How to read a Lennox serial number
Lennox serials have ten characters: four digits, a letter, then five digits [1][2][3].
| Position | Meaning | In 5898G07568 |
|---|---|---|
| Digits 1–2 | Plant code | 58 |
| Digits 3–4 | Year | 98 = 1998 |
| Letter 5 | Month | G = July |
| Last 5 digits | Unit number | 07568 |
The month letters skip I, so J is September and M is December:
| Letter | A | B | C | D | E | F | G | H | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
Don’t mix it up with Carrier
Carrier serials have the same shape (four digits, letter, digits) but put the week in digits 1–2 and the year in 3–4. Decode a Lennox serial as a Carrier and you get a wrong date, so make sure Lennox is selected. (See theCarrier page for its format.)
Worked examples
| Serial | How it reads | Made | Example from |
|---|---|---|---|
5898G07568 | 98 = year ’98; G = month letter (July) | July 1998 | Honest Home Inspections, LLC |
5205J24862 | 05 = year ’05; J = month letter (September)Source caption says 2004; digits say 2005. | September 2005 | PickHVAC |
1606B13871 | 06 = year ’06; B = month letter (February)Example serial; decoded by applying the shared rule. | February 2006 | Building Intelligence Center |
One published example, 5205J24862, is captioned September 2004 on its source page, but its year digits read 05[2]. We follow the digits: September 2005.
Where to find the serial number
On the rating plate on the cabinet. On a furnace, open the blower door (power off first). Older Armstrong units use the same chart.
What this page doesn’t decode
Lennox serials from before 1973 and some letter-heavy styles from the 1960s to 1990s. Sources describe these differently, so the decoder reports them as not recognised.
Common questions
How do I tell the age of my Lennox furnace or AC?
Skip the first two digits. The next two are the year, and the letter after them is the month.
What does the letter in a Lennox serial number mean?
It’s the month of manufacture: A = January through M = December, with no I.
Can a Lennox serial mean two different years?
Only a century apart, so no. The decoder also refuses dates in the future.
Not affiliated with Lennox. The brand name is used only to say which serials this decoder reads.
Sources
- Building Intelligence Center: Lennox HVAC AgeUsed for: Plant code + year + month letter (4 digits, letter, 5 digits).
- PickHVAC (Rene Langer, 2025-06-03): Lennox HVAC Age: Serial Number DecodingUsed for: Plant code + year + month letter (4 digits, letter, 5 digits).
- Honest Home Inspections, LLC (hosted by InspectAPedia): How to determine Water Heater and Furnace Age (PDF, last updated 2009-01-06)Used for: Plant code + year + month letter (4 digits, letter, 5 digits).