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Lennox family

How old is my Lennox furnace?

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 furnace 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

Turn the furnace off at its switch, then remove the upper access door (the burner or blower compartment). The rating plate is on the inside wall of the cabinet or on the back of the door.

The plate lists the model number and serial number; many newer plates also print a date of manufacture.

Lennox furnace 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 furnace last?

A typical furnace lasts about 15–20 years. That's a general estimate: maintenance, usage, climate and installation all matter, and age alone doesn't tell you a unit's condition.

Range from NAHB / Bank of America, Study of Life Expectancy of Home Components (2007, archived copy), Carrier: how long does a furnace last? and Lennox: how long does a furnace last?.

Lennox furnace recalls

No Lennox furnace recalls were found in U.S. Consumer Product Safety Commission data when this page was checked (2026-09-29). A decoded result checks your model number live.

Questions

How can I tell how old my Lennox furnace is?
Find the serial number on the rating plate and enter it above. Lennox furnaces 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 furnace?
Turn the furnace off at its switch, then remove the upper access door (the burner or blower compartment). The rating plate is on the inside wall of the cabinet or on the back of the door. The plate lists the model number and serial number; many newer plates also print a date of manufacture.
How long does a furnace usually last?
A typical service life is about 15–20 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 furnaces 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 furnace, Ducane furnace, AirEase furnace, Concord furnace

Other Lennox equipment: air conditioner, 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.