---
title: "Lennox AC and Heat Pump Age: Read the Serial Number"
description: "Find the year and month your Lennox air conditioner or heat pump was built from the serial on its outdoor sticker, and tell the two apart by model number."
canonical: "https://whatyearis.com/lennox-ac"
markdown: "https://whatyearis.com/lennox-ac.md"
last_updated: "2026-10-19"
---

# How old is my Lennox air conditioner or heat pump?

A Lennox air conditioner or heat pump carries its build year in the third and fourth digits of the serial number, followed by a month letter that runs from A for January to M for December with no I. So 1823K means October 2023, at Lennox’s Saltillo cooling plant. The serial is on the white sticker near the service panel of the outdoor unit.

By [the What Year Is team](https://whatyearis.com/editorial) · Updated October 19, 2026

## How do you read a Lennox AC serial number?

Lennox uses one serial layout across its products: a two-digit plant code, a two-digit year, a month letter, then a five or six digit unit number. Dating the furnace instead? See [how old is my Lennox furnace](https://whatyearis.com/lennox-furnace).

Sources: [Model Numbering Systems: Lennox model and serial nomenclature](https://www.lennox.com/lib/legacy-res/pdfs/lennox_model_and_serial_nomenclature.pdf), checked October 19, 2026; [Model Number Identification (Current Product): Furnaces, All R-454B and R-32 Products, Boilers](https://www.lennox.com/dA/d1bf319d00/ehb_model_id_r-454b-r32_2503.pdf), checked October 19, 2026.

| Characters | What they mean | In 1823K01234 |
| --- | --- | --- |
| 1 and 2 | Plant code | 18: Saltillo, Mexico (cooling) |
| 3 and 4 | Year, two digits | 23: 2023 |
| 5 | Month letter (A to M, no I) | K: October |
| 6 onward | Unit number, five or six digits | 01234 |

## What will the home systems age report add?

The report will list your outdoor unit with its build year and the typical life of an air conditioner or heat pump, then do the same for the furnace, water heater and every other major system, with a budget for when each may need replacing.

## More about Lennox AC serial numbers

- [How old is my Lennox furnace?](https://whatyearis.com/lennox-furnace): Where the label sits inside the furnace cabinet, and how to read a Lennox furnace model number.
- [How long do AC units last?](https://whatyearis.com/how-long-do-ac-units-last): Typical air conditioner and heat pump life across brands.

## Where is the serial number on a Lennox air conditioner?

Lennox mounts the plate on the outside of its outdoor units, so there’s no panel to open. Look near the service panel for a sticker or plate with black letters on a white background. The serial is the 10 or 11 character code on it.

## How can you tell a Lennox AC from a heat pump by model number?

On current Lennox outdoor units, the letter after the refrigerant code says what the unit is: C for an air conditioner, P for a heat pump, and L for a cold-climate heat pump. Lennox’s sample, `SL25KCV-036-230A-1`, reads like this:

| Characters | What they mean | In SL25KCV-036-230A-1 |
|---|---|---|
| SL, EL or ML | Tier: Dave Lennox Signature, Elite or Merit | SL: Signature |
| Two digits | Nominal efficiency | 25 |
| K | R-454B refrigerant | K |
| C, P or L | Air conditioner, heat pump, or cold-climate heat pump | C: air conditioner |
| 1, 2 or V | Compressor: single-stage, two-stage, or variable | V: variable |
| Three digits after the dash | Cooling capacity: 024 is 2 tons, 036 is 3, 048 is 4 | 036: 3 tons |

Older units use HS for an air conditioner’s outdoor unit and HP for a heat pump, with the size after the dash in an older code: 26 is 2 tons, 31 is 2½ and 41 is 3.

## What do some Lennox AC serial numbers decode to?

All three rows are made up to show the rule and don’t belong to real units. The plant codes come from Lennox’s current list.

| Serial | Plant | Year | Month | Built |
|---|---|---|---|---|
| `1823K01234` | 18, Saltillo, Mexico (cooling) | 2023 | K | October 2023 |
| `5712F00456` | 57, Marshalltown, Iowa (cooling) | 2012 | F | June 2012 |
| `5605D33310` | 56, Stuttgart, Arkansas | 2005 | D | April 2005 |

## How long do Lennox air conditioners and heat pumps last?

Lennox gives a different range for each. It says most air conditioners last 15 to 20 years with proper maintenance, and heat pumps typically last 10 to 15 years with proper care. The model number tells you which one you have: C for an air conditioner or P for a heat pump on current units.

## Common questions about Lennox AC serial numbers

### Is my indoor coil dated the same way?

Lennox’s guide says all Lennox products use one serial coding system, and its current plant list includes a code for air handlers and coils (15, Saltillo). Read the coil’s own rating plate, since it may not be the same age as the outdoor unit.

### Why does the build date matter for the warranty?

For Lennox’s Merit series basic warranty, coverage starts on the installation date. If that date can’t be verified, the warranty period begins six months after the build date, so the installer’s invoice or the registration record is the better proof.

### What if the outdoor unit’s serial doesn’t fit this pattern?

Lennox’s warranty page shows serial shapes that open with a letter, and no Lennox document we checked decodes them. Give the model and serial to Lennox customer support and ask for the build date.

## What will the home systems age report cost?

The home systems age report will cost $19 per home. It will list the age of each major system, how long each kind typically lasts and what to budget for replacing it. The report isn’t open yet. Join the waitlist and we’ll email you the day it opens.

## Sources

- [Model Numbering Systems: Lennox model and serial nomenclature](https://www.lennox.com/lib/legacy-res/pdfs/lennox_model_and_serial_nomenclature.pdf), Lennox Industries Inc.. All Lennox product uses one serial coding system (plant, year, month letter, serial number, sample 5178M 99989); on products other than furnaces the plate is mounted on the outside of the unit; HS is a condensing unit and HP a heat pump outdoor unit, and in older model numbers -26 is 2 tons, -31 2½ tons and -41 3 tons.. Checked October 19, 2026.
- [Model Number Identification (Current Product): Furnaces, All R-454B and R-32 Products, Boilers](https://www.lennox.com/dA/d1bf319d00/ehb_model_id_r-454b-r32_2503.pdf), Lennox Industries Inc.. Current AC and heat pump model numbers (sample SL25KCV-036-230A-1): tier SL, EL or ML, nominal efficiency, K for R-454B, C for air conditioner or P for heat pump, 1, 2 or V cooling stages, and capacity (036 = 3 tons). Serial plant codes include 15 Saltillo (AH and A coils), 18 Saltillo (Cooling) and 57 Marshalltown IA (Cooling).. Checked October 19, 2026.
- [Lennox Warranty Lookup](https://www.lennox.com/residential/owners/assistance/warranty/), Lennox. Serial numbers are printed on a label on the equipment, and Lennox lists three serial shapes: ####A#####, A##A###### and AA##A#####.. Checked October 19, 2026.
- [How can I confirm the model and serial numbers for my cooling system?](https://www.lennox.com/residential/support/faqs/model-serial-numbers-cooling), Lennox. On a Lennox cooling system, the model and serial numbers are on the nameplate or sticker near the service panel, which should have a white background with black letters.. Checked October 19, 2026.
- [How Long Do Air Conditioners Last?](https://www.lennox.com/residential/lennox-life/consumer/how-long-do-air-conditioners-last), Lennox. Lennox says most AC units last 15 to 20 years with proper maintenance.. Checked October 19, 2026.
- [How Long Do Heat Pumps Last?](https://www.lennox.com/residential/lennox-life/consumer/how-long-do-heat-pumps-last), Lennox. Lennox says heat pumps typically last 10 to 15 years with proper care.. Checked October 19, 2026.
- [Lennox Equipment Basic Limited Warranty (W900000-02e)](https://www.lennox.com/dA/60a7c17d7a/W900000-02e.pdf), Lennox Industries Inc.. If the original installation date can’t be verified, the warranty period begins six months after the date of manufacture.. Checked October 19, 2026.

## About this document

This is the Markdown version of a page on What Year Is (https://whatyearis.com). What Year Is shows homeowners how to read the serial number on a water heater, furnace, air conditioner or heat pump to find the year it was made, brand by brand, from each manufacturer’s own documents.

- Site index for AI assistants: https://whatyearis.com/llms.txt
- About What Year Is: https://whatyearis.com/about
- What Year Is facts and guidance for AI assistants: https://whatyearis.com/facts
- All systems: https://whatyearis.com/#products
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