How old is my Lennox HVAC?
On the long-running Lennox format the first two digits are a plant code, the next two are the year, and the fifth character is a letter for the month (e.g. 5621M… is 2021, M = December).
The same decoder works for: Aire-Flo, Armstrong Air, Ducane, AirEase, Concord.
Free. The serial is on the metal nameplate or rating label, usually near the bottom of the unit.
Lennox serial-number formats
| Era | Format | Confidence |
|---|---|---|
| 1973–present | a 10-character serial — two plant digits, two-digit year, a month letter, then five digits (used since 1973) | medium |
Worked examples
5621M99989
→ manufactured December 2021 (about 5 years old). Read as a 10-character serial — two plant digits, two-digit year, a month letter, then five digits (used since 1973). Decoded year 2021 (digits "21"), month December (letter "M") → manufactured December 2021, about 5 years old.
Frequently Asked Questions
How do I find the age of my Lennox unit?
On the long-running Lennox format the first two digits are a plant code, the next two are the year, and the fifth character is a letter for the month (e.g. 5621M… is 2021, M = December). Enter the serial from the unit's nameplate into the decoder on this page and it returns the manufacture date and age instantly. Always confirm against the manufacture date printed on the nameplate.
Where is the serial number on a Lennox HVAC?
The serial number is printed on the metal data plate or rating label, usually near the bottom or on the side of the unit. It is labeled "Serial," "Serial No.," or "S/N." Do not confuse it with the model number.
Is the Lennox serial-number date exact?
It identifies the manufacture date, not the install date — a unit is often installed weeks or months after it was built. Lennox has used more than one serial format (1973–present), so some decodes are approximate and flagged accordingly. Verify against the manufacture date on the nameplate when one is printed.
Developers & AI agents: the Lennox decode rules are available as JSON.
Serial-number decoding is approximate. Results are estimates based on published manufacturer formats; always verify against the manufacture date printed on the unit nameplate. ApplianceAge is a CarWhere tool and is not affiliated with, endorsed by, or sponsored by any manufacturer. Brand names are used only to identify the equipment a serial belongs to.