How old is my Lennox furnace?

By Dana Whitlock. Updated October 2026. Widely documented.

Your Lennox furnace's build date is in characters three through five of the serial: a two-digit year and then a month letter. A furnace serial of 5898G07568 means July 1998. The model number next to it tells you the stages, efficiency and size, and that context matters when you decide whether an expensive repair makes sense.

Where to find the label on a Lennox furnace

Lennox furnaces carry the rating plate inside the cabinet, behind the blower or burner access door. On an upflow furnace the blower door is at the bottom; on a horizontal furnace in an attic or crawlspace, it faces the side you service from. Set the thermostat to off, switch off the service switch, then pull the door. Read the plate by flashlight and leave everything inside untouched. Plates on older Lennox units, like the 1998 Pulse furnace discussed on InterNACHI, can be grimy; a dry cloth on the plate surface is fine.

How to read the code

Lennox serials are ten characters: a two-digit plant code, a two-digit year, a month letter, and five digits. The month letters run A to M without I, so H is August and J is September. On the model number, take SL280UH070V36 as the pattern: SL is the Dave Lennox Signature Collection, 2 means two-stage heating, 80 is the AFUE, UH is upflow/horizontal, 070 is the nominal input in thousands of BTU per hour, V is a variable-speed blower, and 36 is the cooling airflow the blower can support, about 2 to 3 tons. Extra letters can flag a California low-NOx version, a cabinet width, or a K for refrigerant-detection-ready.

Real serials in this code

What age means for a Lennox furnace

DOE's analysis puts the average gas furnace life at about 21.4 years. Other industry charts put gas furnaces anywhere from 15 to 25 years. Many Lennox units run longer than that with good care, but a furnace past 20 is living on borrowed time. As furnaces age, the parts that tend to fail are the ignition components, the inducer motor, the blower motor and the control board, and eventually the heat exchanger. Heat exchanger checks belong to a licensed HVAC technician during the yearly tune-up. Before you pay for a large repair on a Lennox, look at the warranty terms for your model. The current SL280 carries a limited 20-year heat exchanger warranty and 10 years on covered components in residential use, but those terms vary by model and registration. Ask for the failed part and the diagnosis in writing. If the repair is a heat exchanger on a furnace near the end of its expected life, the replacement range above may be the better use of the money.

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

• Count the stages: a 2 after the series letters means two-stage heat, which usually means a quieter, more even furnace worth keeping if the rest is sound.
• Match the 36 at the end of the model to your AC. That blower supports about 2 to 3 tons, so a bigger air conditioner needs a different blower.
• An AFUE of 90 or above means a condensing furnace with a condensate drain. A clogged drain is a common service call. It doesn't mean the furnace is done.
• Replace or check the filter monthly in heating season.