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Lennox family

How old is my Lennox air conditioner?

Enter the serial number from the label. The date comes from Lennox's documented serial rules, not a guess.

Look for “Serial No.” or “S/N” on the label. Spaces and dashes are fine.

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How to read a Lennox air conditioner serial number

Lennox serials are 10 characters: a two-digit plant code, a two-digit year, a month letter, then a five-digit sequence. Allied Air brands (Armstrong Air, AirEase, Ducane, Concord) have used the same format since 1993.

  • Plant + year + month letter (Lennox since the 1970s; Allied brands since 1993)

    Example: 5178M99989 (Lennox) was made in December 1978. Lennox-family serials start with a two-digit plant code (51), then the year (1978) and a month letter ("M" = December; the letter I is skipped, so J is September).

Not decoded yet: Older all-numeric Ducane/Concord serials, pre-1973 Lennox and 1970s AirEase formats, and Lennox ductless units are not decoded yet.

Where to find the serial number

The rating plate is on the outdoor unit, usually on the side or back of the cabinet near the electrical disconnect or the refrigerant line connections.

Don't open the unit's panels to look for it. If the plate is faded, the indoor coil or the installer's paperwork may also list the model and serial.

Lennox air conditioner model numbers

If the brand name isn't visible on the label, the model number can help. Model numbers like these follow Lennox's published nomenclature:

How long does a air conditioner last?

A typical air conditioner lasts about 10–15 years. That's a general estimate: maintenance, usage, climate and installation all matter, and age alone doesn't tell you a unit's condition. Some manufacturer pages cite 15–20 years for well-maintained systems.

Range from NAHB / Bank of America, Study of Life Expectancy of Home Components (2007, archived copy) and Carrier: AC replacement.

Lennox air conditioner recalls

From the U.S. Consumer Product Safety Commission, checked 2026-09-29. A decoded result checks your model number against recalls live.

Questions

How can I tell how old my Lennox air conditioner is?
Find the serial number on the rating plate and enter it above. Lennox air conditioners use the Lennox family serial format, which includes the manufacture date. For example, 5178M99989 was made in December 1978: Lennox-family serials start with a two-digit plant code (51), then the year (1978) and a month letter ("M" = December; the letter I is skipped, so J is September).
Where is the serial number on a Lennox air conditioner?
The rating plate is on the outdoor unit, usually on the side or back of the cabinet near the electrical disconnect or the refrigerant line connections. Don't open the unit's panels to look for it. If the plate is faded, the indoor coil or the installer's paperwork may also list the model and serial.
How long does a air conditioner usually last?
A typical service life is about 10–15 years. That's a general estimate: maintenance, usage, climate and installation all matter, and age alone doesn't tell you a unit's condition.
Do Armstrong Air, Ducane, AirEase and Concord air conditioners use the same serial format?
Yes. Lennox, Armstrong Air, Ducane, AirEase and Concord use the same serial-number conventions (Lennox family), so the same decoding rules apply.
How reliable is the result?
The Lennox family rules come from documented sources, but they're in Beta while we check more real-world examples, so results are capped at medium confidence. If a serial doesn't match a documented format, we say so instead of guessing.

Same serial format: Armstrong Air air conditioner, Ducane air conditioner, AirEase air conditioner, Concord air conditioner

Other Lennox equipment: furnace, heat pump

Sources

Rules last reviewed 2026-09-28. These rules are in Beta while we verify more real-world examples, so results are capped at medium confidence.

Manufacture dates are decoded from manufacturer serial-number conventions and available documentation. Serial formats can change. Verify critical equipment information with the manufacturer when necessary.