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BigRedFan 10-26-2012 12:32 PM

OTR Microwave Venting
I am planning on venting our new microwave outside. Currently we have no venting so anytime we do any searing on the stove or any hardcore cooking there is no ventilation.

Since our range is on an interior wall my plan is to go vent straight up into the attic and then instead of going through the roof, run the ductwork ontop of our blownin insulation through the aluminum siding on the back of our house.

my question is what kind of wall cap do i need for a microhood?

BigRedFan 10-26-2012 12:39 PM

Also what tools will I need:

6" HOle Saw
Duct Tape

Evstarr 10-26-2012 01:02 PM

Why not just go up and 90 right out thru the kitchen wall?
No cloth duct tape. Mastic or the metal tape!
Wall cap should have an insect screen and a flapper of some kind to close it when not in use to keep the cold out.

Love Neil Stephenson's books.

BigRedFan 10-26-2012 01:09 PM


Originally Posted by Evstarr (Post 1038376)
Why not just go up and 90 right out thru the kitchen wall?
No cloth duct tape. Mastic or the metal tape!
Wall cap should have an insect screen and a flapper of some kind to close it when not in use to keep the cold out.

Love Neil Stephenson's books.

that is what I will be doing. It will be up to the attic and then out immediately to the side of the house which is about 10ft

BigRedFan 10-26-2012 01:10 PM

will i need crimpers or can I get away with using some sort of coupling pieces instead?

funfool 10-26-2012 02:38 PM

Pretty sure your microwave will give you options to exhaust out the back or the top like you want. It will have a knock out in it that you remove and attach a flange to it, comes the oven.
This will get you into the cabinet. you then need to get a boot to attach to the flange.
Just a guess that the flange will be 4"x12" I forget, but get a boot that connects to it and goes to 6" round., read the directions, have seen some high end units that require 7" duct.

With 6", they make a flexible aluminium duct, they claim it is 8' long when you stretch it out, ends up being 6' :( You can use this to go straight up through the cabinet and make a gentle bend in the attic to point it in the direction you need. I section of duct will not get you the 10'.
So will need a 6" coupling and another section of flex duct.

No special tools needed for the duct work, just depends on the siding of your house and how you will mount the wall cap.
A 6" hole saw will not help you, you can cut a square hole and wall cap will have a flange on it. That will be the tricky part sealing the flange.
In a perfect world, wall cap goes on first, aluminium siding goes over and around it.

BigRedFan 10-27-2012 08:49 PM

I have used the rigid duct for most of the duct work but there is a few sections where using the flexible stuff would really help

is there any problems with that??

funfool 10-27-2012 09:40 PM

No problems using it. Others may have opinions on it. It is what I would use.

You can hard pipe it, the real difference I think is in the air flow.
If you look at the flexible duct I am suggesting, it has ridges down the side.
This will slow down the cfm airflow. Will be such a small amount you will never notice.
If you hard pipe it, will be enough turns and elbows in it, will actually slow down the cfm more.

You could flex pipe it and be done in less then one hour.
Hard pipe it and take 4 hours and 3 trips to the hardware store.

The flex is actually designed and made for what you are doing, I would use it.

hvac122 10-28-2012 07:17 AM

Flex is against code for a range vent. Also no screws allowed for joints.

Its A Good Life!

beenthere 10-28-2012 07:39 AM


The flex is actually designed and made for what you are doing, I would use it.[/QUOTE]

No it isn't. It has no rating as a range vent duct. And is against fire code.

ben's plumbing 10-28-2012 07:49 AM

do not use flex duct for venting microwave.. as already mentioned its against code..... 2 nd and 3 rd that motion....ben sr

oh'mike 10-28-2012 08:04 AM

Depending on your wall structure---and whether you want to run the duct inside the wall or inside the cabinet--

You can use 3 1/4 x 10 inside the wall or cabinet---then switch to 6" round when you get up into the attic--
These are easy to add a finished woo box around---you don't want to see the tin work inside your cabinet--

There are lots of heavy duty vents available----stay away from the flimsy ones designed for dryers---

funfool 10-28-2012 10:00 AM

1 Attachment(s)
I have not ever had a inspector complain about it here.
Agree that it may be against code in some areas. Especially in multi unit apartments where stricter codes are required.
This is same product sold to exhaust dryers. I have heard of more fires caused from plugged dryer vents then a microwave.
If it is against code in your area, by all means you should go code.

beenthere 10-28-2012 11:01 AM

From hart&Cooleys website. Their one of the manufacturers of flex duct.

Can Flexible Air Ducts or Connectors be used to vent clothes dryers?

No. The International Mechanical Code & Residential Code state that "Exhaust ducts for domestic clothes dryers shall be constructed of metal and shall have a smooth interior finish...." Additionally, most if not all appliance manufacturers also state this in their installation instructions. Often, the clothes dryer warranty is void if it is not vented properly. None of our flexible duct products can be used to vent clothes dryers.

So if your venting a drier with flex duct, your against code. Your area may not enforce many codes.

funfool 10-28-2012 12:10 PM

Maybe a better topic for general discussion.
Go to the hardware store and buy a dryer kit, is 4" flex duct with a couple clamps and a few other worthless goodies.
Have a brand new washer and dryer delivered and installed, installers will connect it with a flex duct.
95% of dryers I need to remove, connected with flex duct.

I could see where the manufacturer may make a disclaimer for some reason, cover liability.
But it is packaged and sold for this purpose.
Real question is it code in your area? I would bet a large percentage of inspectors also have their dryers connected with flex duct and out of code also then.
Is no excuse for me to offer bad advice, if not legal.

Back to the microwave, and final co inspection. I like to build a chase around the vent for a finished look, many times there is not.
They never had said anything about it. I do not use screws, just metal tape. I have never seen them check for that either, maybe just a grey area that they are not concerned with.

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