For any hood that is installed directly underneath or inside of a wood hood or cabinet, charcoal filters only are used to recirculate the hood. A set of 2 filters is the only part that comes in a recirculating kit.
How the kits work:
Hoods that use the ductless conversion kit require absolutely no ductwork at all to bring air back into the kitchen.
Hoods that are recirculating (except the Agio, Cylindra, Cylindra Isola, Matrix and Mirror), require a small amount of ductwork to divert the air to the top of the cabinet or the face or side of the soffit. However, the ductwork does not have to exit the home, but rather return the air back into the kitchen. Click here to see a picture of a Cristal HC with a recirculating installation. Click here to see the ductless Agio with no ducting required.
The charcoal filters for all hoods should be replaced approximately every 6 months depending on usage as the charcoal's odor absorption decreases over time.
See the technical specs page for specific model number information, which hoods take a ductless conversion kit, or charcoal filter kit only and to find specific instructions for installation. Replacement charcoal filter information can also be found there.