I have a similar situation to this one. I have two Trane XB80 gas furnaces and one gas hot water heater. Last year I had a Trane fresh air exchange unit installed on the largest unit. Planning to get open cell foam and would like to make attic as tight as possible. Foam installer is concerned about combustion air for furnace and hot water heater.
House interior walls are not foam insulated. Attic is at least 2400 cubic feet. I'm wondering if I have some of the stale house air exhaust from the air exchanger diverted back into the attic instead of outside, it would provide sufficient combustion air for the furnace and hot water heater (exchanger runs both periodically 24 hrs/day and when the unit is running). I would have a net gain in air supply by bringing in fresh outside air and not exhausting all of the inside air.
I liked the idea of a dedicated air intake with a fan that runs when the unit runs, but what about combustion air for the hot water heater?