I'm not sure this will answer your question but it may help.
We recently installed a new over the range with outside vent connection. The
vent pipe was 7 in. in size - and we had to buy an attachment that had a round
hole to fit the vent on one end and it changed to a rectangle shape on the
other end to fit the microwave.

We were putting in the oven to replace a regular vent hood over the stove so
yours may be different. We had a choice of venting it to the outside or having
it just filter the air and blow back into the room.

Someone on here can probably explain it better.


Dorothy