Forced / Gravity Venting to Common Flue

[inspectorwill]

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.

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[inspector57]

I think that is technically not a forced draft but an induced draft furnace, a different category.

I believe it is allowed since it still relies on the buoyancy of the warm gas to draft rather than pressure from the forced draft. But I could be wrong, I was one time before.

[kurt]

Correct.

That's not a "forced vent" furnace. It's a Category I furnace, which relies on the buoyancy of the hot air to provide draft.