How old is my Lennox furnace or AC?
Enter the serial number from the label. The date comes from Lennox's documented serial rules, not a guess.
Lennox serials are 10 characters: a two-digit plant code, a two-digit year, a month letter, then a five-digit sequence. Allied Air brands (Armstrong Air, AirEase, Ducane, Concord) have used the same format since 1993.
The same rules apply to Armstrong Air and AirEase and Ducane and Concord furnace, AC and heat pumps.
By equipment type
Lennox furnace age
Typical life: 15–20 years
Lennox air conditioner age
Typical life: 10–15 years
Lennox heat pump age
Typical life: 10–15 years
Where to find the serial number
Furnaces: the plate is inside the blower access door.
Outdoor units: look on the side panel near the service valves or electrical cover.
Questions
- Which letter is which month?
- A is January through M for December, skipping I. So J is September, K October, L November and M December. Mixing that up is the most common decoding mistake.
- My Armstrong Air serial looks completely different.
- Armstrong Air and AirEase units from 1978–1992 use a letter format where the 7th character is the month and the 8th is the year. We decode that too.
Same serial format
Sources
- Lennox model and serial nomenclature (archived 2010)
- Allied Air warranty procedures WP20565203 Rev 10 (2015): serial number formats
- Building-Intelligence Center: Lennox HVAC age
- Electrical-Forensics: Air conditioner date codes
Rules last reviewed 2026-09-28. These rules are in Beta while we verify more real-world examples, so results are capped at medium confidence.
Manufacture dates are decoded from manufacturer serial-number conventions and available documentation. Serial formats can change. Verify critical equipment information with the manufacturer when necessary.