The Williams thin wall collar kit is for use when installing a direct vent furnace on walls with thicknesses from 1" to 5". It installs with a vent kit with Williams Direct-Vent models including 1403621,1403622, 2203621, 2203622, 3003621 and 3003622.
It can also be used to extend the vent cap away from the outside wall when mounting the vent cap onto vinyl siding or greenhouse coverings. This prevents damage for the exhaust on these materials.
The four pieces fit together forming a square/rectangle.