Vent Duct Problem

The GE Spacemaker over-the-range microwave requires a 6" round duct for proper venting over an "effective length" not to exceed 140ft.
I can't fit a 6 inch round duct; I can only use a 4" one because of space limitations.
I have to run it up 2 feet elbow it 90 degrees and then out 10' to the back of the house.
How less effective is a 4" round duct when a 6" duct is called in the description above?
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How about an oval duct? TB
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BN wrote:

I can fit a 3 1/4" x 10" rectangualr duct most of the way, but I can only use a 4 inch opening to the outside? How much are you losing if you funnel a 3 1/4" x 10" rect. duct and funnel everything out through a 4" round duct at the wall to the outside?
The microwave is capable of 315 CFM.
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The 3 1/4" x 10" duct is about the same area as the 6" round duct (32.5 sq inch vs 28 sq inch). However, if you go down to a 4" round duct (12.5 sq inch) you lose about 60% of the surface area.

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140 feet from a GE microwave......... I do not think so more like 14 feet. Yes I pulled up the pdf spec sheet. Yep it says 140 feet, I can not imagine that a 315 cfm fan could push air 140 feet.
If you ignore the instructions the exhaust function will diminish substantially. My last house had one that vented up and then out the wall. Total length of the run >3 feet, but there were 2 90's in the run. Outside you could feel air, but it never really moved much air. I called it the noise maker, instead of Space Maker.
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I would expect a 6" duct 140ft long to have more resistance to airflow than a 4" duct only 12 feet long. Sounds like you can get away with it.

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It won't work correctly and when you burn out the fan motor and the tech comes to fix it under warranty, he's gonna laugh...
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