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Re: Mid efficiency furnace combustion air
The IRC implies, based on the drawings, that when obtaining combustion air from a crawlspace that the lower opening is allowed to vent from the ventilated crawlspace, but that the upper opening is to be vented out through the attic - Figure G2407.6.1(1).
The code text, Section G2407.6.1 does not explicitly say that. The code wording simply treats "vertical" ducts, whether vertically into an attic or vertically into a crawlspace, as "vertical" ducts. The code does differentiate the difference between vertical duct and "horizontal" ducts as there is an obvious loss of air flow through a horizontal duct as related to a vertical duct.
The intent, whether through a vented attic or a vented crawlspace, is to provide at least one high and one low vent opening which will allow for air circulation within that confined space and provide more efficient combustion air flow. Either way, the combustion air will be drawn from the negative pressure being developed at the burner, drawing air from both vents.
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