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My basement utility closet currently uses the "Two-permanent-openings method" with one 5" duct from the attic terminating 12" from the floor and another 6" duct terminating near the ceiling. My understanding of IRC G2407.6.1 (304.6.1) Two-permanent-openings method is that the lower duct is supplying fresh air from the outside and the upper duct is "venting" out to create a convective air current in the room. (source)

Why is the 2nd, higher duct required for venting? Aren't the flue vents doing that already?

Reading up on G2407.6.2 (304.6.2) One-permanent-opening method it seems that this would be a simpler method of providing combustion air. Per the same source:

Research has shown that for modern appliances, a single opening to the outdoors will perform as well as the traditional two-opening method. The one-opening method described in this section depends on a reduced pressure being created in the enclosure by the draft created by the venting system. This reduced pressure causes combustion air to enter the enclosure through the single opening.

I understand how both codes work and the sizing requirements, but it's unclear what considerations must be taken into account when choosing one method over the other. Is the one-opening method simply a more updated approach and the two-opening method no longer necessary?

Picture of my utility closet for reference: enter image description here

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