Microwave ovens produce heat and moisture which must be vented away from surrounding cabinets. How a microwave with a vent should be positioned over a stove depends on the location of the microwave vent. Some microwaves have front exhaust vents that eliminate the need for duct work in the house walls. Other microwaves have a top or back vent which connects to exhaust duct work. A microwave positioned over a stove is mounted to both the back wall and the underside of the upper cabinet, regardless of vent placement.
Microwaves with a front vent are the easiest to install over a stove because the unit vents into the kitchen instead of through an outer wall or the house roof. It is tempting to install a trim or lower valence kit so that the bottom front of the microwave matches the decor of the surrounding cabinets. However, any trim cannot block the front vents on the microwave. A charcoal filter should also be installed on a front-vent microwave to eliminate cooking odors.
A top-vented microwave oven connects to duct work that runs vertically through the house wall to exhaust heat and moisture into the attic or through a roof vent. The microwave should be mounted onto the studs of the back wall with a mounting plate and onto the underside of the upper cabinet so its front does not extend beyond the surface of the surrounding cabinets. Check the installation instructions of your specific unit to ensure that the required air-space needs are met.
A back-vented microwave oven connects to a horizontal opening in the house's rear wall, which exhausts heat and moisture to the outdoors. The horizontal opening cut in the house's rear wall must be capped and sealed around the cap to prevent moisture and insects from entering the house through the opening. You may also have to adapt the microwave blower so that the unit exhausts correctly. The fan blade openings should point towards the back of the microwave.
Use caution when positioning a microwave over a stove. It usually takes two people to lift the microwave into place. The installation space can be cramped, especially on a back-venting microwave where the vent is flush with the rear wall. This makes it difficult to adapt the blower or to thread the unit's electrical cord through its receptacle. It is very easy to pinch the wiring on the blower or the electrical cord which can cause the microwave to malfunction. It is also easy to pinch fingers between the wall and the mounting plate.