Superior 4" Outside Air Kit for BRT40ST See-Through Gas FireplaceWhat's Included Outside Air Kit Features For use if outside combustion air is required or desired. Must be used in conjunction with locally purchased, non combustible Class 1 or Class 0 flexible duct. For use with Superior BRT40ST B Vent See Through Gas Fireplace Specifications Model Number FOAK 4LD Catalog Number 81L88 Diameter Size 4" Compatible Fireplace BRT40ST