A Type B gas vent cap (also called a rain cap, vent top, flue cap) is a fitting installed at the top termination of a Type B (B‑vent) gas vent system. Its purpose is to:

Prevent rain, snow, debris, and animals from entering the vent pipe

Provide a safe termination for combustion gases to exit the vent system

Allow gases to be expelled safely while maintaining draft

Protect the venting system from external elements and ensure compliance with codes

This cap is specifically meant for Category I gas appliances that use a Type B venting system, which is a double-wall, air-cooled metal vent system for gas furnaces, water heaters, boilers, etc.