While all microwaves require ventilation, under-counter microwaves do not necessarily need to be vented through a wall or ceiling. Under-counter or above-range microwaves can be vented through the front, the back or the top of the oven.
The fans that ventilate these microwaves are adjustable, so they can be turned to blow in the desired direction. During installation, the fan can be turned to blow through the top of the oven for a ceiling exhaust system or turned to blow through the back of the oven for a wall exhaust system. If the fan is turned to blow through the front of the oven, the ventilation functions much like that of a counter-top microwave.