How old is my A. O. Smith water heater? Reading the serial number

A. O. Smith, one of the largest U.S. water heater makers, uses a simple numeric date code on most tanks built in roughly the last fifteen years. It also applies to State and several other brands made by A. O. Smith.

Where to find the serial number

Find the rating label on the side of the tank, usually in the upper third, near the gas control valve on gas models or the upper access panel on electric models. Do not confuse the serial number with the longer model number printed nearby.

How to read it

1. Take the first two digits as the year: 15 means 2015.
2. Take the next two digits as the week of the year, 01 to 52.
3. The rest is the production number.

Weeks near the start or end of a month can fall either side, so treat the month as approximate.

Serial numbers that start with a letter

Older A. O. Smith and State units, built before roughly 2008, often start with a letter and use a different date code. If yours starts with a letter, the heater is very likely old enough that replacement should be on your radar regardless of the exact date. The manufacturer's product registration and support pages can confirm it.

What the age tells you

Most tank water heaters last about 8 to 12 years. If yours is past ten years and showing signs such as rusty water, rumbling or moisture at the base, compare any repair with the water heater replacement cost. A. O. Smith and State gas models report faults with a blinking status light; our A. O. Smith error codes page explains the patterns.