The 4" vent cap can be used in a separated or non-separated unit heater. In a separated unit, it would be used as a cap to the combustion air terminal or air intake, or with the use of a concentric vent kit(included). In a non-separated unit, it would be used as a cap to the vent on the combustion air terminal. For use with all Separated and Non-Separated Axial Fan Unit Heaters and Duct Heaters of 250,000 BTU of lower. ONLY WORKS WITH ROBERTS-GORDON LLC PRODUCTS.