Furnace and Water Heater Sharing Vent

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.

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.

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.