Lennox serial number lookup: age, tonnage and furnace size
A Lennox serial number starts with a two-digit plant code, then two digits for the year and a letter for the month, from A for January to M for December. In the example on Lennox's identification form, 59 is Marshalltown, Iowa, 21 is 2021 and C is March. The decoder reads the current and the older Lennox plant lists and, from the model number, the nominal tonnage or the furnace input.
Where the serial number is
The nameplate is on the outdoor cabinet or on the furnace casing. It lists the model number, the serial number, the electrical data and, on cooling equipment, the refrigerant.
The current layout
Lennox form 210965 explains the example 59 21 C 05716. The form prints it with an extra letter O, one character too many, and the decoder never drops or swaps characters on its own, so type the serial exactly as your plate shows it. Current location codes are 15, 17, 18 and 19 for Saltillo, Mexico, 20 for Gainesville, Florida, 56 for Stuttgart, Arkansas and 57 to 59 for Marshalltown, Iowa.
The older layout
Lennox's older Serial Numbering System uses plant codes 51 for Columbus, 53 for Des Moines, 54 for Fort Worth, 55 for Heatcraft, 56 for Stuttgart, 58 for Marshalltown and 63 for Toronto. Its diagram reads 5178M99989 as Columbus, year 78, December, and its month table runs from A for January to M for December without I.
Centuries and shared codes
Both layouts print two year digits. We read the older plant codes with years 70 to 99 as 1970 to 1999 and the current location codes as this century; both are our reading, not Lennox's. Codes 56 and 58 appear in both lists, so a serial like 5885C12345 is checked both ways: as a current serial it would be 2085, which is impossible, so March 1985 remains.
Model numbers
In cooling models such as EL18KCV-036, the three digits after the first dash are the nominal capacity: 036 is 3 tons. Lennox also uses 041 or 042 for 3.5 tons, 047 or 048 for 4 tons and 059 or 060 for 5 tons.
Furnace input codes are not the code times 1,000 in Lennox's table: 045 is 44,000 BTU/h, 070 is 66,000, 090 is 88,000 and 135 is 132,000.
How to read the plate, step by step
- Read the first two digits as the location or plant code.
- Digits 3 and 4 are the year; the letter after them is the month.
- Leave out spaces if you like, but do not add or remove letters.
- Add the model number for the nominal tonnage or the furnace input.
Serial formats the decoder reads
| Format | How it is read |
|---|---|
| Lennox location + year + month letter (current) | Lennox, current layout: two digits for the location, two for the year, a month letter (A = January, B = February, C = March and so on), then 5 or 6 digits (Lennox form 210965). |
| Lennox plant + year + month letter (older) | Lennox, older layout (Serial Numbering System): a two-digit plant code, the two-digit year, a month letter (A = January to M = December, I skipped) and a sequence. |
Model number codes the decoder reads
Only the codes listed here are translated. A code outside this list is shown as "not in the table", and you read the size or input from the plate instead.
| Nomenclature | Position | Codes |
|---|---|---|
| Lennox split AC or heat pump | Lennox split AC and heat pumps such as EL18KCV-036: the three digits after the first dash are the nominal cooling capacity (036 = 3 tons; Lennox also uses 041/042 for 3.5, 047/048 for 4 and 059/060 for 5 tons). | 018 = 1.5 tons, 024 = 2 tons, 030 = 2.5 tons, 036 = 3 tons, 041 = 3.5 tons, 042 = 3.5 tons, 047 = 4 tons, 048 = 4 tons, 059 = 5 tons, 060 = 5 tons |
| Lennox gas furnace | Lennox gas furnaces such as the ML180UH series: the three digits after the series letters are the input code, and Lennox's table does not use code times 1,000 (070 = 66,000 BTU/h). | 135 = 132,000 BTU/h, 045 = 44,000 BTU/h, 070 = 66,000 BTU/h, 090 = 88,000 BTU/h |
Worked examples
Each serial below is either printed in a source or built by us strictly by the source's rule (marked "by the rule"), and each is an automated test of the decoder. Readings that depend on today's date are as of October 27, 2026.
| Serial | Result | Why |
|---|---|---|
| 5921C05716from a source | March 2021Decoded | Lennox's example reads location 59 (Marshalltown, heating), year 21, month C = March. Lennox prints it as 59 21 CO 05716, one character too long; we test it without the O. |
| 5921CO05716by the rule | Format not in our database | The example exactly as printed, with the letter O, is not a valid layout; we do not drop or swap characters on our own. |
| 5178M99989from a source | December 1978Decoded | The example on Lennox's older diagram: Columbus plant, year 78, December. That 78 means 1978 is our reading of the century. |
| 5885C12345by the rule | March 1985Decoded | Code 58 is in both lists; as a current location, year 85 would be 2085, so only the older reading (March 1985) is left. |
| 3021C05716by the rule | Not confirmed by the manufacturer | 30 is not a Lennox location code in our sources. |
Edge cases and limits
- Lennox's warranty page lists two more serial shapes, a letter, two digits, a letter and six digits, or two letters, two digits, a letter and five digits, without explaining them; the decoder recognises them and reports them as not confirmed.
- A location code that is in neither list, or a fifth character outside A to M, means the serial is not in Lennox's documented layouts.
- Lennox says its older model codes, such as -41 for 3 tons, are no longer accurate measurements, so the decoder reads only the current three-digit codes.
- Furnace input codes that are not in our transcription of Lennox's table, such as 110, are not translated.
Questions
Why can't I just multiply a Lennox furnace code by 1,000?
Because Lennox's own table does not. Code 070 is listed as 66,000 BTU/h and 090 as 88,000. Multiplying would overstate the input, so the decoder uses only the published values.
My Lennox serial starts with 58. Which plant is it?
Marshalltown, Iowa, in either list. The year decides the layout: a year that would lie in the future as a current serial is read with the older system.
Does the Lennox model number show the refrigerant?
Lennox's newer nomenclature has a refrigerant letter, but we have not confirmed its position for every series, so the decoder does not read it yet. The refrigerant is printed on the nameplate.
Related pages
- Trane: Trane serials carry a fiscal week and, since 2010, a plant letter at the end.
- Carrier and Bryant: Carrier and Bryant put the week and the year first.
- Goodman, Amana and Daikin: Goodman, Amana and Daikin also give the month, in digits.
- All HVAC brand families: Every brand family we decode, and the brands we do not decode yet with the reason.
- Where to find the data plate: Where the serial number sits on each type of equipment and how to copy it without mistakes.
- Refrigerants and EPA dates: Why the date of manufacture alone does not tell you whether a unit uses R-22, R-410A or R-454B.
- Service life estimates: What ENERGY STAR publishes about typical ages, and what kind of number each figure is.
Sources
Every rule on this page comes from the documents below, published by the manufacturer or a U.S. government agency. The date is when we last checked each one.
- Model and serial number identification, form 210965 (03/2025) Lennox International. Checked .
- Bulletin 210941 (08/2022) Lennox International. Checked .
- Lennox model and serial nomenclature (page 3, Serial Numbering System) Lennox International. Checked .
- Engineering handbook EL18KCV (2025) Lennox International. Checked .
- Engineering handbook ML180UHV (input table) Lennox International. Checked .