How to read A. O. Smith water heaters serial numbers

This covers A. O. Smith, AO Smith, State, Reliance. The serial number is on the rating plate on the side of the tank.

Brand on the labelSerial number

Reading it yourself

Since about 2008: 11 to 15 characters that start with four digits. The first two digits are the year and the next two are the week of the year.
Example: 1210A002243 was made in week 10 of 2012.
About 1989 to 2008: the second character is a letter for the month (A is January, I is skipped, M is December). The two digits after it are the year.
Example: AF04A093001 was made in June 2004.
Before about 2008, mostly State tanks: the first character is a letter for the month (A is January, I is skipped, M is December). The two digits after it are the year.
Example: M07A009387 was made in December 2007.

This is the date the unit was made. It was usually installed soon after, but it can be later. If a serial doesn’t fit any of these patterns, look on the label for a printed manufacture date.

Sources

Checked in October 2026.

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