Auxiliary Condensate Drain Pan (IRC M1411.3.1)
The overflow protection behind the primary drain, and it turns on one misread sentence.
Example
You enter
- Unit or coil width (in) 21
- Unit or coil length (in) 45
- Pan width (in) 24
- Pan length (in) 48
- Pan depth (in) 1.5
- Method (M1411.3.1) pan-with-drain
You get
- Required width (in) 24
- Required length (in) 48
- Width deficit (in) 0
- Length deficit (in) 0
- Depth deficit (in) 0
- Misread width (in) 27
- Misread length (in) 51
- Pan area (ft²) 8
Details, formula, and sources
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.
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.
The pan dimensions and the four methods are short prescriptive provisions reproduced in state adoptions and inspection guidance. No table is reproduced.
Estimate. AHJ and licensed professional govern.
Field names used by the API: unit_width_in, unit_length_in, pan_width_in, pan_length_in, pan_depth_in, method, required_width_in, required_length_in, width_deficit_in, length_deficit_in, depth_deficit_in, misread_width_in, misread_length_in, pan_area_sf
- Pan size unit or coil dimension + 3 in in width AND length - overall, not per sideIRC M1411.3.1
- Pan depth 1.5 in minimumIRC M1411.3.1
- Four methods only two use an auxiliary pan; the others use an overflow line or a shutoff deviceIRC M1411.3.1
- Overflow line connects ABOVE the primary connection, discharging to a conspicuous pointIRC M1411.3.1
- Detection device UL 508, above the primary connection and below the overflow rim, shuts the equipment offIRC M1411.3.1
- Applicability where damage may occur from overflow - the prior question, not checked herestated scope limit