[Domo]

disconnect the vent above the microwave and see if the fan sucks and air blows up. If it does, the problem is the vent run (length, elbows, stuck flapper at the end) not the microwave. If it does NOT suck and blow the trouble is the microwave.

 

[Domo]

The microwave definitely blew air out of the top before the duct was connected. So then it must be the vent run.

Have them replace the flapper in the microwave that they took out - it's there for a reason - you don't want a column of attic cold/heated air dumping into the kitchen.

Check the operation of the outside flapper - I've seen many that are welded shut by hornet nests/bird poop, mud wasps, etc. I've also seen the type that have 4-5 vanes where one has come loose and jams the rest from opening...

 

[Domo]

Good for you! Cook something nice and share the recipe!