Gas furnace induced draft exhausts through roof… no big deal. However, hot water tank with natural exhaust flow, joins furnace exhaust in attic. Wouldn’t this cause possible back flow through water heater exhaust or at least create a pressure that restricts the water tank exhaust. I can’t find any documents preventing this.
What type of vent did the furnace have? if it wasn’t plastic, most likely it’s category I which is nonpositive and can be mixed with natural draft water heater as long as the vent piping is installed correctly. You cannot mix positive with negative draft. The draft inducer only helps get the gasses through the heat exchanger, it does not pressurize the flue in category I appliance. Post pics of the furnace and or look up its model to see what category furnace it is.
As Simon said my gas furnace is a category 1 with a draft inducer and my natural draft water heater joins that flue to exit through the roof, no problem.
Thanks guys. I found it right after posting… Y’all rock