This belongs in our long list of "stupendously dumb things salespeople say". No, you cannot put a 30" OTR over a 36" cooktop. It is against any Building Code in the country. The 3" of exposed cabinets that will be on either side of the MW are combustible. So you will be having 6" of cabinetry exposed to the gas flame. Your salesperson is not only dumb, but giving dangerous advice. You should call his/her manager and report this, so as to avoid this salesperson giving some unsuspecting person this advice, and putting a fire hazard in their house.

Now, there are some 36" OTR MW's made. The interiors are just a 30" MW, and the manufacturer has put "wings" on each side. They would also be a very poor use of your money. You still will not have proper coverage or cfms for your 36" range.

The rough rule of thumb is to count up your cooktop's btu's and divide by 100 to get the rough amount of cfm's you need. So (for example) if your 6 burners total 90,000 btu's, you would need a 900 cfm hood, though you could squeak by with less, but certainly not 300 cfms of a MW.

The other issue with an OTR MW is that they are not built to be over high btu's like the 17K you mentioned. Even if your range was not 36", the 17K btu burner would fry the MW above it. You really should get a proper hood over your 36" cooktop. Try to buy one with baffle filters (not mesh).

Thank you, Shannonplus2. I appreciate your response very much.

You are totally right on the size issue. Sounds like I need a hood as opposed to an OTR micro. But that still leaves the CFM issue. The BTUs on the cooktop I want are 17K, 9500x2, and 5000. That adds up to 41000, which -- based on your recommendation -- would necessitate a 410 CFM hood.

Am I correct? If so, it seems like something like this
http://therangehoodstore.com/36i-kf-wall-hood-197.html
would be excessive. And would probably require some type of additional air source (I don't get how that works, anyway).

Would a more standard 400 CFM hood work? If so, recommendations? Has anyone used The Range Hood Store?

Here is a link that might be useful: The Range Hood Store 36-KF Baffle

CurlyElle you need induction, not gas.

Here is a link that might be useful: Induction Venting Questions

You said 5-burners, but only listed 4. 5k sounds pretty small to me.

Weedmeister-- thank you, but I'm afraid I'm old school and want to go with gas.

Davidrol -- right you are. I forgot about the 12K burner. That means I'm at 530 CFMs. It seems like I have to decide between 400 and 750. One's not enough, the other's too much and means I definitely need make-up air. I live in Minnesota, so bringing in cold air is NOT appealing (yes, I know it's not supposed to actually cool down your house, but still...).

''I live in Minnesota, so bringing in cold air is NOT appealing''

Whether it is 400 CFM or 750 CFM, if you are blowing air our of your house from a hood, air will be coming into your house somewhere else. If not through a proper MUA, then round windows, down chimneys or flues, etc. Air drawn in round windows will still be cold; air drawn in down chimneys or flues may be warm, but will also be full of smoke or combustion fumes - potentially deadly.

Bottom line, you really should have MUA anyhow, regardless of code requirements. If you accept that, then obviously you may as well get a properly sized hood.

IMHO, there is no such thing as too much CFM in a vent.

Go for the 750 and use it on a lower setting. It will be quieter than the 400 on high.

"IMHO, there is no such thing as too much CFM in a vent. "

Obviously, you've never had to install heated MUA.