Over-the-range microwaves with vent fans are designed to perform two main functions: venting the air and removing cooking odors and steam, and providing additional ventilation for the microwave itself to prevent overheating. The way the vent operates can vary depending on the specific model and the installation setup.
Some over-the-range microwaves are designed to recirculate the air back into the kitchen after passing it through a charcoal filter to remove odors. This configuration is known as a "recirculating" or "non-vented" setup. In this case, the hot air from the microwave is blown back into the kitchen.
Other models may be set up for "vented" installation, where a duct system is installed to carry the hot air and odors outside the house through a vent in the wall or roof. This setup effectively expels the hot air outside, which is generally more desirable for removing heat and odors from the kitchen.
Whether your over-the-range microwave vents the air outside or recirculates it depends on how it was installed and the specific model's configuration. Many microwaves come with both options, and the installer or homeowner can choose the desired setup during installation.
So, to answer your question, both configurations are common, but if you prefer the hot air to be vented outside of your home, you'll need to check if your microwave supports a vented installation and have the appropriate ductwork installed if it's not already in place. Consult your microwave's manual or a professional installer for guidance on your specific model and installation options.