My GE SpaceMaker over-the-range microwave requires 6 inch duct pipes.
If you use 6 inch ducts for all 10 feet to the wall of the house but use a 4 inch opening to the outside - is it as bad as having 4 inch ducts all the way?
Do you want it to work properly? If you do, use 6 inch all the way. I see it kinda like wanting to water a large area of your yard with an 85PSI water supply and choking the flow down at the sprinkler.
If it requires 6 inch duct, it also requires 6 inch opening through the wall. Just put the right size hole in the wall. If you don't know how, have it done. You are spending money and effort for a good vent, why cheap out at the end and screw up the job?
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