Water heater vent pipe piped through an old (not working) gas heater to the exterior.
Put it this way.
If the furnace was working would it still bother you ?
Simple answer is they are tied in all the time ,and this is not an issue.
Needs T replaced with a Y pipe.
If that gas heater is no longer working… don’t you guys think that the diameter of the old flue is too large for just the water heater gases. If it’s too large of a diameter, won’t the gases condensate before they hit the exterior?
Jeff
Better larger than smaller 
Anyway why would it matter if the furnace is working Jeff as the furnace is not always running anyway and what is the formula for max diameter allowed on that?
I sure am not aware of it.
Far as the y connection goes ,that is a good point but hard to see angle from that view .
I believe they call it an ‘orphaned water heater’. After looking at the info… the diameter of the larger flue cannot be 7x larger than the I.D. of the water heater flue.
Jeff