These are water heaters, furnaces, and boilers that use propane, oil, or natural gas for fuel. They exhaust their flue gasses up a pipe without the aid of a fan. For use in BPI field exams, the vent pipes exiting these must be single-walled metal pipes – not double-walled or plastic. Newer models (since 1995) are less likely to use this technology.
The first image to the right shows the top of a naturally vented gas-fired water heater. Fuel may be propane or natural gas. Flue gasses exit the heater and make their way up the vent pipe without the help of any fan.
The bottom images show details of different configurations for naturally vented furnaces and boilers.