So the mechanical room is located on the lower floor of a house constructed on a downslope lot and has 2 - 100,000 BTU, mid-efficiency furnaces. Two 4x14" screened vents opening into the foundation crawlspace provide combustion air. If each furnace requires a minimum of 100 square inches, these two vents are inadequate. Right? Thought so.

Now the real question. I was flipping through my code check and found an interesting note that I had not noticed before. "Only lower opening connected to underfloor area". This indicates that only one combustion air vent can go down into the foundation crawlspace area, the other has to go up into the attic or out an exterior wall. However, perusing the IRC, it looks like there is no restriction on both venting into the foundation crawlspace area, just cannot share an opening or duct. Anyone have an answer?

__________________
We have enough youth, how about a fountain of Smart?