How do you read a Lennox serial number?
Lennox uses one serial layout across its products. The first two digits are the plant code, the next two are the year, then a single letter for the month, then a five or six digit unit number. Lennox’s month letters run A for January to M for December and leave out I, so J is September.
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 5814B04321 |
|---|---|---|
| 1 and 2 | Plant code | 58: Marshalltown, Iowa |
| 3 and 4 | Year, two digits | 14: 2014 |
| 5 | Month letter (A to M, no I) | B: February |
| 6 onward | Unit number, five or six digits | 04321 |
What will the home systems age report add?
The home systems age report will put your Lennox furnace 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 furnace and every other major system, each with its age, typical life and replacement budget.
Where is the serial number on a Lennox furnace?
Lennox puts the rating plate inside the furnace, behind the top front service panel, on the left side of the cabinet or on top of the blower deck. Which one depends on the furnace model and how it’s installed. A technician can read it for you at the next service visit if you’d rather not turn the power off and remove the panel.
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 |
5814B04321 |
58, Marshalltown, Iowa | 2014 | B | February 2014 |
5808G12345 |
58, Marshalltown, Iowa | 2008 | G | July 2008 |
5921J00456 |
59, Marshalltown, Iowa (heating) | 2021 | J | September 2021 |
Lennox’s plant list has changed over the years, so the plant name matters less than the year and month. The current list includes 59 for its Marshalltown heating plant and 15 to 19 for its plants in Saltillo, Mexico.
How long do Lennox furnaces last?
Lennox says furnaces typically last 15 to 20 years with proper care. That’s a typical range, not a promise for any one furnace: installation, maintenance and how hard the furnace works all play a part.
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 furnace 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 a Lennox year of 08 from 1908 or 2008?
The year has only two digits, so read it in context. A furnace with 08 in the year position was built in 2008. Lennox’s older guide uses 78, for 1978, as its sample.