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Discussion Starter #1
We're getting a new microwave and all the instructions seem pretty straightforward, except I'm not sure exactly how I'll connect the microwave to this existing duct. There are two small issues:
1. The cabinets have a 1 1/8" lip, and the existing duct only comes down about 1/4", so without some sort of connector the new microwave's vent will not connect to the existing duct (the old microwave had nothing special here, and that would explain why it never seemed to work well as an exhaust fan! lol)
2. The existing duct is cut really rough and I"m not sure if that's going to make it any harder to for a connection

What will I use to connect the existing duct to the microwave vent, which will be 7/8" (1 1/8" - 1/4") under this existing vent, considering the existing vent is completely enclosed in the cabinet above, and therefore I can't really monkey around with it?
 

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retired framer
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Usually the duct in the cupboard is not boxed in and you make the connection from there.
 

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You gotta figure out how to remove that box in the cabinet. You'll need to seal the duct to the adapter with the flapper on top of the microwave. I would for sure even out that duct edge—that's a real hack job if ever I saw one!
 

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Discussion Starter #4
Hmm, well, ok that shouldn't be too much more work. It's just a plywood box. I'll get the oscillating tool and cut out a section. I probably can even replace it after the ducts are all connected.

How doe the existing duct connect to the microwave damper? Is the damper's collar slightly smaller than the opening of the existing duct, so that the existing duct just slides onto the damper? Does it click on or just rest there?

Or do I need to buy some of of adapter to connect the existing duct with the microwave's damper?
 

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retired framer
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Hmm, well, ok that shouldn't be too much more work. It's just a plywood box. I'll get the oscillating tool and cut out a section. I probably can even replace it after the ducts are all connected.

How doe the existing duct connect to the microwave damper? Is the damper's collar slightly smaller than the opening of the existing duct, so that the existing duct just slides onto the damper? Does it click on or just rest there?

Or do I need to buy some of of adapter to connect the existing duct with the microwave's damper?
If the hole thru the shelf is big enough you may be able to install the damper before you install the MW It stick up about an inch. So barely thru the shelf.
Then it is usually a boot that changes to a 6" round.
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Discussion Starter #6
If the hole thru the shelf is big enough you may be able to install the damper before you install the MW It stick up about an inch. So barely thru the shelf.
Then it is usually a boot that changes to a 6" round.
The existing duct is rectangular, 3" x 10" I think, whatever that standard is.

The cabinets have a 1 1/8" lip, and the hole in the bottom of the cabinet shelf through which the existing duct passes is about 4 1/2" x 12", so there should be plenty of room for me to tile the microwave up into place with the damper installed. I think that is actually the sequence suggested by the installation guide.

So let's say the damper collar is 1" proud of the top of the microwave. That still leaves it 1/8" below the level of the shelf. I'll have to get the oven, take the damper to Lowes and see how the ducts fit onto it.
 

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retired framer
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The existing duct is rectangular, 3" x 10" I think, whatever that standard is.

The cabinets have a 1 1/8" lip, and the hole in the bottom of the cabinet shelf through which the existing duct passes is about 4 1/2" x 12", so there should be plenty of room for me to tile the microwave up into place with the damper installed. I think that is actually the sequence suggested by the installation guide.

So let's say the damper collar is 1" proud of the top of the microwave. That still leaves it 1/8" below the level of the shelf. I'll have to get the oven, take the damper to Lowes and see how the ducts fit onto it.
If the front of the cupboard is 1 1/8" lower than the shelf you would also want a couple blocks at the back the 1 1/8" so when you bolt up the micro wave it will pull up solid.
 

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Discussion Starter #8
If the front of the cupboard is 1 1/8" lower than the shelf you would also want a couple blocks at the back the 1 1/8" so when you bolt up the micro wave it will pull up solid.
You mean blocks up front, right? In the corners, where the bolts come down? Pine should be ok?
 

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retired framer
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You mean blocks up front, right? In the corners, where the bolts come down? Pine should be ok?
The bolts will pull it up in the front to the face frame but you can keep tightening and lift the back out of the bracket at the back. Anything the right thickness will do.
 

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Your duct is 3-1/4x10 inches, and so is the damper on your microwave. Get yourself some duct seal tape—not duct tape, the UL 181A type with the foil backing—and seal up around the damper/vent joint.
 

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Discussion Starter #11
Alrighty! it was pretty straight forward so far exposing the duct. I'll even be able to put the removed piece back, adn if I need to I can also cut away those remaining side pieces of the enclosure, to give my fingers more room to work.

I just tried putting the template up against the bottom of the cabinet. The holes for the bolts that secure the oven to the cabinet are 11" from the back wall. My cabinets are only 12" deep. That seems like the bolt hole is awfully close to the very edge of the cabinet shelf's edge. That's ok? Part of my concern, is that the cabinet has a lip on the bottom. So yes the total depth is 12" but that is including the width of the cabinet lip, if you know what I mean. So those bolt holes are really close to the edge of that cabinet shelf. . . .l guess having those blocks up in the corners will help?
 

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retired framer
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the face frame is 5/8 this and you are drilling a 1/4" hole 11 inches from the back so the hole will be 1/4 away from the face frame, nothing to worry about.
I would make those spacer blocks 10" long and put one on each side
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With that amount of room, it looks like it'll be hard for you to seal the duct to the microwave properly.

Also, I'm not sure it's kosher to run anything else off a microwave circuit—assume that is some kind of under-cabinet lighting? I mean, it's probably fine practically, but not quite code.
 

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Discussion Starter #14
With that amount of room, it looks like it'll be hard for you to seal the duct to the microwave properly.

Also, I'm not sure it's kosher to run anything else off a microwave circuit—assume that is some kind of under-cabinet lighting? I mean, it's probably fine practically, but not quite code.
Yeah that's under cabinet lighting - it's LED tape. It worked ok with the previous oven for about 5 years, can't remember exactly when we had that put in. It'll work ok with this over too, I guess. You're right about properly sealing the duct, I'm not going to be able to get my fingers behind that duct work very well, and taping the duct to the damper is going to be tricky if not impossible. There's a chance I might still completely remove that wood box, but even then, taping the duct to the damper will be a pain because the top of the oven will be 9/8" under the bottom of the cabinet shelf, which is itself 5/8" thick, so I'll need to scooch my fingers down 14/8" (1 1/4") to tape that connection. The previous oven was not taped, heck I don't even think there was really any connection to the duct, can you believe it? No wonder it never seemed to be venting the cooking smells very well.

I went to the store, got everything except the wood I need for the blocking. I need something 1 1/8 thick. That's how deep a recess is created by the lip of the cabinet. Lowes didn't have anything that size, seems like they're not stocking the stores much these days. In stead I got two pieces of wood, 3/4" and 1/2" for a total of 10/8" thickness that I thought I could hand plane or sand down to size. But dang if I'll be able to get that level . . .

What I just thought of, is that I can get two pieces of plywood (3/4" and 3/8"), glue them together and cut to size to make something 9/8" thick. That's going to be a better option. Back to Lowes tomorrow lol.
 

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Discussion Starter #16
I'm stumped on how to connect this duct work. Any ideas? I got this thing and some of this stuff but gawddangit if I could figure out how to make these things connect. That first item basically wouldn't fit over the oven damper. The problem is that the damper is not centered in the existing hole in the cabinet shelf. I was able to snip away and get one of the rectangular stacks onto the damper, not perfect but workable, but then I could not figure out how to connect the existing rectangular stack. There are some tricks to cutting this stuff to get it to fit together?
 

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Discussion Starter #17
Basically, the question I need to answer is this:

How do I insert duct “in-line”?

I have the existing duct above, and I have the oven damper below. I can extend the height of the damper so that it’s top edge is visible and accessible is the cutout I have in the box that surrounded the existing duct. But I’m a little stumped on how to insert another piece of duct to connect everything.
 

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Discussion Starter #19
I don't suppose you have access above the cabinet.
There IS a soffit above the cabinets. So I suppose I could cut into that. You’re thinking it it would be easier to connect the vertical duct to a horizontal duct, rather than trying to do this in-line connection?
 

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retired framer
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What is the length and width of the old duct and the damper?
 
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