Originally Posted by
Gunnar Alquist
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.
Gunner, if the crawlspace is vented, it is considered to be venting directly to the outdoors. "The inlet and outlet air openings shall each have a free area of not less than 1sq in per 4,000 BTU per hr of the total rating of all the appliances in the enclosure." NC Gas Code Book old 1988 but I think it is still the same.
If the crawlspace is not vented it is 100 sq in per 1000 and the 100 sq in minimum code requirement is just that "100 sq in min. total" Your 200,000 would still need 200 sq in though.
I can't find an answer to the question of both combustion and makeup vents to the crawlspace, but the absence of any diagram with this configuration suggest it is not allowed. Sorry for lack of positive ya or na.