I am planing to install a new microwave which has the possibility of a back/top exhaust vent. I want to chose the back vent. Is it OK to do a straight vent to the outside, or does it need to go up first then out ?
I installed mine back, down across under floor then out. As long as the equivalent lengths of duct don't exceed the rating you can go any direction. Straight out is simplest if you have exterior wall. Trick is if you have a stud in the way.