# Venting over the range exhaust



## swindmill (Feb 8, 2016)

As part of my kitchen remodel, I'd like to go ahead and vent my OTR microwave outside. It is not an exterior wall, so I will being going straight up, or as straight as I can go depending on rafter locations. The kitchen has a sloped ceiling with no attic above it, so I'm hoping I'll just need to go straight though drywall/insulation/roofing. I know generally how to do it, and I've done some Google research on the process, but I'd like to ask a few things before I start. First is supplies, which I will go ahead an pick up; I'd like to do this on the coming weekend. I believe I'll need a standard aluminum roof vent, 6" round ductwork, some length of 3.25"x10" to attach to the OTR, and a transition piece. Luckily, I did save the piece that came with the OTR that I'll need. The cabinet above the microwave is 30"x15"x30", but because the ceiling is sloped, it does not reach the ceiling. I plan on using the transition inside the cabinet, and then going out of the top of the cabinet with 6". I may need a couple 90 degree elbows to avoid a rafter. Then through the ceiling/roof and into the roof vent.
So, I'm thinking I need: applicable duct work, roof cap, and I assume roofing tar. The roof is asphalt shingle.

For the process, I'm thinking I locate the spot I'd like to go through the ceiling, remove a small cutout of drywall to ensure there's nothing in my way (electrical, etc.). Drill straight through the roof with a small bit. Get on the roof and cutout a 6" hole. slide the 6" duct through the roof and into the kitchen. Install roof vent per manufacturer instructions, using roofing tar to make it water tight. Then it's just a matter of running duct work inside and connecting to the OTR. I'm not sure how or if I'll hide the ductwork. The cabinets will be semi-custom. They're frameless, so I'm making all the fronts, end panels, and some shelving and odd sized cabinetry. I could enclose what's visible in matching wood, but I guess I can make that decision when I see what it looks like. I know it'll be difficult to have a clean exit where the duct goes through the drywall ceiling.

Any comments or suggestions are appreciated. I just need the confidence that I'm doing it right before I start cutting a hole in my roof.


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## nealtw (Feb 8, 2016)

Find the rafter first, if you are as luck as I am it will be right in the middle, go to the closest side. Two adjustable bends one at the ceiling, one off the boot and just cut the hole in the top of the cupboard off center to fit.
If you end up with a bigger hole in the drywall than you wood like make a sheet metal ring or cut a hole out of a piece of sheet.
The normal is to build the box and drywall it for the fires top and you and cover that with anything even just a matching veneer.
You want a dedicated 15 amp outlet in the back of the cupboard. That is also a good place for the outlet for low voltage lighting too.


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## frodo (Feb 8, 2016)

i use a 24'' long bit,  in my drill,  i drill thru sheetrock at the point i desire,  then put a level on the drill ti guide it up to the roof.

drill thry roof.

using the hole as a center mark,  place duct on ceiling,  draw around it   cut a neat round hole

stick something thru hole in roof to flag it

go on roof, find flag.  mark round hole.  cut shingles, wood everything,  

install pipe thru hole, cap, collor, flashing

buy 2 collars,  use the 2nd collar on ceiling as a beauty ring,to cover rock edge


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## swindmill (Feb 8, 2016)

Thanks of the responses. The diagram is helpful. What type of roof cap do I use? I was thinking the same kind that is used for bathroom exhausts (aluminum box-looking piece with vents around it).


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## nealtw (Feb 8, 2016)

different fan but good info
[ame]https://www.youtube.com/watch?v=8TFc0Trl9fw[/ame]
[ame]https://www.youtube.com/watch?v=9egwOazPJis[/ame]


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## swindmill (Feb 9, 2016)

It looks like, depending on the power rating of my OTR, I may not want to use the type of box cap I was considering. I probably need to find a proper supply store, as the big box stores seem to have a limited selection from what I could tell.


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## nealtw (Feb 9, 2016)

He was moving a lot of air, this is what we see most of the time.


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## swindmill (Feb 9, 2016)

OK. I probably won't be moving nearly as much air with an OTR microwave. The pictured vent cover is what I had in mind.


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## nealtw (Feb 9, 2016)

Bigger duct is less noise., 6" if you can.


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## swindmill (Feb 9, 2016)

I was planning on 6". It seems the only downside to a bigger duct is less cabinet space, but I can live with that.


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## nealtw (Feb 9, 2016)

That cupboard is pretty useless with what ever size duct.


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## swindmill (Feb 9, 2016)

That's unfortunate, because I really need all the storage space I can get in this kitchen. But, I don't see an alternative. The recirculation method doesn't seem to be too helpful.


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## nealtw (Feb 9, 2016)

If you are on an outside wall you could straight out the back. But that is different set of pains.


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## swindmill (Feb 9, 2016)

I'm not on an outside wall, unfortunately. Straight up is the only way to get outside from the OTR location.


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## nealtw (Feb 9, 2016)

I saw one installed, I am not sure what they were trying to get around but they Raised the cupboard another 3" or so and brought it out 4 " the duct must be rectangle run against the wall, but there was no duct in the cupboard.
Then they put a cornice molding around the top of that cupboard..
Not sure if they were after a better look, saving space or working around a problem but it looked good.


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## swindmill (Feb 9, 2016)

The duct coming out of the OTR is 3.25"x10" I believe. I could take that up through the top of the cabinet and then convert to 6". That would seem to save space. And if needed, I can make a molding on top to cover the ductwork. That may be the way to go as I think the transition to 6" round will take up the most space.

I don't think I can bring the cabinet out 4", but when I have them, I could look into that possibility. It will depend on how much it seems to be in the way of the cooktop.


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## nealtw (Feb 9, 2016)

When I did the suite I am in I bought used cupboards, it has that hole in the top, I didn't do that, but that is what they had. That would save about an inch of space, the inch in the back doesn't count.


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## slownsteady (Feb 9, 2016)

If the unit has a 4" outlet hole, there's not much point in putting a 6" pipe on it. The bottleneck is already established. Did the vent come with instructions? They will tell you the proper pipe sized for the unit. If not, you can figure it out from the CFM.

Same thing with the square pipe. That guy was talking specifically about his product. And his main point was sweeping turns instead of hard right angles.


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## nealtw (Feb 9, 2016)

slownsteady said:


> If the unit has a 4" outlet hole, there's not much point in putting a 6" pipe on it. The bottleneck is already established. Did the vent come with instructions? They will tell you the proper pipe sized for the unit. If not, you can figure it out from the CFM.
> 
> Same thing with the square pipe. That guy was talking specifically about his product. And his main point was sweeping turns instead of hard right angles.



3.25x10 = 32.5 sq in.

6" duct 28 sq in

4" duct 12.5 sq. in.
I had a tight spot so I ran 4" only about 3 ft with two 90*s and the top 2 speeds don't move more air than the third speed.


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## slownsteady (Feb 9, 2016)

nealtw said:


> 3.25x10 = 32.5 sq in.
> 
> 6" duct 28 sq in
> 
> ...



Are you calculating area in the cabinet or cubic in. in the pipe?
Your installation could have as much to do with the 90s as with the pipe diameter, and you didn't tell us the size of the unit, so YMMV.


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## nealtw (Feb 9, 2016)

slownsteady said:


> Are you calculating area in the cabinet or cubic in. in the pipe?
> Your installation could have as much to do with the 90s as with the pipe diameter, and you didn't tell us the size of the unit, so YMMV.



What do want to know about the size of my unit?

Size does matter and yes mine would calculate as 13 ft with the 90*s
32.5 sq. inches is what you have with the micro wave over the stove so changing to 4" cuts the sq. inches to almost a third. Anytime it is straight forward and easy, go with 6"


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## frodo (Feb 9, 2016)

as long as you have the 32.6 sq inch.  it does not have to be round. it can be rectangle.or square
2x16=32

4x8=32


we use 3x14 inside wall cavities often  when space is critical


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## slownsteady (Feb 9, 2016)

Somehow I got the impression that the exit from the OTR was round - probably from the video . if it is rectangular, then it's my bad. But I still stand by my point about the book being the authority on what size to use.

Neal: I hope your unit is not rectangular, but other than that, it's none of my bizness:hide:


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## frodo (Feb 9, 2016)

jeeezzzzz...ever hear of a transition ?


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