A microwave oven designed to be mounted on a cooktop is usually equipped with a ventilation system to handle cooking odors, smoke and moisture that arise from the cooktop. The ventilation system can lead to external ventilation, or you can simply recirculate the air back into the kitchen. Recirculating ventilation has advantages in terms of ease of installation and energy efficiency but pays for those advantages at the cost of effective ventilation.
An externally ventilated microwave mounted on a hob uses a fan to extract the air through the microwave openings and an exhaust duct leading to the outside of the building, so most of the moisture, heat, the smell and particulate matter the cooking generated come out directly. A non-ducted recirculating vent pulls air through a filter and exhausts air back into the kitchen. Some units are equipped with a charcoal filter containing some particles and smells, but most of the heat and moisture, along with some types of particles, remain in the house. Other units use aluminum filters that hold the grease but not much more. On the other hand, recirculating openings do not exhaust the air conditioner to the outside, so they can help make a kitchen more energy efficient.
The installation of a microwave with a recirculating vent is usually much easier and cheaper than the installation of an externally ventilated unit, which requires the installation of a duct and an external vent plug, as well as a perforation of a Wall, a roof or a ceiling. External ventilation requires access to the outside, so it is more comfortably on an outside wall, but a recirculating unit can be easily installed on an interior wall and can be moved with relative ease, too.
The ventilation requirements
When installing an externally ventilated unit, the vent must conform to common construction standards to be safe and effective. The ventilation duct must be made of galvanized steel, stainless steel or copper, and must have a smooth interior. The vent should always be exhausted, not in an attic or in any other interior space. In some places, local building codes may specify a minimum distance between an outside vent and any adjacent door, window, or other structure. Because recirculating openings do not have external components, they are not subject to these requirements.
The charcoal filter in a recirculating vent will lose effectiveness over time and use, and need to be replaced periodically. In general, a filter will last about six to 12 months, but frequent cooking can reduce its lifespan. Aluminum filters require regular cleaning with a grease cleaner or dishwasher.