I have always understood that a forced vent and a gravity vent may not share a common flue. Frequently, I have been seeing furnaces equipped with a mechanical drafting system and a naturally vented water heater connected to a common flue. Contractors are installing such configurations and municipal inspectors are signing off on it. Is there something I am missing or a variance in the codes which allows for such a configuration. Click to Enlarge 52.78 KB