George E. Cawthon wrote:

... A gas furnace requires 1 square inch of opening for a certain number
of BTU's depending on the exact installation.

More precisely, 1 ft^3 of gas has a heating value of about 1000 Btu, and
burning it takes about 9.6 ft^3 of air, so a 50K Btu/h furnace burns about
50 ft^3 per hour with about 9.6x50 = 480 ft^3/h or 8 cfm of combustion air.

If you have a furnace siting in the open in a very large space you don't
have to have this ventilation because infiltration would be sufficient.

An average 2400 ft^2 US house leaks about 0.5x2400x8/60 = 160 cfm, 20 times
more than 8 cfm, so vents for combustion air or gas leak dilution seem
completely unnecessary, unless you install the furnace in a tight closet.

Nick