Shared HWT & Furnace Flue (High Efficiency)

[Rich Sumen]

The situation;
A 6 year old gas, American Standard, 120,000 BTU, furnace (high efficiency) type furnace is present. (venting with PVC through the wall) The exhaust 2" pipe runs up to the ceiling where it sizes up to a 3" and is then joined by the flue from the 75 Gal HWT. (Also high efficiency, fan induced, PCV type flue discharge). Then the two appliances of course, both vent to the ext. via the one 3" pipe.

I wrote it up, as the furnace needed cleaning and there was some small damage (unrelated to the flue issue). I noted it needed "cleaning and further evaluation by an HVAC company". One company said the "Shared flue" was improper and the units needed seperate flue pipes all the way to the exterior, the other company said, it was OK.

Does anyone know about this?

Thanks / Richard

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[Michael Thomas]

Oops... wrong link...

[Michael Thomas]

As both appliances are venting through PVC, they are both Cat IV (condensing systems), and cannot share a common vent.

Though not at applicable to this particular installation, there is some additional IMO interesting information about inspecting systems with various Categories of water heater is combination with "High Efficiency" furnaces here.