How to read a Lennox serial number (find the build date)

Decode a serial number now (works offline).

The rule

Digits 1–2 = plant code, digits 3–4 = year, character 5 = month letter (A = January … M = December, skipping I). Ten-character serials, in use since 1973.

The maker doesn’t publish this rule where I could find it, so it rests on inspector and HVAC-guide sources and may be wrong for some units: InterNACHI forum and HVAC guides (not the maker).

Worked examples

Applies to: Lennox furnaces and air conditioners. Always check the result against the label.