How old is my Armstrong Air furnace or air conditioner?

By Dana Whitlock. Updated October 2026. Widely documented.

Find the lone letter in the middle of an Armstrong Air serial. That letter is the month, and the two digits just before it are the year. So 1605G24159 is a July 2005 unit, G for July and 05 for the year. This ten-character layout covers units built since 1992.

Armstrong furnaces from the 1980s carry an older nine-character serial. The tool reads that one too, with thinner documentation behind it. Plenty of those older units came out of Bellevue, Ohio, Armstrong's home base for decades.

Where to find the label on a Armstrong Air furnace

On an Armstrong Air furnace or air handler, the rating plate is inside the cabinet behind the access door. Switch the unit off before opening the door. Outdoor condensers and heat pumps put it on the side of the cabinet near the electrical panel, readable from outside. Armstrong plates often print the serial with a space after the month letter, like 1605G 24159; leave the space out when you type it.

How to read the code

Current format, ten characters: two digits for the plant, two for the year, one month letter, five-digit sequence. Month letters go A for January through M for December with I skipped, so J is September, K October, L November and M December. Older format, nine characters, roughly 1978 to 1992: a letter, five digits, a month letter in seventh position, a year code in eighth, and a final letter. The year code runs 8 for 1978 and 9 for 1979, then A for 1980 through N for 1992, again skipping I.

Real serials in this code

When the format changed

The newer layout is well supported. An inspector reported that Armstrong confirmed by phone that serial 8499A13317 was built in January 1999, which matches the rule. The older nine-character table rests mostly on one forum post of maker-style guidance plus a few dated examples, such as A0346HKA, which InspectAPedia lists as August 1989 from Bellevue, Ohio. Older results are solid, but check them against any printed date on the plate. A fifth character that is I, or a letter past M, means a misread or a different label.

Who makes Armstrong Air

Armstrong Air is sold by Allied Air Enterprises, a Lennox International company. Lennox's 1999 annual report listed the Bellevue, Ohio plant as Armstrong's headquarters, factory and distribution center. In 2006 Lennox announced it would close Bellevue and consolidate Allied Air in South Carolina. Armstrong reaches homeowners through wholesale distributors and local dealers, the same two-step route as its sister brands AirEase, Ducane and Concord, which share its serial format.

How sure is this?

Widely documented. Armstrong Air 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 Armstrong Air's warranty page before you spend money on the answer.

Quick notes

• Register within 60 days of installation; for a home you just bought, the 60 days run from closing.
• Unregistered Armstrong equipment defaults to a 5-year parts and 20-year heat exchanger limited warranty.
• Allied Air's Extended Warranty Lookup, linked from Armstrong's site, can confirm coverage on registered units.
• If you have the old nine-character serial, the plate is probably from before 1993, so plan around an older system.