Range hood installation and dampers
November 14, 2019 8:41 AM Subscribe
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.
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.
Best answer: Either option would work. You don't really need two dampers as long as the outside one is reliable. But trimming the internal damper with a pair of tin snips is also standard practice. In fact the installation instructions for some blowers require you to trim the damper slightly depending on whether you are connecting to a horizontal or vertical flue pipe.
Whatever you decide to do, make sure the dampers operate easily and smoothly when installed or else you will be impeding the airflow.
posted by JackFlash at 5:41 PM on November 14, 2019
Whatever you decide to do, make sure the dampers operate easily and smoothly when installed or else you will be impeding the airflow.
posted by JackFlash at 5:41 PM on November 14, 2019
Response by poster: So, in the end I decided to leave out the inside flapper as the outside one is a good one that seals completely and the duct is only 11" long. The crappy one that came with the range hood didn't seal at all and kept either jamming or falling apart, as it was just cheap and thin stamped tin. I checked at local hardware stores, and I couldn't even buy a better replacement if I wanted to. They didn't sell 3.5"x 10" in-line dampers separately from the range hoods, only round ones.
I checked local building codes, and while a damper is required there is no specification on whether it is at the start or end of the run, it just needs one to stop intrusion of outside air.
posted by fimbulvetr at 12:48 PM on November 18, 2019
I checked local building codes, and while a damper is required there is no specification on whether it is at the start or end of the run, it just needs one to stop intrusion of outside air.
posted by fimbulvetr at 12:48 PM on November 18, 2019
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posted by The_Vegetables at 10:35 AM on November 14, 2019