I can’t wait to replace mine with an under cabinet hood.

The primary difference between the correct CFM and hood size over the cooking zone, and the same CFM pulled through a ceiling register is that the latter will only remove the grease, moisture, and odor exponentially through air changes, and this process will provide time for the grease and moisture to be deposited on all surfaces exposed to the cooking effluent.

As you are now recognizing, microwave ovens have too small a fan, and too restrictive a duct path to be effective.

I have to agree with jmm1837. Pull it out and put in a proper hood. Over the range microwaves can be dangerous if you have hot pans cooking below you. They can also be dangerous, when you are moving very hot dishes from the microwave itself. If you are both very tall, it may be a bit safer to remove stuff, unless something is cooking on the stove below the microwave. You have a nice kitchen, and this is a bit more expense, but, in the long run, its a much better option, to just remove it and install the vent hood. Perhaps, you can get some money back by selling the over the range microwave.

I had this in my old house. I would just open a window . If I was frying -- happened rarely--I'd put in a window fan set to exhaust. Not a terrible option. Now I have a "real" range hood, but the air return is near the kitchen. My house has way more cooking smells now than my old house w/the microwave fan. Lots of elements at play . Good luck!@

Is it still possible under code to install a wall fan - one of these things?

This is one reason why I use a camp stove to fry bacon in the garage or on the patio. The other reason being that I use a cast iron skillet which doesn't do as well on an electric range.

And yes, through-the-wall kitchen exhaust fans still exist.

They do make OTR microwaves with fans that vent outside....

" They do make OTR microwaves with fans that vent outside...."

OP has that. Just because OTR microwaves have external venting doesn't take away from the fact they don't vent very well.

Are you sure that the microwave is actually venting? Did you go outside and feel the air flow? Have you considered cooking less stinky things?

Ahhh, you're right - I missed that in the original post. Reading is hard, apparently, lol

As @Kaseki mentioned, other ventilation will be quite poor even with 1200-2000 CFM's in a ceiling exhaust. The grease and odors have already spread out.

I'm not aware of any OTR microwave that does a remotely decent job of exhausting effluent.

I'd recommend talking to your wife, finding a new place to put a MW and install a proper hood.

More: https://www.gardenweb.com/discussions/5161173/hood-faq

What Lawrence^ said: Are you sure that the microwave is actually venting?

When I removed my OTR microwave a few years ago (that didn't vent worth a hoot & was deafening loud), I discovered that whoever installed it didn't remove a baffle inside the MW that (would have) allowed it to vent. All of the vent pipe that it was connected to was in pristine new condition. The inside of the pipe wasn't even dusty! (The microwave had been manufactured in 1987 so I assume that it had been in service for over 20 years by the time it was removed.)

That is my only experience with a venting MW. I know that they don't have a good reputation even under the best of circumstances but do double check to make sure that yours is actually blowing exhaust outside.

I replaced with a Zephyr Typhoon hood that I'm not real happy with. Looks nice but the design isn't good for capture because of the swooped bottom. Plan to replace it with a hood that has a boxed capture area, a 42" hood over a 36" rangetop. Instead of the 36" Zephyr. Probably a Victory hood.

>> Are you sure that the microwave is actually venting? Did you go outside and feel the air flow?

Not the OP but I did a housesit where the appliances were installed the day before we started. Fancy Samsung unit that you could even pull out a flat extension over pots/pans on the front burner Thing couldn't even vent water vapour. I went outside to check the wall vent, and yes, the louvres were open and flowing out air, just not venting any vapour from the pasta pot. That was my first experience with how useless those things are.

>> Have you considered cooking less stinky things?

Awfully limiting unless you just want to cook/reheat boil in bag.

I knew someone who had that issue with a new microwave. He had someone take a look at it & found out the vent was not turned the right way. Some of these microwaves have an option to vent inside & or outside. The guy switched the mechanism to vent outside & it worked great afterwards. Hopefully that's all it needs to breathe better.

Yup. For example, GE sells model JNM3161MFSA which is set up to recirculate (but can be changed to vent externally) and the nearly-identical JVM3160RFSS which is set up to vent externally (but can be changed to recirculate).