I would love any help you could give me with regards to deciding how much make up combustion air i need for my master bedroom area. The natural gas, room air fed units are a 90% 80k btu furnace (oversized), and two (overkill) 50 gallon hot water heaters, in a 700 sq ft unfinished basement (insulated ceiling).

It connects via an open 3/4 door passage to an area where my wife works out (rarely i must add). There were two screens in place of concrete blocks for vents in the equipment room. In winter the equipment area is freezing cold, and the cold drifted over to my wife's workout area. I perhaps stupidly sealed off the two screen vent/holes.

I would love to have the minimum safe air possible. What would i need in winter? Maybe just one open. What about summer (only the hot water heaters).