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Old 01-04-2011, 12:00 PM   #1
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Microwave roof venting


We recently had our roof replaced with metal roofing. Prior to that time, we did not have any issues with our over range microwave vent. Since then, I have had water in the microwave. Not just a few drops, I mean cups. The first time was after a hard rain and the roofers resealed the vent. The most recent time was when the outside temps were in the teens. I used the microwave for maybe 10 minutes and water started gushing out of the openings in the top. The roof vent has been checked and sealed numerous times. The only thing that changed is they put an extension on the vent, approximately 2'. Previously the vent cap sat flush on the roof. The length of the vent pipe is about 4' from the microwave to the roof. Could this be a condensation problem, due to the extended length of the vent pipe or to I have a roof problem that has not been properly diagnosed? The installer is aware of the problem but they have not found a solution yet. Should the vent piping in the attic be insulated?

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Last edited by grandmalois; 01-04-2011 at 12:14 PM. Reason: add more information
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Old 01-04-2011, 09:11 PM   #2
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Microwave roof venting


1. What type of vent was installed ?(rooftop)Explain what it looks like, or take a picture from the ground or have someone familiar with heights take it for you.
2.What type of sealant was used ? (type,brand)
3.What type of pipe was used for the extension ?
4. How was the pipe connected ?

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Old 01-05-2011, 12:27 AM   #3
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Microwave roof venting


Wouldn't be a condensation issue. Pictures would definitely get you a long way if possible.
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Old 01-05-2011, 12:45 PM   #4
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Microwave roof venting


First off, let me say, I am not impressed with the vent. It looks like crap. That will be addressed when the weather is better. At first he had a chimney type cap but we told him it had to be a flapper vent. The pipe is standard duct and it was sealed with clear silicone. I have been in the attic and the above roof duct was inserted inside the ducting in the attic. The roof is south facing is that matters.

The photos might not be very helpful since I couldn't get any closer. It's hard to tell from the photo, but there is about 3" of snow on that part of the roof. Any help will be appreciated since the roofer doesn't seem to know what he is doing. He was adamant about not having the vent flush mounted because of snow buildup. As I said earlier, we never had a problem with the flush mount before and we have lived here for 25 years. I wonder if the other roof vents are improperly sealed and don't know it.
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Old 01-05-2011, 08:44 PM   #5
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Microwave roof venting


With the flapper do you mean it is a damper vent style with a baffle that opens when the exhaust is initiated and closes when the fan is off?
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Old 01-05-2011, 11:24 PM   #6
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Microwave roof venting


Why does a microwave need to be vented externally?
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Old 01-06-2011, 10:18 AM   #7
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Yes, it is a damper style. I can hear it closing after the microwave has been running.


The reason it needs to vent externally is because it is a combination microwave/range hood.
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Old 01-06-2011, 05:19 PM   #8
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Microwave roof venting


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Yes, it is a damper style. I can hear it closing after the microwave has been running.


The reason it needs to vent externally is because it is a combination microwave/range hood.
Personally, I would never put a range hood exhaust through the roof....I would put it through an exterior wall. The last thing you want is kitchen grease all over the roof.
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Old 01-06-2011, 05:28 PM   #9
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Microwave roof venting


Ya, I know, but the way our house was constructed - piecemeal - the wall venting is not an option. There is not room to run the vent because the kitchen portion was added to the original house - at a 90 degree angle - and the old roof and soffit were never removed. Therefore, there is no space for a wall vent. Not my choice, it was that way when we bought the house. And grease isn't a big issue for us because we rarely fry anything.
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Old 01-06-2011, 05:31 PM   #10
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Microwave roof venting


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Why does a microwave need to be vented externally?
The microwave is mounted above the stove and has a built in vent hood to exhaust the fumes from the stove.
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Old 01-06-2011, 07:08 PM   #11
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Ya, I know, but the way our house was constructed - piecemeal - the wall venting is not an option. There is not room to run the vent because the kitchen portion was added to the original house - at a 90 degree angle - and the old roof and soffit were never removed. Therefore, there is no space for a wall vent. Not my choice, it was that way when we bought the house. And grease isn't a big issue for us because we rarely fry anything.
Do you know whether there are code issues with this setup?
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Old 01-07-2011, 05:42 PM   #12
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Microwave roof venting


The style of oven you have isn't like the ones from years ago that would create an issue of grease on the roof.,the oven you have is designed to go thru the roof.,with the technology of today the air/exhaust wil be much cleaner.The older models have the fans that would only remove about 70% of the exhaust and you can guess where the other 30% goes? throughout your house and on your ceilings cabinets etc.,Majority of the ovens like yours are designed to exhaust thru the roof.,I have seen some go thru the wall to the exterior but if you have concerns about any exhaust or grease issues then imagine what it would do to your siding,soffits, besides when the exhaust is being forced vertical naturally gravity and pressure take over and heat always rises,in my opinion you have a much better chance of maintaining a better airflow vertical rather than horizontal because depending on your climate (wind etc) some of that exhaust could push back into your oven/house. Your oven has forced air from the exhaust so when you turn the oven on the damper vent will open and when its turned off it closes.,Unfortunatley you have a situation with the pipe.,Has anyone got into your attic to see where it is coming from? To try and guess is pointless,O.k so you agree that since it has already leaked and a controlled leak is the obvious course of action.,PROVIDING it isn't below freezing have someone get into the attic in the approximate area of the leak.,then have someone on either the ground or the roof PROVIDING no ice/snow etc with a garden hose and create a simulated rainfall.,I would not recommend spraying directly down but create a shower.9 times out of 10 you will find your leak.And if you have as much liquid coming out of your oven I would refrain from using it until the leak is corrected because like you know water and electricity refuse to be friends.

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Old 01-08-2011, 02:35 PM   #13
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Microwave roof venting


I have to agree with the roofer on the vent extension. How good does a flush vent work when you have 8" of snow on the roof ?
I think your statement that nothing has changed besides adding a 2' extension is a false assumption. Have you crawled into the attic to check that area ?
And I would say that yes, your duct going through the attic should be insulated. A stove vent would have high moisture and your attic temperature will be equal to outside temperature. Warm moist air hitting cold steel vent equals condensation. However its difficult for me to figure how you could possibly vaporize "cups" of water in 10 minutes.
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Old 01-08-2011, 03:20 PM   #14
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Microwave roof venting


Just my two cents, but I would say that you definitely have a condensation issue. A roof leak would more likely pour water into the attic rather than your microwave. The extra pipe is probably only a part of the problem. The biggest culprit is likely the damper in the huge, ugly cap. As soon as you turn the fan off, the flapper closes and all the hot moist air is trapped inside the duct and cap. As the air cools the moisture condenses and falls back inside the microwave. My microwave/hood combination has a damper as well, but it is located at the microwave, allowing the warm air to continue to rise naturally when the fan is turned off. An interim remedy might be to let the fan run for ten minutes or so after cooking.
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Old 01-08-2011, 05:41 PM   #15
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Just my two cents, but I would say that you definitely have a condensation issue. A roof leak would more likely pour water into the attic rather than your microwave. The extra pipe is probably only a part of the problem. The biggest culprit is likely the damper in the huge, ugly cap. As soon as you turn the fan off, the flapper closes and all the hot moist air is trapped inside the duct and cap. As the air cools the moisture condenses and falls back inside the microwave. My microwave/hood combination has a damper as well, but it is located at the microwave, allowing the warm air to continue to rise naturally when the fan is turned off. An interim remedy might be to let the fan run for ten minutes or so after cooking.
Interesting point about the moist hot air after the fan is turned off...

Looking at the damper, it looks exactly like the model that HD sells for $35. I have one, and it's supposed to be used as a dryer exhaust vent. The different with the dryer exhaust is that by the time the fan stops, all the moisture from the dryer has probably been evacuated.

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