Lennox furnace and AC age by serial number

Reviewed 2026-10-155 primary sourcesSee sources

A Lennox serial starts with a two-digit plant code, then the last two digits of the year and a letter for the month. Lennox’s older nomenclature document and its current product guide both show this layout.

Covers: Lennox residential furnaces, air conditioners, heat pumps and coils with serials made of two digits, two digits, a month letter and a unit number.

Where to find the serial number

• Furnace: the label is behind the top front service panel, on the left side inside the cabinet or on top of the blower deck, depending on the model and airflow direction.^[4]
• Air conditioner or heat pump: the white nameplate with black letters is near the service panel.^[5]

Date formats Lennox documents

Plant + year + month letter + unit number1978 example to current products

Lennox doesn’t give the century; we read the two-digit year within 1975–2074, which covers Lennox’s 1978 example and its current 2019–2023 range. Lennox’s current example shows an extra O after the month letter that it doesn’t explain; the decoder ignores it.^[1]

What Lennox says

• Current plant codes: 15, 17, 18 and 19 are Saltillo, Mexico; 20 is Gainesville, Florida; 56 is Stuttgart, Arkansas; 57, 58 and 59 are Marshalltown, Iowa.^[2]
• Older plant codes: 51 Columbus, 53 Des Moines, 54 Fort Worth, 55 Heatcraft, 56 Stuttgart, 58 Marshalltown and 63 Toronto.^[1]
• Lennox’s warranty lookup also accepts serials shaped like A##A###### and AA##A#####, which no Lennox document we found explains.^[3]

Worked examples

• 5178M 99989: Plant 51 (Columbus), 1978, December (Lennox’s older example).^[1]
• 5921CO05716: Plant 59 (Marshalltown, Iowa, heating), March 2021 (Lennox’s current example).^[2]

What the serial can’t tell you

• Neither Lennox document says which years this layout covers; a two-digit year repeats every century.^[1]
• Plant codes outside Lennox’s two lists are unknown.^[2]
• The meaning of Lennox serials shaped A##A###### or AA##A#####.

Ask Lennox

• Lennox warranty lookup: Search by serial number for warranty coverage.^[3]