How old is my Lennox furnace or AC?

Type the serial number from the Lennox label to see when it was built.

Example

5178M99989

51Plant: 51 = Columbus, Ohio
78Year: 78 = 1978
MMonth: M = December

Built December 1978

47 years, 10 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, 51, are the plant code (Columbus, Ohio).
The next two digits, 78, are the year: 1978.
The letter M is the month: December. A is January and the letter I is skipped.

Format: Plant code, year and month letter. Sources: Lennox model and serial nomenclature (PDF), CPSC 2013 Lennox Hearth recall (serials 6412D to 6412H), CPSC 2020 Lennox ductless heat pump recall, Building Intelligence Center: Lennox.

Reading a Lennox serial number

Lennox serials have 10 characters: two plant digits, two year digits, a month letter, then a running number. 5178M means December 1978 at plant 51.

The month letter starts at A for January and skips I, so M is December.

About Lennox

Dave Lennox founded the company in Marshalltown, Iowa in 1895 to build a riveted steel coal furnace. Lennox International is now based in Richardson, Texas.
Lennox brand equipment is sold straight to independent dealers and through company-owned Lennox Stores, not through wholesale distributors.
Lennox has built home heating and cooling equipment in Marshalltown, Iowa and Orangeburg, South Carolina.

Sources: Lennox International Inc. Form 10-K for 2006, Lennox International Inc. Form 10-K for 2024

Plant code, year and month letter

Used about 1974 to today. Example: 5178M99989

Lennox serials have 10 characters. The first two digits are the plant, the next two are the year, and the fifth character is a month letter (A is January, I is skipped). 5898G means July 1998 at plant 58.

Worked example: 5178M99989 was built December 1978, so it is 47 years, 10 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: Lennox model and serial nomenclature (PDF), CPSC 2013 Lennox Hearth recall (serials 6412D to 6412H), CPSC 2020 Lennox ductless heat pump recall, Building Intelligence Center: Lennox

Lennox recalls

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

Year	Recall	Hazard
2020	Lennox ductless heat pumps	Electrical parts can fail and the unit can overheat and catch fire.
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.

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

Where the serial is on a Lennox

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.