When venting to the outside of your home is not an option, the recirculating kit takes air that is meant to be exhausted outside, and passes it through a charcoal filter. Removing any unwanted odor and purifies the air to make it breathable again. This kit can be fitted to a circular duct with 7" diameter. Compatibility DK-SMALL: 011, 01A, 021, 02A, 03A, 04DK-LARGE: 06WP Ducting: 7" Diameter