Sure, it will help. Some. But your problem with the proposed solution is the distance between where the grease is being vaporized at just above waist level, and where those vapors must travel to escape your kitchen, a small intake several feet above your head. The grime is going to disperse all over your house.

Here's the thing. People will argue that perfect ventilation is a must. But it isn't. You can get by without it. It is just that your house is going to be smellier for longer after you cook and you are going to be seeing more grease accumulating on the cabinets and other appliances. Nothing some glade and a couple bottles of Formula 409 can't lick. But what you are trying to do is do something better than that, right?

For that reason, if I were you, I would not settle for anything less that a real range hood sucking the smoke and grease out from a height of 30" above your cooking surface. Lacking that, I would try to find the best OTR microwave exhaust setup I could buy. Also, if you frame up the enclosure and run the ductwork, it should be really cheap to get a drywall guy in to finish it up and texture it for you.

Before my present kitchen reno, we had a Litton cook center that included an OTR microwave with a vent mesh on the microwave base over the cooking zone. It also had a "brow" type manual opening above the microwave (more mesh) to capture additional cooking effluent. This combo was vented via a fan in the brow cavity to the roof via modest diameter ducting.

The original fan motor in the brow above the microwave was, shall we say, also modest, perhaps capable of besting a clock motor in a torque contest.

I eventually put in a roof fan to pull on this ducting, and later a ceiling register to capture some of what the brow missed. All in all, it was only an improvement and far from what we now know is needed for adequate ventilation if cooking scope extends beyond boiling eggs.

Adequate means capture and containment of all effluent rising conically from the cooking pots and pans (and all combustion effluent where applicable). This will force use of either a vertical duct that penetrates the cabinets and ceiling, or possibly a rectangular duct within the wall cavity between studs. This second option can use a back-facing hood exit, and allows salvage of some cabinet space above the hood.

Conceivably, in some house configurations, the hood and cabinet would hide the duct and no sheetrock repair would be needed. The cabinet could be of reduced depth so that the duct could be larger than 3 inches front to back.

In all cases of adequate hood (which needs some vertical distance for good containment) and ducting, the microwave would end up somewhere else.

I recommend that before picking from among Fantech's fans, the ducting and hood questions be resolved so the pressure drop can be estimated. Then given Fantech's fan curves, and a goal for actual cfm, the fan can be selected.

kas

You know, another option is simply to remove/replace the cabinets above the stove. Reconfigure the setup to include a "nook" for a built-in microwave to the right or left of the stove and that would leave room for an honest-to-goodness hood, as wide or wider than your cooking surface. With the cabinets gone, you can find a wall mount hood with an integrated vent and vent cover either in stainless or painted (or other finish to suit you decor) that runs all the way to the ceiling. This gives you straight-shot efficiency and no need to hire drywallers.

Admitedly this solution takes away cabinetry and space that will have to be made up somewhere else in your kitchen, but running a duct concealed on the inside of a cabinet (that's how our Reagan-era tract house was originally ducted) means that there is little to no usable space in the cabinet anyway. Plus, those cabinets above your present microwave can't be that tall, right? 12" or so? To me, the key question is "is there a place to relocate your microwave?"

Not enough info to answer that one. Anyway, just a suggestion for you to evaluate. Best of luck with whatever you decide to do.

mojavean's cabinet replacement suggestion above is the best one from a functional point of view. Unfortunately, kitchen renovation is like skimming along the event horizon of a black hole; one small perturbation and one risks being trapped in an inescapable pit of expense.

kas

to add to the above posts, I'll say the OP image reminds me of bathroom fans like ILF200 from Broan / Nutone. I have one of these. The steam from a long shower doesn't get sucked away even at 200 CFM. But leaving doors closed and relying on high suckage to pull new air into the bathroom through the crack under the door, it can be observed that the shower steam disappears gradually, but faster than without the fan

Of course, one can state that it helps. I'm sure I could say that too.

zirconx you haven't written out where the replacement air will come from. (E.g. open windows year round?) There are also many other factors to consider.

In my kitchen I have a Fantech inline. Before telling you whether or not it's the smallest one or some other size, I'll reinforce what was written above: figure out the ideal situation and then go hunt up parts and pieces later. Think about a conical catchment / capture basin.

What about taking the venting down, into basement/crawlspace and out of the house that way?

Thanks for all the replies. I did think briefly about venting it downward, but I don't know how I'd even start to go about that. Put a register in the wall? I'm trying to avoid the expense and hassle of ripping out cabinets and installing a hood.

Really what I'd like is to just hook ductwork to the microwave vent, then install an inline fan in the attic, sucking air in through the microwave venting system. That way I'm pulling the air right at the stove. And it would be a quieter system since the fan is in the attic. I could even do this in 2 steps - first ducting the microwave, then later installing an inline fan & wall control. Do you think this setup would work?

But I'm having trouble finding people do to this. I have trouble getting any contractor to show up at all, the economy must be doing well! I don't do drywall, and when I've gotten estimates in the past for drywall work, its been very expensive since they have to come back 3-4 times to get it completely finished (drying time between coats, sanding, finishing, painting). I was quoted somewhere around $400+ to fix a little 1 foot by 1 foot patch in a ceiling at my last house.

For make up air I would probably crack a kitchen window when running the fan on high.

I am in the same situation and came across your post. Were you able to connect an inline fan to the microwave vent system?

I did exactly the above...disconnected the fan in my microwave then used an inline Fantech and silencer in the attic. The thing is whisper quiet