Lennox furnaces, air conditioners and heat pumps use a ten-character serial number where the date of manufacture sits in the middle rather than at the start. The first two digits identify which Lennox manufacturing plant built the unit, not the date. The third and fourth digits are the two-digit year of manufacture, and the character immediately after them is a letter standing for the month, from A for January through M for December, skipping the letter I. A serial reading 5621M in its first five characters was therefore built in December of 2021 at the plant coded 56.
Lennox confirms that its serial numbers carry model and production information and publishes a model-and-serial nomenclature reference, but the exact plant/year/month breakdown used here comes from multiple independent HVAC-trade sources that agree on the same reading, not from directly quoted Lennox text.
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Decode your own serial number
What this covers
- Lennox residential furnaces, air conditioners and heat pumps with a 10-character serial (four digits, one letter, five digits) on the rating plate
- Reading the two-digit year from positions 3–4 and the month from the letter in position 5
- The plant code in positions 1–2, which does not affect the date reading
What it does not cover
- ×Lennox equipment whose serial does not fit the four-digit, one-letter, five-digit pattern
- ×A guarantee that the plant-code table (positions 1–2) is complete — this page only confirms the date fields
- ×Ducane or other Lennox-family brands not confirmed to share this exact layout
Try in this order
0 of 4 completedLocate the rating plate
ReversibleOn a furnace, check inside the front access panel with the unit off. On an outdoor condenser or heat pump, check the exterior panel near the service valves.
Skip the first two digits
ReversibleThese identify the manufacturing plant, not the date, and do not affect the year/month reading.
Read positions 3–4 as the year
ReversibleThese two digits are the two-digit year of manufacture.
Read the letter in position 5 as the month
ReversibleA = January through M = December, skipping I. A serial reading 5621M means December 2021.
Evidence ledger
Sources behind this answer
Where the manufacturer's article stops. Lennox's own support page confirms the location of the serial number and that a nomenclature reference exists, but does not itself spell out which characters mean plant, year or month in consumer-facing text. That exact breakdown is sourced from an independent reference here, with the evidence grade set to reflect that difference.
Official Lennox page confirming where the rating plate and serial number are located; it does not itself state the digit-position decode rule, which comes from the independent source below. Verified Sept. 10, 2026.
Independent reference stating the exact reading used below: positions 1–2 are the plant code, 3–4 are the year, and the letter in position 5 is the month. Verified Sept. 10, 2026.
Exact questions
Questions about Decode a Lennox serial number
Why don't the first two digits of a Lennox serial mean anything about the date?+
They identify the manufacturing plant. The year and month are read from positions 3–4 and the letter in position 5, further into the serial.
Why does the month use letters and skip I?+
Lennox's month code runs A through M for the twelve months and skips I specifically to avoid confusion with the number 1, a convention shared with several other manufacturers' letter-coded fields.
Is this rule confirmed directly by Lennox?+
Lennox confirms the serial carries production information and where to find it, but this exact digit breakdown comes from independent HVAC-trade sources that agree with each other, which is why this page's evidence grade is B rather than A.
Where do I find the Lennox unit serial number?+
Look for the rating plate or label on the unit, on the unit's rating label. Photograph it before you start, since the label can be hard to read once the appliance is installed.