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How old is my Armstrong Air furnace or AC?

Type the serial number from the Armstrong Air label to see when it was built. Armstrong Air units are built by Allied Air, a Lennox company, so the serials follow that company's format.

Example

8499F63244

84Plant code: 84
99Year: 99 = 1999
FMonth: F = June

Built June 1999

27 years, 4 months old

Past typical life. Past the usual 15 to 25 year lifespan. Watch for rising gas bills, uneven heat, a yellow or flickering burner flame, rust or soot, and a carbon monoxide alarm.

Plan the replacement

Plan the replacement. Parts for older units get harder to find, and older AC units may use R-22 refrigerant, which is no longer made.
A common rule of thumb: if the repair cost times the age in years is over 5,000, replacing usually makes more sense.
Ask about rebates. Many utilities and states pay toward a high-efficiency replacement.
Some home insurers and home buyers’ inspectors ask the age of the HVAC system. Florida 4-point inspections record it.
Most HVAC parts warranties run 5 to 10 years and require registration within 60 to 90 days of install. Check with the installer.

How this was read

The first two digits, 84, are the plant code.
The next two digits, 99, are the year: 1999.
The letter F is the month: June. A is January and the letter I is skipped.

Format: Plant code, year and month letter. Sources: InspectAPedia: how to determine furnace age, with Ducane’s explanation (PDF), InterNACHI forum: Armstrong furnace date code, Building Intelligence Center: Armstrong, Building Intelligence Center: Ducane, Inspector Handbook: Armstrong Air, Lennox model and serial nomenclature (PDF).

Reading a Armstrong Air serial number

Newer Armstrong serials use the Lennox layout: plant, year, then month letter. 8499F means June 1999.

From 1978 to 1992 the serial ends in three letters: month, year and plant.

About Armstrong Air

Lennox Furnace Company started Armstrong in Columbus, Ohio in 1928.
Armstrong was based in Bellevue, Ohio for decades. Allied Air moved its work from Bellevue to South Carolina by early 2007.
Armstrong units are now assembled in Orangeburg, South Carolina.
Armstrong is sold through independent wholesale distributors, unlike the Lennox brand.

Sources: About Armstrong Air, Lennox International Inc. Form 10-K405 for 1999, Lennox's Allied Air To Consolidate Operations, Lennox International Inc. Form 10-K for 2024

Plant code, year and month letter

Used about 1974 to today. Example: 8499F63244

Armstrong Air, Ducane, AirEase and Concord use the Lennox layout: two plant digits, two year digits, then a month letter (A is January, I is skipped). 8499F means June 1999.

Worked example: 8499F63244 was built June 1999, so it is 27 years, 4 months old as of October 2026.

Month letter (fifth character)
Letter	Month
A	January
B	February
C	March
D	April
E	May
F	June
G	July
H	August
J	September
K	October
L	November
M	December

Sources: InspectAPedia: how to determine furnace age, with Ducane’s explanation (PDF), InterNACHI forum: Armstrong furnace date code, Building Intelligence Center: Armstrong, Building Intelligence Center: Ducane, Inspector Handbook: Armstrong Air, Lennox model and serial nomenclature (PDF)

Armstrong month letter and year letter

Used 1978 to 1992. Example: A25175KKA

Older Armstrong serials end in three letters. The first is the month (A is January, I is skipped), the second is the year (8 is 1978, A is 1980, K is 1989), and the last is the plant. A25175KKA means October 1989.

Worked example: A25175KKA was built October 1989, so it is 37 years old as of October 2026.

Month letter (third from last)
Letter	Month
A	January
B	February
C	March
D	April
E	May
F	June
G	July
H	August
J	September
K	October
L	November
M	December

Year character (second from last)
Character	Year
8	1978
9	1979
A	1980
B	1981
C	1982
D	1983
E	1984
F	1985
G	1986
H	1987
J	1988
K	1989
L	1990
M	1991
N	1992

All-digit serial with year and week at the end

Used about 1990 to the 2000s. Example: 4140720328

Some Ducane, AirEase and Concord serials are 10 digits with no letter. The last four digits are the year (2 digits) and the week (2 digits). Ducane gives 9933 as the 33rd week of 1999.

Worked example: 4140720328 was built Week 28 of 2003 (around July), so it is 23 years, 3 months old as of October 2026.

Armstrong Air recalls

Recalls from the U.S. Consumer Product Safety Commission that name Armstrong Air. Whether a unit is included depends on its model and build date; the checker above flags matches.

Year	Recall	Hazard
2023	Armstrong Air and AirEase 96% gas furnaces	A part assembled wrong can make the furnace produce high carbon monoxide.
1998	Plastic HTPV vent pipes on gas furnaces	Plastic vent pipes can crack or separate and leak carbon monoxide.

Standing at the unit? Use your phone

Point your phone’s camera at this code. The scanner opens and reads the label for you.

Same serial number format as Armstrong Air: AirEase, Concord, Ducane.

Formats on this page were last checked against their sources in October 2026. Armstrong Air is a trademark of its owner. This page is independent and not affiliated with or endorsed by Armstrong Air. For warranty questions, contact Armstrong Air directly.

Where the serial is on a Armstrong Air

On a furnace or air handler, remove the front door: the rating plate is on the inside wall of the cabinet. On an outdoor unit, look on the side of the cabinet near the electrical panel.

Turn off the furnace switch before removing the front door. The outdoor unit’s plate is usually on the side panel near the electrical connections, sometimes on the back.