How do you read a Lennox AC serial number?
Lennox uses one serial layout across its products, including its air conditioners and heat pumps. The first two digits are the plant code, the next two are the year, then a single month letter, then a five or six digit unit number. The month letters run A for January to M for December and leave out I.
Sources: Model Numbering Systems: Lennox model and serial nomenclature, checked October 19, 2026; Model Number Identification (Current Product): Furnaces, All R-454B and R-32 Products, Boilers, checked October 19, 2026.| Characters | What they mean | In 1823K01234 |
|---|---|---|
| 1 and 2 | Plant code | 18: Saltillo, Mexico (cooling) |
| 3 and 4 | Year, two digits | 23: 2023 |
| 5 | Month letter (A to M, no I) | K: October |
| 6 onward | Unit number, five or six digits | 01234 |
What will the home systems age report add?
The home systems age report will put your Lennox air conditioner or heat pump next to every other major system in your home, each with its build year, how long that kind of system typically lasts, and what to budget for replacing it.
Get the home systems age report
We’re building a home systems age report: your Lennox AC and every other major system, each with its age, typical life and replacement budget.
Where is the serial number on a Lennox air conditioner?
The rating plate is on the outdoor unit, near the service panel. Lennox describes it as a sticker or plate with black letters on a white background. The model number comes first; the serial number is the 10 or 11 character code below it.
What do some Lennox serial numbers decode to?
The first row is the sample printed in Lennox’s own guide. The others are made up to show the rule and don’t belong to real units.
| Serial | Plant | Year | Month | Built |
|---|---|---|---|---|
5178M99989 (Lennox’s sample) |
51, Columbus | 1978 | M | December 1978 |
1823K01234 |
18, Saltillo, Mexico (cooling) | 2023 | K | October 2023 |
5712F00456 |
57, Marshalltown, Iowa (cooling) | 2012 | F | June 2012 |
5605D33310 |
56, Stuttgart, Arkansas | 2005 | D | April 2005 |
Plant codes come from Lennox’s current list, and the list has changed over the years, so the year and month are the part to rely on.
How long do Lennox air conditioners and heat pumps last?
Lennox says most air conditioners last 15 to 20 years with proper maintenance, and that heat pumps typically last 10 to 15 years with proper care.
Why does the build date matter for the warranty?
For Lennox’s Merit series basic warranty, coverage starts on the installation date. If that date can’t be verified, the warranty period begins six months after the build date. The installer’s invoice or the registration record is the better proof.
Common questions about Lennox AC serial numbers
What do the Lennox month letters mean?
A is January, B February, C March, D April, E May, F June, G July, H August, J September, K October, L November and M December. Lennox skips the letter I.
What if my Lennox serial starts with a letter?
Lennox’s warranty page lists two other serial shapes that start with letters, and the Lennox documents we checked don’t explain them. Lennox’s customer support can date those units from the model and serial number.
Is my indoor coil dated the same way?
Lennox’s guide says all Lennox products use one serial coding system, and its current plant list includes a code for air handlers and coils. Read the coil’s own rating plate, since it may not be the same age as the outdoor unit.