Everyone has opinions. My suggestion is to ditch the old vent and get a new one, 600cfm or better. Think about putting a MW in the bottom of a cabinet (leave an opening of sufficient height) to free up counter space. And there is always induction...

Harrietthehomeowner, I replaced my BRAND NEW OTR MW with a hood about 3 months after I installed the OTR MW in my brand new kitchen. I really detested that MW. I think you said you already have the ductwork, right? Then I don't know why you believe you'll need electrical work (the MW needs electricity, so if you already have the electricity for the MW you don't need more electrical work), carpentry (why?) and drywall repair (why?). The biggest expense in my replacement was the hood itself. Countertop MW's these days are pretty low-cost. I bought a large countertop MW, with actually bigger interior space than my OTR MW, with all the bells and whistles like sensor cook, several defrost modes, etc., except convection (I didn't want convection). That Sharp countertop MW cost me about $120. I put my MW on a shelf in my pantry. The other expense was tiling. I had a backsplash up to where the OTR MW was. But the hood is higher, and so I needed to bring my tile guy back to fill in that space, I think it was 3-4 rows, 30" width, so wasn't that big of a deal. Luckily, I had plenty of extra tile from when he did the backsplash in the first place. I sold my OTR MW on Craig's List, which wasn't too hard since it was practically new, although I could have also donated it to Habitat for Humanity, but it didn't occur to me at the time. Here's why I detested my OTR MW: - OTR MW's just have a couple of little mesh screens for exhaust, very inadequate. - capture area is a joke. Since the MW only hangs over the back burners, it can't capture anything coming from the front burners, and even the back burners are inadequately captured, given the little mesh screens I already mentioned. - my OTR specified 300 cfms, but hardly exhausted anything. - little mesh screens got clogged very quickly, since they are so small, making the exhaust even less effective than it already was. - as a short person, I found it awkward trying to peer up into the MW to check on things cooking. - OTR MW is really noisy. My replacement hood isn't dead silent either, and in fact I believe no hood is unless it has a remote or in-line fan. But that OTR MW exhaust fan was like having a 747 in my kitchen. I do understand that in a small kitchen, sometimes the only spot for a MW is OTR. But barring that, I wouldn't recommend one.

Thanks! I noticed that the grille was greasy and when I removed it, the top filter obviously had been filtering. The fact that the holes on either side of the top filter had airflow and were completely open (there were no filters on them) made me think that something was wrong.

@acm I hadn’t thought about into the sunroom then out the side. That would be easiest, but would mean two bends. I’ll have to look into that. For wall cab plans, If you can make out my very hastily (not to scale) drawing. We have one hanging cabinet planned between our front door and window and an easy reach corner wall cabinet (the kind that wraps around corner, not angled) in the corner next to the range. The picture with the MW on the metal shelf is where that first wall cabinet would be and I agree that is the best spot. I’ve liked it there. It’s very accessible, you have a place to set things when you take them out. Its close to my cooking area but not right in it. This morning I told DH we absolutely could not hang a MW under the corner cabinet. It’s such a bad spot. You can’t actually get in front of things in that corner. I can’t imagine trying to pull hot things out of a MW from to the left or right of it. That does sound dangerous. He has agreed to put it in the wall cabinet that corresponds to where I have it now, but only if it can be inside the cabinet. The cabinets are shaker but he is definitely throwing in some craftsman elements and he really wants to try to get that uniform look that you see in craftsman furniture. This has been really hard because our space is odd and in order to make it work symmetry has had to go out the window. He is wanting to try to make the uppers as uniform as possible. I’ve done lots of sketching trying to figure out how things will work. Here is one when I was trying to imagine how to make the microwave not look completely out of place there above the range ending at the window. I just could not do that to my satisfaction. A hood will not only be more functional but it will look so much nicer in that spot. Here I adjusted a drawing to make the wall cabinet have an open shelf for the MW. He doesn’t like how that it would make our only two wall cabinets no longer match in style, which I get. I’m not super excited about the idea of having to open a door to use the microwave, but I’ll do if I have to. I’ll work on him some more first and see if I can get him to come around to either an open shelf or a built in. GE does have a smallish MW that can be built in or hung and would fit on that shelf and we decided to make them slightly deeper to accommodate a MW more easily. Since our prep areas are under the windows the deeper cabinets won’t affect us. I could hang it under the cabinet but then I’d have to move my coffee pots and I really like them there. @kaseki I hadn't thought to try that, though the MW is currently in a box. I was going to call GE however since its been decided to put the MW in the wall cabinet, no need to figure it out I’ll just sell the MW. Thanks for the tip about wind direction. I’ll keep that in mind.