over-the-range microwave exhaust upgrade.

[Vince Shriver]

Microwave ovens have very poor exhaust fans. Can I just put an in-line 600 cfm fan in the 8" ducting that presently exists in the attic? Would I have to remove the fan that's built in the appliance, or just leave it? If left in, I would switch on both fans I suppose. Any thoughts much appreciated. I would add a switch on the wall for the upgraded fan.

[Alan Bienlein]

Most of the ones that I've seen that had poor exhaust didn't have the fan oriented correctly for the specific duct work installation. There are three ways the fan can go depending if you exhaust out the top, the back or recirculate thru the built in charcoal filters.

[Matt Meiser]

I just vented ours through the wall--was in recirc mode so I pulled it down and cut the hole. It moves a lot of air with 8" of 3x10 or whatever it is duct. Are you sure the duct ins't the problem?

[Wade Lippman]

I've never been lucky enough to have one where it could be vented anywhere, but I would be reluctant to exhaust into the attic. Do you mean through the attic?

[Vince Shriver]

Good point, Wade. Sorry I wasn't more specific; yes the present duct goes in to the attic to a cut out in the roof where it vents outside the house. Maybe it would behoove me to just replace the microwave with a hood that has some real ventilation capability -- but I would really miss the convenience of having the microwave where it is.

[Matt Meiser]

What size pipe? Sounds just like the original setup at our old house which used too small corrugated hose and was worthless. They did it exactly the same as the bath fans which are about half the CFM.

[Vince Shriver]

I'm sure the present ducting is 7".

[Robert Engel]

Seems like it should work.
If there was some way to remove the existing fan it would work better.

[Alan Bienlein]

I'll restate this again. All the over the range microwaves I have installed come with the exhaust fan in the re circulate position. When they are to be installed to exhaust out the back wall or thru the cabinet above the fan needs to be removed and rotated to either exhaust out the back or the top. Did you install this above the range microwave?

[Vince Shriver]

Alan, thanks for the input. The exhaust exits thru the top and eventually thru the roof. I found a picture of the inside of a micro/w and there is a labyrinth (like an obstacle course) the air has to travel to get to the ductwork. I'm thinking if I do this, I'd have to get something like a 900cfm blower and remove the restriction of the present fan motor in order to get enough vacuum where I need it. Then IF it all worked, there would be a lot of grease and moisture that would get sucked up into the little corridors of the machine and into the duct work. And no replaceable filters except the wire mesh that's there now. Geesh! I guess I'm beginning to think myself out of the project, and visualizing a good strong OTR hood in its stead.