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Microwave Range Hood Vent: 6" or 4"?

rasher's picture

I have an over-the-range microwave vent hood thing that I'm retrofitting to vent out the wall of the house (currently, it's set to recirculate the air). The outlet on the microwave is 3-1/4" x 10". Do I transition that to a 6" or a 4" wall vent? Obviously, I'd rather cut a 4" hole in the wall of my house, but the instructions call for a 6", which seems awfully big for as wimpy as the fan is...

Also, can I use a plastic vent cap, or does it need to be metal?

(post #83134, reply #1 of 25)

3 1/4" x 10 duct is 32.5 square inches


6" round is 28.26 square inches


4" round is 12.56 square inches


See the problem with using 4"?  You're reducing the duct capacity by almost 2/3.  That wimpy little fan needs all the help it can get to exhaust the air.


Vent could be plastic, but I usually use metal.

(post #83134, reply #2 of 25)

OK. 6" it is. The problem with the 6" metal vent at my local big box is that it's pretty ugly. The metal seems to oilcan pretty easy. I've seen a 6" plastic one on google and it looks like it'd be a little more attractive and might take paint a little better/easier...

(post #83134, reply #9 of 25)

you need to shop for pipe at your local duct supply house where they sell the pipe in 24 ga. instead of the tin foil sold at the junk box.


 


carpenter in transition

carpenter in transition

(post #83134, reply #23 of 25)

Greetings, all:
Just to wrap up the thread, I went ahead and installed a 3-1/4" x 10" metal wall cap (I primed and painted it to match the siding color) which perfectly aligned with my clapboards. Then I just cut up, screwed together and thoroughly foil-taped two flat 90s to get my offset up to a standard 90 into the wall cap. It took a lot of cutting, bending, fighting, cussing, and finger slicing, but I did make it happen. Doesn't seem to be leaking any air into the cabinet at all even though the fit was REALLY tight.

Anyway, the vent sucks range exhaust real well, and it's a little quieter than the blow-it-back-into the room option we'd previously been using. Hopefully it'll be cleaner and less odorous, as well.

Just finished cleaning up this morning and after about 6 hours total over a week, wifey is a just little bit happier she married me.

Thanks for all of the good advice.

(post #83134, reply #24 of 25)

I must be real old school, but I remember fondly the aroma out of gramma's, my mothers, and my wifes kitchen.  To miss it would have put a real dent in what memory I now have.


Don't much care for dryer vent stink with those fabric softener towells tho, while working on someones house.


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A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


(post #83134, reply #3 of 25)

I just did the same thing. I used 7" so as not to reduce the opening. The fittings are a little more difficult to find, but performance is better.

(post #83134, reply #4 of 25)

I made the mistake of using 4" last year for our range hood. On high speed you could hear the fan sputtering from the restriction. I just ripped the whole mess apart last month and installed a 6". It's made a huge difference.

If you can't find a vent pipe that you're happy with, check with a local sheet metal or HVAC shop. They should have a source for a good one.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #83134, reply #5 of 25)

Just throwing this out there, but have you considered a 3-1/4" x 10" rectangle exhaust vent?  May not be what your looking for but I have used them before they're okay.   

(post #83134, reply #6 of 25)

Can you get decent looking vent flap assemblies for that size? I've never seen them, but of course that doesn't mean much. I had to start my vent with a 3.25 x 10 to 6" transition and use round pipe from then on.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #83134, reply #14 of 25)

Check with a duct/ sheet metal supply house that's where I got mine.  It took them a while to find it but they definitely do carry them.

(post #83134, reply #16 of 25)

That's a good option; I've seen them before. The rectangle shape is more pleasing, in my opinion, too.

There ain't NO free lunch. Not no how, not no where!

(post #83134, reply #7 of 25)

why not use a 3x10?

I have an extra in the shop, they are stock at the yards around here, not hard to find at all and look decent, esp with clapboards

 

 


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(post #83134, reply #8 of 25)

Ya know, Broan makes a wallcap that is the same dimensioin as the rectangle vent on the microwave.


Its alot easier if you dont have to transition sizes and you'll have a little more room in the cabinet above the micro.


Edited 7/13/2008 8:58 pm ET by MSA1

 

Family.....They're always there when they need you.

(post #83134, reply #10 of 25)

Well, of course I'd rather use a 3x10 wall cap, but unfortunately, there's a stud running down the wall right in the middle of the where the vent would exit. If theres a way to get a 3x10 straight duct up into the cabinet, then somehow shifted over about 6 inches, I sure can't figure out how...
Any ideas?

(post #83134, reply #11 of 25)

A small header as if you were making a window opening? But this might be a bit much if it's not new construction.

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #83134, reply #12 of 25)

Existing construction, so header is out of the question. We don't really need the cabinet space, so I'll just transition into a 6" round, use an adjustable elbow to get my offset and then a 6" 90 to the wall cap. If wifey doesn't like it, then I'll just build a cover panel and cut the shelves shallow (to maybe 4-6" deep).

(post #83134, reply #13 of 25)

That's exactly what I did. Works fine. Heaps of improvement in noise and volume over the 4".

Scott.

Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

(post #83134, reply #17 of 25)

You could also install a flat 45 3x10 offset (2 curved pieces) ... like a lot of the other stuff, not common, but available.

There ain't NO free lunch. Not no how, not no where!

(post #83134, reply #19 of 25)

Could you elaborate on the "flat 45" thing you're talking about? Would that installation be straight up into the cabinet, 45 over to the right (or left as it may), then up as much as it takes to get over 6" or so, then 45 around to straight up, and finally a 90 into a 3x10 wall cap?

(post #83134, reply #22 of 25)

Sounds right; I think you understand.

There ain't NO free lunch. Not no how, not no where!

(post #83134, reply #25 of 25)

If theres a way to get a 3x10 straight duct up into the cabinet, then somehow shifted over about 6 inches, I sure can't figure out how...

Check out this site.
http://www.nutonesales.com/installation.html
There's a horizontal elbow in the "transitions and elbows" section. If you cut that in half at 45°, flip it over and rejoin it to the other half, I think you'd get about a 5" lateral shift out of it. A little bit of straight 3 1/4 x 10 duct would shift it further if needed, then use a 90° elbow and a wall cap to the outside.

Oops, I didn't notice the last post that makes this easy solution moot.
BruceT


Edited 7/21/2008 11:27 pm by brucet9

BruceT

(post #83134, reply #15 of 25)

(post #83134, reply #21 of 25)

Thanks for the links to the copper caps. They look great! Anyone care to speculate whether or not there's a galvanic reaction problem when you mate the copper cap to steel ductwork?

For my house, though, a copper wall cap is kind of like putting lipstick on a pig. I think the way I'd like to go is get 3x10 wall cap and paint it the color of the clapboards and make it disappear as much as possible. The 3x10 caps at Home Despot look a lot better (to me) than the 6" round and it seemed like the dampers sealed a lot better.

(post #83134, reply #18 of 25)

rasher,


OK, I will be the outside the box thinker and ask why you just don't go with the ventless option if your microwave has it?  I know many new ones do.  You just change the filter every so often and you are good to go.


Don't want to debate the pros and cons of vented vs. ventless as I like venting myself...just thought I'd mention it.


Mike

(post #83134, reply #20 of 25)

We've been living with ventless for 6 years now and I hate it. Our house is very "open plan" and so any time a pan broil or saute anything, it stinks up the whole house. It wasn't until last week that I realized I could just convert the thing to vented and eliminate one more (small) source of stress from my life.