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Question DetailsAsked on 1/18/2016

water leaking from vent into microwave

Its winter and -4 outside, when I start up my microwave I started to here a drip, so I looked at the vent above and noticed water. What could be causing the water to make it into the vent. Should I call a roofing company?

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1 Answer


While it coudl remotely be a roof leak at the vent hood (assuming it vents through the roof), not likely at those temps.

Saying it is -4 and it happens when you start it up means it is almost certainly condensation frosting up in the ducting, which then melts when you run the fan. Could esaily be, at those temps, just household warm moist air frosting there due to the continual minor airflow up the duct (by convection); if heavy and causes more than the occasional drip (like more than a small washcloth can easily soak up) then could mean that the damper (flapper) in the outlet hood is stuck open with lint, rust, frost, etc and letting a LOT of household air out.

Running the fan more often to keep frost from building up (though it wastes energy) can reduce this - and you need to run it for a good 15 minutes or more at a time to let it evaporate out the moisture in the duct as it melts it.

Solution - if the flapper (aka damper) is stuck open (commonly you can see it from the ground with binoculars or blown-up tablet photo - is a relatively flat-lying metal plate insidse the duct mouth, generally can see it open upwards when the fan is turned on, then drop back down onto a sealing lip in the hood when the fan is off) then a roofer or a Heating and A/C contractor can fix it - I would go with a Roofer because if the flapper it shot due to rust he is best qualified to replace it and tie it into the shingles and roofing correctly. Commonly looks like this, though sometimes inside the duct itself like the second image -

If just frost in the ducting because it is cold, then a Heating and A/C contractor can (assuming your vent ducting is accessible) can wrap the ducting in insulation to minimize the frosting, though you still need to run the fan at least daily in very cold weather to minimize this.

BTW - if your kitchen fan ducting goes out the wall rather than up through the attic to the roof, see if the contractor who insulates it can modify the run so it reaches a high point right above the fan unit, then slopes downhill to the outside wall vent hood - so any frosting/condensation in the ducting runs to the vent rather than to the microhood. Be sure he exte4nds the hood out from the wall or makes sure the leakage will not run down the wall and rot and discolor it.

Now safety - see if you can see up into the unit with a flashlight - if the moisture is not free-falling through the fan and vent to the countertop or stove or is hitting the fan motor, it may be accumulating in the microwave and could short it out - so I would not use the microwave until the problem is fixed, then ask the contractor to turn it on to dry out the system.

Answered 4 years ago by LCD

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