Lennox serial number lookup: age of a furnace, AC or heat pump

The serial number is on the nameplate: inside the cabinet of a furnace, or near the service panel of an outdoor unit. In a Lennox serial the third and fourth digits are the year and the letter after them is the month.

Decode a Lennox serial number

Look for “Serial”, “Serial No.” or “S/N” on the rating plate. It is not the model number. What you type stays in your browser.

How to read a Lennox serial number

Built since the early 1970s

Plant code, year, month letter

The first two digits are the plant. The third and fourth digits are the year. The letter after them is the month, A for January through M for December with the letter I left out.

Shape: four digits, a letter and five or six digits.

58plant99yearLmonth17212
Example: serial 5899L17212 reads as November 1999.

Month letter

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

Many websites say a Lennox serial starts with the week and the year. Lennox’s own documents and the serial ranges in recall notices show that it does not: the first two digits are the plant.

Where to find the serial number

On a furnace, remove the top front service panel and look on the left side of the cabinet interior or on top of the blower deck.

On cooling equipment the model and serial numbers are on a nameplate or sticker near the service panel: a white label with black letters.

If the serial number does not decode

  • Lennox’s warranty page shows two other serial patterns, one starting with a letter and one starting with two letters, without explaining them. The decoder does not read those.
  • Lennox plates do not always print a manufacture date. Older units may carry the year on a separate sticker.
  • Gas furnaces from the 1980s and 1990s print the year of the safety standard they were certified to, for example “Z21.47” followed by a year. The furnace was built in that year or a few years after it.
  • The manufacturer can confirm the date from the serial: Lennox registration, warranty or serial lookup.

What the age tells you

EquipmentTypical service life
Furnace15 to 22 years
Central air conditioner10 to 21 years
Heat pump15 to 16 years

What size is it?

The model number on the same rating plate gives the size: the tonnage of an air conditioner or heat pump, or the heating input of a furnace. Lennox model number tonnage shows where the digits sit and reads them for you.

About Lennox

Lennox was founded in Marshalltown, Iowa in 1895. Its Allied Air Enterprises subsidiary builds the Armstrong Air, Ducane, AirEase and Concord brands, which have used the same serial layout as Lennox since 1993.

Brands that share this serial system: Armstrong Air, Ducane.

Questions and answers

Which characters in a Lennox serial number show the date?

The first two digits are the plant. The third and fourth digits are the year. The letter after them is the month, A for January through M for December with the letter I left out. This applies to units built since the early 1970s.

Do the first four digits of a Lennox serial give the week and year?

No. That reading is common online and it is wrong. The first two digits are the plant that built the unit. The third and fourth digits are the year, and the letter in the fifth position is the month. Serial 5899L17212 was built in plant 58 in November 1999.

Sources

Formats last checked against these sources on October 5, 2026. Found a serial that does not fit? Send a correction. How formats are verified: method and sources.