flimsy internal flapper and make do with just the heavy duty and well-sealed external one? I live in Ottawa, Ontario if climate makes any difference.

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