Installing Over The Range Hood - exhausting question

[Konowl]

I'm contemplating replacing my current ancient exhaust hood with an over the range microwave. Presently, the exhaust hood is vented directly to the exterior of the house right above the range. When I install the microwave, the cupboards will have to be replaced and raised to make room for the microwave, and the exhaust will thus be ineffective, so I have to make a new one.

What I had planned on doing (my house is a very small house situated on the lake) was replace all the plastic duct work from teh exhaust fan in the bathroom with metal exhaust, and tie in the exhaust from the microwave into that (installed dampers etc). Someone told me this is against the building code, but I looked it up and there are certain provisions, such as installing connectors into the intake of the exhaust fan (?) and ensuring all connected systems are installed with "back pressure devices" (damper?).

Thoughts? I could install the ductwork myself, relatively handy, but don't feel comfortable installing a roof vent and would frankly prefer to avoid it if possible (aesthetics).

[CanadianMoFo]

Without the back pressure device you could have a situation where the exhaust from the kitchen is directed into your bathroom.

CM

[Pete_Coach]

The microwave has two exhaust options, on the top and in the back. Will neither of these work for your application? Does the present exhaust vent to the outside wall or up to the roof?
I don't know what your building codes are so I cannot say about splicing into another duct but it makes sense that they be independent, I would also hate for bathroom smells to be forced into the kitchen

[MacGyver]

I'm not a fan of OTR Microwaves. The reliability is questionable, so make sure you do your research and find one with a good history. Panasonic is a good brand that may be worth a look. I had a GE/Camco in my house that was there when I moved in; I replaced it with a regular range hood after the electronics blew up! I'm not the only one with a similar experience.

The exhaust performance of OTR's is very poor. I was very surprised that the performance of the cheapest range hood well outperformed the old microwave. My wife and I were unsure if we wanted to switch to a regular range hood and forgo the convenience of the OTR, but the better exhaust performance and improved visibility makes cooking inside much more enjoyable.

Now, about the exhaust. I designed HVAC systems for 10 years. The building code states you are not allowed to combine exhausts of different types except under certain circumstances. In Ontario, sentence 6.2.3.8. of the 2006 Ontario Building Code is the one that applies. I've encountered different interpretations of clauses 8, 9, and 10 from different chief building officials, but the consensus is this: most municipal building inspectors will not accept exhausts of different types being combined at any point, despite what the code may suggest otherwise.

As an HVAC designer, I would never recommend combining a bathroom exhaust and kitchen exhaust. The kitchen exhaust must be constructed of smooth metal ductwork so that grease buildup is not promoted, and can be cleaned more easily down the road. Eventually that grease will build up and limit the effectiveness of a backdraft damper, or even gum it up entirely. Only a backdraft damper at the outlet can be cleaned and visually inspected easily. The existing exhaust duct going outside is probably not large enough to allow both fans to exhaust at the same time. And if both fans do not have similar static pressures, one running fan may overpower the other, rendering it useless. Backdraft dampers are not air tight, so you could have odor and/or grease laden exhaust come back into your kitchen or bathroom, particularly if there is a strong outside wind blowing against the exhaust outlet. Finally, the risk of fire is great and isolating a grease duct fire to one system is important to make sure smoke and flame do not spread throughout the home.

[Konowl]

The microwave has two exhaust options, on the top and in the back. Will neither of these work for your application? Does the present exhaust vent to the outside wall or up to the roof?
I don't know what your building codes are so I cannot say about splicing into another duct but it makes sense that they be independent, I would also hate for bathroom smells to be forced into the kitchen

It presently exhausts to the left of the range - range exhausts up, 90 degree angle to the left, exhaust. Microwave won't work in this situation as I'd have to raise it up, practically blocking the current exhaust.

[Konowl]

I'm not a fan of OTR Microwaves. The reliability is questionable, so make sure you do your research and find one with a good history. Panasonic is a good brand that may be worth a look. I had a GE/Camco in my house that was there when I moved in; I replaced it with a regular range hood after the electronics blew up! I'm not the only one with a similar experience.

The exhaust performance of OTR's is very poor. I was very surprised that the performance of the cheapest range hood well outperformed the old microwave. My wife and I were unsure if we wanted to switch to a regular range hood and forgo the convenience of the OTR, but the better exhaust performance and improved visibility makes cooking inside much more enjoyable.

To be frank, the house is maybe 700 square feet, and I don't cook THAT much, so I'm not overly concerned with the exhaust performance. My main goal in this is to get the microwave off of my 8 foot countertop as it's blockign the view of the lake

I like the high end Panasonic ones, they are nice to look at and ALLEGEDLY go up to 420 cfm.

[GTT1]

If you don't cook a lot and just want to get the Microwave over the stove why not go with a non-vented model where you just have to wash the screens.

Not to my liking but may work for you.

[Konowl]

I'm kind of leaning that way right now - I found some 18 inch cabinets that might work at Ikea (kind of like their kitchens...) which would allow me to utilize the present exhaust.... the only time I want the vent is when I'm cooking pasta/rice, as the humid air steams up my windows like there is no tomorrow (small house and all).

Really like the Panasonic line...