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How old is my Lennox furnace?

Serial number age decoder for Lennox furnaces: 2 format eras, the decode rule in plain English, transcribed tables, and the official lookup.

Serial number decoder for Lennox furnace

Serial no.

Type the serial from the data plate, not the model number. Nothing is sent anywhere; the tables are in this page.

Format example: 5899L17212

Lennox furnace serial numbers in the 1973 to present format encode the year as digits 3-4 (YY) and the month as letter at position 5 (A=Jan .. M=Dec, I skipped) (source: building-center.org).
Example: serial 5899L17212 decodes to November 1999.
Lennox furnace serial numbers in the before 1972 format encode the year as digits 1-2 and the month as digit(s) 3(-4), 1-12 (source: building-center.org).
Example: serial 6512 decodes to December 1965.
Lennox's official lookup at lennox.com returns Your Product's Coverage.

Format eras

1973 to present VERIFIED
####A#####digits 3-4 (YY)
before 1972 VERIFIED
####digits 1-2

How to read the serial

1973 to present formatVERIFIED

Shape ####A##### · example 5899L17212 · confidence high

Year: digits 3-4 (YY)
Week or month: month = letter at position 5 (A=Jan .. M=Dec, I skipped)
Plant: digits 1-2 = factory/plant code

Month code	Month
A	January
B	February
C	March
D	April
E	May
F	June
G	July
H	August
J	September
K	October
L	November
M	December

Example: serial 5899L17212 decodes to November 1999.

Trap: Looks identical to Carrier style 1 (WWYY L #####). Century ambiguous (99=1999 vs 2099 not an issue, but 06=1906/2006 needs era). Lennox official lookup accepts formats ####A#####, A##A######, AA##A##### which implies newer alpha-prefixed formats NOT covered by building-center; treat those as low-confidence until sourced.

before 1972 formatVERIFIED

Shape #### · example 6512 · confidence low

Year: digits 1-2
Week or month: month = digit(s) 3(-4), 1-12

Example: serial 6512 decodes to December 1965.

Trap: 3-4 char serial; ambiguous month split (e.g. 651 = 1965 Jan or ...)

Where the data plate is on a furnace

Inside the furnace: remove the upper (burner) access door and look on the inside of the door, on the blower compartment wall, or on a sticker facing you at the burner deck.

General guidance, not brand-specific: the owner's manual or the installer can point to the exact spot. See the data plate guide.

For scale only: the InterNACHI Standard Estimated Life Expectancy Chart for Homes lists Furnace at 15 to 25 years of expected service (nachi.org). That chart is a guideline, not a prediction for your unit.

Official lookup: the authority

Open the official Lennox lookup takes serial (####A#####, A##A######, AA##A#####); returns Your Product's Coverage; URL verified live

This site is a reference built from published tables. The manufacturer's own lookup wins whenever the two disagree.

Sources

Frequently asked questions

How do I tell the age of a Lennox furnace from the serial number?

In the 1973 to present format the year is digits 3-4 (YY) and the month as letter at position 5 (A=Jan .. M=Dec, I skipped). Example: serial 5899L17212 decodes to November 1999.

Where is the serial number on a Lennox furnace?

On the data plate. Inside the furnace: remove the upper (burner) access door and look on the inside of the door, on the blower compartment wall, or on a sticker facing you at the burner deck.