condensation in my rear-venting over-the-range microwave

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Old 01-22-11, 09:00 AM
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condensation in my rear-venting over-the-range microwave

I have recently completed my kitchen remodel. During the remodel we re-arranged the layout such that the range is now on an exterior wall allowing us to install an over-the-range microwave with venting through the exterior wall instead of recirculating as we had before. This was great... until winter came.

Now that the temperature is regularly near or below freezing, we constantly have condensation on the back wall of the microwave interior. I also presume that there is condensation in the interior of the venting area between the back wall of the interior and the back wall of the unit. We're concerned that all of this condensation will not only cause mold issues, but may also short out the unit. Last night it got down to 8 degrees below zero F and we awoke to ice frozen to the back of the microwave. Needless to say, the convection feature of this microwave really is not working because it is not powerful enough to compensate. Which is a disappointment.

I installed the unit following directions. There is a thin metal damper on the rear of the microwave itself. Then it vents through about a 10" pipe through the exterior wall and a brick veneer. The cap (Home Depot variety) has a thin, spring-loaded damper. When operating the fan, plenty of air moves through the system and we haven't had a troubles with either of the dampers opening. Of course, the problem isn't when the fan is operating... it's of course when it is NOT operating. The vents appear to be closing properly... they just aren't insulating enough to prevent cold air from entering.

We do not have the option to vent through the roof. The kitchen is on a first floor with one of our 2nd floor bedrooms overtop it. Furthermore, the 2nd floor overhangs the first floor on this exterior wall making ducting a bit more complicated. So we either vent directly outside horizontally... or we extend the run up or down either on the inside or outside and then run directly outside horizontally.

Does anyone have a suggestion for how to deal with this condensation problem? We have had one person suggest extending the run on the outside of the house in a downward direction using some kind of insulated piping. The theory here was that by extending in the downward direction, you could use the fact that cold air sinks to provide some amount of resistance to enter the home. This of course would only work if the house wasn't under too much negative pressure. But I still wonder if that would do any good. The only way I can see to eliminate condensation is to either do a better job of blocking airflow when the fan isn't running or to extend the run such that the temperature differential occurs across a much longer length of pipe (which is what venting through the roof might do... but we can't do.)
 
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Old 01-22-11, 11:33 AM
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Hi jb,
To review, condensation forms when warm moisture laden air is cooled below its dew point and can no longer hold all of its moisture. In a two story house, there is a natural stack effect where warm air leaks out through various paths in the upper half of the building and replacement air leaks in through the lower half. So, there is probably a negative pressure in this problem area. An incense stick (smoke test) may help to confirm.

The other aspect you mentioned is the overhang, they are notorious for air leakage. Whether this is contributing in some way to this problem is unknown, but keep it in mind. In conjunction with the overhang, is there a drop ceiling above this microwave. You know, the box that usually fills the space above the cabinets. If so, that boxed in area is often open to the joists above, which in your case extend out over the wall and may be open to the outside via leakage.

The flapper style vents are never very good. There are a couple of dryer vents that seal better, one is a ball type and the other is a can style. I have never seen the can style, but the ball type can be found with a search.

In addition to addressing the vent itself, reducing air leakage in the upper half of the home will lower the negative pressure. But, turn on the dryer, your furnace or bath fans and replacement air will be looking for a way to get in.

You will need to do some testing to see if the cold is via air leakage through that vent pipe, conduction, or something to do with the overhang.

Bud
 
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Old 01-22-11, 05:51 PM
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I believe the condensation is mostly a result of cold air infiltration. If you remove the filter under the microwave and place your hand about 2 inches from the opening, you can feel a gentle flow of cold air falling down.

I don't know what we can do to seal up the perimeter of the house much more. The 2nd floor bathrooms do not have vents and we also have new high-efficiency windows.

I'm guessing our biggest cause of negative pressure in the house is the variable speed blower on our new heating system that runs at low speed 24x7. The duct work in the house is basic aluminum without any insulation or joint seals. So, I'm guessing air is leaking out of the furnace supply lines into the joist space under the 2nd floor. This air then finds its way to the exterior wall, around the insulation and out under the siding. If the rest of the house construction is similar to what I saw when I removed the soffits in the kitchen, there is no air/vapor barrier between the joist area and the overhang... just some R13 insulation shoved out there. I did completely drywall above and behind the cabinets before hanging them.
 
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Old 01-22-11, 07:09 PM
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I haven't looked for awhile, but I believe there are powered closures that could close and air seal when not in use.

Also, for reference, here is a link on air sealing. opens slow, but has a lot of info.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
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