How old is my Lennox furnace or AC?

By Dana Whitlock. Updated October 2026. Widely documented.

A Lennox serial starting 58 was not built in 1958. The first two digits are the factory code. Digits three and four are the year and the letter in position five is the month, so 5898G07568 reads as plant 58, 1998, G for July.

The month letters run A for January through M for December, with I left out: A Jan, B Feb, C Mar, D Apr, E May, F Jun, G Jul, H Aug, J Sep, K Oct, L Nov, M Dec. This reading is Widely documented by home inspectors and matches the serial ranges Lennox published in a CPSC recall.

Where to find the label on a Lennox furnace

On Lennox furnaces and air handlers, the rating plate is inside the cabinet behind the blower or burner access door. Turn the thermostat to off and switch off the furnace's service switch before lifting that door, and use a flashlight to read the plate without putting your hands inside. Outdoor Lennox air conditioners and heat pumps have the plate on the side of the cabinet, near the electrical access panel. Read it from the outside only: the electrical panel itself is for a licensed HVAC technician. Older Lennox units sometimes carry a separate sticker with the date printed on it.

How to read the code

The modern Lennox serial is ten characters: two-digit plant, two-digit year, one month letter, five-digit sequence. Labels often add a space, as in 5898G 07568, so strip spaces before reading. 5896A36571 is January 1996, and 5894L24805 is November 1994. If position five holds a letter after M, or the letter I, the serial was misread or belongs to some other format, so do not force a date out of it. For the year pair, 70 to 99 means the 1900s and 00 to 69 means the 2000s.

Real serials in this code

When the format changed

Lennox has used the plant-year-month layout since the early 1970s, which is a long run without a change. The best outside check comes from a 2011 CPSC recall of Lennox garage heaters: serial ranges starting 5604G and 6004G belonged to units sold from July 2004, and 04 plus G decodes to July 2004. Because the year is two digits and the format has run for five decades, a 1970s Lennox and a 2070s Lennox would look alike, but that is not a practical problem yet. A real ambiguity is reading a smudged letter, especially telling G from C or J from I. If the letter is unclear, look for a dated sticker, the install invoice, or use Lennox's warranty lookup.

Who makes Lennox

Lennox International builds Lennox-brand equipment, sold through Lennox dealers. Its Allied Air Enterprises division makes Armstrong Air, Ducane, AirEase, Magic-Pak and Concord. Newer Armstrong Air, AirEase and Ducane units use the same plant-year-month serial as Lennox, but some older Allied brands used a 9-character code or an all-numeric code, so check the brand page for those.

How sure is this?

Widely documented. Lennox doesn't publish this code; it comes from recall notices, service literature and inspector references. We tested the lennox allied rules against 14 serials, 7 of them with a date known from a recall notice, a printed label date or the maker. None decoded to a wrong year; codes whose year mark repeats show every possible reading. Always confirm with Lennox's serial lookup before you spend money on the answer.

Quick notes

• Ignore the first two digits; they only identify the factory.
• For the month, count letters from A and skip I. G lands on July.
• Photograph the plate with the flash off and side lighting so the month letter is legible.
• Look for a separate date sticker on older Lennox units before decoding.