Hoffman WFHD3VB6830, Vented Base, 24.00 X 68.56 X 30.00Vented bases are designed to be used with Hoffman's Impeller packages and include a stainless steel mesh to prevent the ingress of debris. The vented bases have a provision to easily install a standard furnace filter. If vented bases are used, a maximum airflow furnace filter is recommended. Filters are not provided. Solid bases are designed to be used with a closed-loop cooling solution.