Auxiliary Condensate Drain Pan (IRC M1411.3.1)
The overflow protection behind the primary drain, and it turns on one misread sentence: the auxiliary pan shall be not less than 3 in larger than the unit or coil in WIDTH AND LENGTH -- 3 in on the OVERALL dimension, not 3 in per side. A 21 in air handler wants a 24 in pan, not 27. Read it the generous way and you buy a bigger pan than the code asks for in an attic where space is not free; read it the other way and you fail. Plus the 1.5 in minimum depth, and the choice among the four methods the section allows -- only TWO of which involve a pan at all, the others relying on an overflow line above the primary connection or a UL 508 shutoff device, which is often the practical answer in a tight space. Whether any of it applies (damage may occur from overflow) is the prior question.
Formula and source
required pan width = unit or coil width + 3 in; required pan length = unit or coil length + 3 in; minimum pan depth 1.5 in. The 3 in applies to the OVERALL dimension, not to each side, so the per-side misreading would demand width + 6 and length + 6.
IRC M1411.3.1 auxiliary and secondary drain systems, with its four permitted methods and the auxiliary pan dimensions.
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