i don't have an answer for your question but i'm making myself crazy trying to select an otr microwave. can you tell me what it is you had that didn't function well?

Microwaves do not good range hoods make. Since almost the entire interior of the microwave is taken up by the cooking chamber and electronics, cooking fumes have little recourse other than to fill your kitchen with their distinctive funk.

Sometimes, there is no place to put the microwave other than above the range, so you have no choice. But one should not expect the microwave to perform like an honest-to-goodness range hood. If you have enough room to do something else, move the microwave elsewhere and get a real range hood. Otherwise, don't worry about it too much. Maybe open a window when you stir-fry.

For the original poster, one possible method of determination is if you can find the total btus/hr rating of your range, divide by 100, then that will be what you need in CFM rating. Example, 45,000 BTU/hr rated range is adequately served by 450CFM hood.

I have to say that I did not learn that in engineering school, it came off the internet, so you might want to double check with your contractor. I have read that it is not an exact science due to spacing, added oven fumes, hood shapes, ducting variances, etc. I would say that if you are going to err, lean toward excess capacity. Every brand of hood has a multi-speed fan nowadays, and the max rating is always the highest blower speed. If it turns out you don't need as much suckage going on, turn the fan down. It'll make it easier to hear J-Lo's trenchant comments on American Idol.

I have just finished ordering all new kitchen appliances for a gut/remodel of our kitchen. I got the same info re CFMs that Mojavean cites above in an excellent article, I think it was in Fine Homebuilding - Kitchen and Bath issue.

I purchased a ventahood on many many recommendations - one feature I like (which was also mentioned) is that I can select fan power to match the # of BTUs I am using, so not running it at full power if only using one or two burners of my 5 burner cooktop. I am told it is also much quieter than other brand hoods.

Another thing - they have a 'magic lung' design (don't ask me to explain it!) so there are no filters needed, which would impede airflow - hence the unit is more efficient, and easier to clean.

My appliances won't be installed for at least another month, so I will have to let you know about the quietness!

Good luck!

"If it turns out you don't need as much suckage going on, turn the fan down. It'll make it easier to hear J-Lo's trenchant comments on American Idol. "

I knew that there was some reason I wanted low noise at uber-kilo cfm, and now mojavean has clarified what it was.

The Vent-a-hood (VAH) fan collects grease (over part of the particle size spectrum) by centrifugal impingement, which is the same principle that baffles use to collect grease (over part of the particle size spectrum). The VAH effectiveness versus particle size has not been published, to my knowledge, so a comparison with baffles and other filtering techniques with published data will have to wait until it is.

In any case, all blowers (at full speed, say) have a flow rate (cfm) versus pressure across the fan capability that is normally presented by a fan curve. (Haven't seen one of those for VAH fans either.) This curve has the general characteristic of decreasing flow rate as the differential pressure increases.

The pressure differential across the fan is due to friction losses and momentum changes in the air flow traveling from the outside of the house to the stove to the outside of the house. It includes losses proportional to duct length, duct bends, duct and hood transitions, the hood aperture transition to the air above the stove, and any filters, dampers, etc. in the path.

It also includes the pressure drop from getting outside air inside, whether via window screen loss, house leakage loss, or imperfectly balanced make up air (MUA). Without perfect MUA the house pressure goes negative, further increasing the differential pressure and thus reducing fan output.

At best, the losses at the hood itself have been accounted for by VAH in specifying the hood cfm, but the rest of the duct losses and house losses are surely not accounted for as they vary with the configuration.

I would like to believe that if the 600 cfm VAH fan were operated outside the hood at zero static pressure, the flow rate would be 900 cfm as VAH claim when touting their magic factor. However, this 2/3 factor for operation in the hood without the other duct and house factors is in the ball park of roof fans working against baffle hoods with ducting and modest house negative pressure.

Hence, I conclude that the grease separation process of the VAH does not provide a significant flow rate advantage over baffle hoods. If there were a significant efficiency advantage, the VAH approach would be used commercially.

Noise and cleaning convenience of baffles versus the VAH approach are other factors that are best evaluated by the customer.

kas

My wife & I had to go with a OTR microwave because our kitchen is so small and there is no place else to put a microwave plus the cost factor of buying a hood and having it installed was not in our budget.
The microwave we chose works just fine however, it is 400CFM.
A LG LMHM2017ST with the extenda vent and a radiant heating element for keeping food warm or heating up plates.

We have it over a NXR NRG 3001 gas range with 4 15,000 BTU burners.

Thank you all so much for the information! This site is becoming a valuable -- if sometimes overhwelming -- resource. :)
Celinahex, I originally had a Kenmore micro/hood combo that came with the house and died at around the 10 year mark. It never pulled steam, odors, etc. up from the range top. I replaced it with a Whirlpool Gold model that was nothing but trouble (from the handle falling off to again, no fan suction). We now have another Kenmmore. The microwave works fine, and the fan pulls a little air, but not enough to accurately vent the range. I think it's a matter of finding a good one, which it sounds like Nunyabiz1 has done. good luck

I consider this site to have been indispensable for my kitchen renovation planning. I spent months here (mainly at the appliance and kitchen forums) absorbing facts and opinions and getting ideas for further research before settling on requirements and plans. It was possible to learn more here than many kitchen designers and planners knew.

It seems I was lucky to have started my research in 2007 when renovation was going full bore; at present the economy seems to have significantly cut the number of renovations and hence reduced the traffic here. Many well-informed contributors, both private and commercial, have reduced their presence. The site's limited storage capacity has caused a lot of really useful information from that period to fade out, but what can be found here or learned by asking is still indispensable.

kas