It should work with a 4" vent connector. I believe he has 3" at the moment.Thanks john recalculated and he needs 27 square inches of additional intake (which he has) and your right thats minimum should work then right? especialy if he is just using water heater alone then it should work fine no problem at all . I had only figured on the outside air and not accounted for the space of 4368 cu ft of basement with a need of 8500 . agree 40k heater dosent need a 4 inch vent thats why they come with 3 inch collar.