A.O. Smith Water Heater Age: Serial Number Decoder

Type the serial number from the rating plate to get the build year, build month and age. It reads the four-digit year-week style and one older letter format.

Last updated: 9 November 2026

How this is calculated

A.O. Smith publishes a manufacturing-date guide on its website. For serial numbers that start with four digits, the first two digits are the year and the next two are the week. Its worked example, a serial starting 2037, is week 37 of 2020.

Style 1: four digits first (year and week)

1. Year = 2000 plus the first two digits. A year after the as-of date means we cannot date it.
2. Week = digits 3 and 4. We read it as an ISO 8601 week, so a week must exist in that year: week 00 does not, and week 53 exists only in years that have 53 ISO weeks (2020 does, 2021 does not). An impossible week is an error.
3. The build date is taken as the Monday of that week, and the build month is the month of that Monday. The page shows the Monday-to-Sunday span so you can see the range.
4. Age is whole completed years from that Monday to the as-of date.

Worked example: 2037A123456 is year 20 (2020) and week 37, which runs 7 to 13 September 2020, so the build month is September and the age on 9 November 2026 is 6 years.

A.O. Smith's own example is a commercial model. We assume residential units with the same four-digit start decode the same way, but we have not confirmed it.

Style 2: the letter format in the 2004-2005 recall notice

The US Consumer Product Safety Commission's 2005 notice about A.O. Smith water heaters describes serial numbers with a month letter and a two-digit year. We decode only that format, and only for 2004 and 2005: an optional prefix M or A, a month letter from A to M (no I; A is January, M is December), then 04 or 05. So MA04, AA04 and A04 all read as January 2004, and G05 reads as July 2005. The build date is taken as the first of the month.

We do not present this as a general rule for letter serials. Any other letter style, and anything that looks like a 1990s serial, shows Can't date this reliably and no year.

Where the figures come from

1. Manufacturing date device, A.O. Smith. The four-digit year-week style and its worked example.
2. A.O. Smith recall of water heaters (2005), US Consumer Product Safety Commission. The 2004-2005 letter format.

We are independent of A.O. Smith. Follow the first link for the manufacturer's own explanation.

Which style do I have?

Look at the first four characters of the serial number on the rating plate. If all four are digits, use style 1. If it starts with a letter and a two-digit year of 04 or 05, it may be the recall-notice format. Anything else cannot be dated here. The guide to finding the rating plate on a water heater shows where the serial is printed.

Age and expected life

A build date is not an installation date, and an age is not a verdict on the heater. To compare an age with typical ranges, use the remaining life calculator; its ranges come from InterNACHI.