Furnace and Water Heater Sharing Vent

[Eric Hyde]

Hello, I was curious, since I could not find reference to this situation in any of my training manuals, as to whether or not a gas furnace and water heater can share the same exhaust vent?

Any help would be greatly appreciated, thanks in advance.

[David A. Andersen]

The main vent must be the size of the largest appliance vent, plus 50% of the second.

You can not put a forced vent appliance with a natural convection water heater flue. It may back draft at the air break.

[Russel Ray]

The main vent can be the actual vent for the largest appliance (BTU-wise) as long as it is also 50% larger than the second vent beginning at the point where they connect, as shown here.