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Cold air pouring in Microhood vent over range.

Post in 'The Green Room' started by Seasoned Oak, Dec 3, 2011.

  1. Seasoned Oak

    Seasoned Oak Minister of Fire

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    Any ideas from forum members?
    Got a convection microhood over a range. cold air pours in through both air doors,one at the back of the range hood and the other on the exterior of the wall. Both doors have springs that hold them closed when not in use but a lot of cold air gets by anyway. (poor design)So much so that the air condenses water drops on the back wall of the microwave interior. In the morning theres a puddle of water in the microwave. Possibly why the magnetron and capacitor had to be replaced after only 6 months. As the house is a 2 story air likes to chimney here that is come in on the first floor where ever and Warm air exit on the 2 nd floor from where ever.
    After several years of this i am considering closing up the whole thing and using the indoor vent feature.

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  2. DickRussell

    DickRussell Burning Hunk

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    You might look at using a motorized damper so as to have a positive seal when the fan is not in use. Here is one by Broan: http://www.broan.com/display/router.asp?ProductID=100744
    Of course you need the space to put the damper in the duct, and that depends on your configuration.
  3. yooperdave

    yooperdave Minister of Fire

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    you need to check out the outside termination of the micro wave vent system. this should also have a damper that is free and working...and a bird screen
  4. Seasoned Oak

    Seasoned Oak Minister of Fire

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    Yep got a spring loaded door there as well , not a very good seal there even when its working properly.
  5. Seasoned Oak

    Seasoned Oak Minister of Fire

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    The vent goes directly thru the outside wall about 6-7" total wall thickness and is about 3" by 10" rectangle.
  6. begreen

    begreen Mooderator Staff Member

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    I'm wondering if this is negative pressure. If you crack open a window .5" next to the stove, does the cold air influx at the oven stop?
  7. laynes69

    laynes69 Minister of Fire

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    I had problem after problem with cold air entering ours from the microwave over the stove. I ended up removing the microwave, pulling a couple of screws and rotating the blowers. We bought a charcoal filter and I plugged and insulated the old hole. Ours was so bad it felt like an open window. Our microwave had 2 models and both are the same, it's just how the blowers were configured. I tried magnetic covers, and I replaced the exterior damper but it didn't work well.
  8. semipro

    semipro Minister of Fire

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    There are some better dampers out there that do seal well. I found an inline version with neoprene seals. Search for "Aldes 6" Backdraft Damper - BDD6". That's just one brand. There are others. If you can't install an inline unit I'll be you can find a better sealing rectangular unit.

    As much as I had air leaks in my house I hate non-venting range hoods even more. The charcoal filters do little and get saturated very quickly, then cost to replace. The pervasive smell of cooked bacon hangs around a long tiime and gets old.
  9. semipro

    semipro Minister of Fire

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    duplicate post deleted.
  10. DickRussell

    DickRussell Burning Hunk

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    I see only two possibilities here. One would be to find a better wall cap for the outside (the least amount of work). I selected this one for ours a year back, the Broan 639 rectangular: http://www.broan.com/display/router.asp?ProductID=2927

    It is a well made unit, with screen and backdraft damper that seems to close fairly well. It's size (3.25x10") seems to match what you have.

    The only other thing you could do would be major surgery on the wall, turning the fan outlet down 90 degrees and to the basement (forget this option if there is no basement), where you may have more roof to transition to a motorized damper before sending it to the outside. I suppose if you wanted to sacrifice a cabinet above the microwave you could use that space for the damper, if things would fit.

    The root of the problem, beyond dampers that don't close well (ones built into range hoods are useless, from what I've seen), is a house that is too leaky. As you've surmised, cold air leaks in at lower levels, below the neutral pressure plane, while warm air above the neutral plane leaves through myriad cracks and holes in the thermal envelope of the house. A thorough air-sealing job on the house could reduce the extent of the "chimney effect" through the house, and make the vent leak less of a problem. Sealing would be done, using can foam and caulk at every conceivable air leak, much of which may be addressed under attic floor insulation where wiring passes through the top plates of wall partitions and where lights are hung. Often there is a lot of leakage around a chimney, and sealing around this should be done with sheet metal and fire-rated caulk.

    Vent fans in the kitchen often do create cold spots around the range, for the reasons you've noted. This is an area that doesn't get enough attention in designing a new house. Worst is a fan that is ducted up and out through the roof to a cap with a simple flapper. Next is straight out also to a cap with simple flapper. Better still is a path that goes down before going out, as long as the house is reasonably tight, the reason being the downward path creates a cold air trap that resists inflow when the fan is not in use. Best is any path that involves a tightly-sealing motorized damper wired into the fan.
  11. Seasoned Oak

    Seasoned Oak Minister of Fire

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    I think Its the pressure imbalance that is causing the majority of the problem cuz it really comes in like crazy when its colder outside. Add to that this is a 100 year old house.Im really thinking about just going with the indoor vent option on the microhood. Only thing stopping me is the wife is asian and cooks some types of fish that smell up the whole house from time to time. But i do have a whole house fan i could employ in those cases.

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