Bob,

I have to admit there are some sticky points to my comment. Under the Standard Gas Code 610.3.5 "A gas appliance vented by natural draft shall not be connected into a vent, chimney or vent connector on the discharge side of a mechanical flue exhauster."

The IFGC 503.10.4 - Two or more appliances connected to a single vent states "...Vent connectors serving category I appliances shall not be connected to any portion of a mechanical draft system operating under positive static pressure...".

My first impression of the description of backdrafting led me to think that the mechanical draft was operating under positive static pressure. If the furnace mechanical draft proves to be a negative static pressure the installation would be ok. Then the question would be has the vent above the WH connector been sized properly based on the combined BTU input and vent height.

As to the issue of backdrafting thru the WH draft hood, the vent system needs evaluation and make any necessary repairs.