A. O. Smith water heater age by serial number
The serial number is on the rating plate on the side of the tank. On an A. O. Smith water heater built since 2008, the first two digits of the serial are the year and the next two are the week.
Decode an A. O. Smith serial number
How to read an A. O. Smith serial number
A. O. Smith has used 3 serial formats. Find the one that matches the shape of your serial.
Year and week, first four digits
Skip a leading letter if there is one. The first two digits are the year and the next two are the week of that year.
Shape: four digits, then a letter and six digits, or nine more digits. Some serials carry one extra letter in front.
1210A002243 reads as week 10 of 2012 (around March).
Month letter, then two-digit year
The first letter is the month, A for January through M for December with the letter I left out. The two digits after it are the year.
Shape: one letter, two digits, then a letter and six digits or six to eight digits.
E07A135491 reads as May 2007.
Month letter
- A January
- B February
- C March
- D April
- E May
- F June
- G July
- H August
- J September
- K October
- L November
- M December
State and Reliance used this layout for decades. A. O. Smith-branded heaters carry it only from July 2004, when the leading factory letter was dropped, to 2007.
Factory letter, month letter, two-digit year
The first letter is the factory. The second letter is the month, A for January through M for December with the letter I left out. The two digits after the letters are the year.
Shape: two letters, two digits, then more digits. Older plates print it in hyphenated groups.
GG03-1495366-S29 reads as July 2003.
Month letter
- A January
- B February
- C March
- D April
- E May
- F June
- G July
- H August
- J September
- K October
- L November
- M December
Where to find the serial number
The rating plate is on the outside of the tank jacket. On gas models it is next to the gas control valve near the bottom of the heater.
Newer plates print a build date under the serial number. When it is there, read it directly.
If the serial number does not decode
- Water heaters sold in Canada under the A. O. Smith, GSW and John Wood names used a different layout before 2008: year digits followed by month digits. The formats on this page are for heaters sold in the United States.
- Serials from the 1970s are printed in short hyphenated groups. They are not covered by the decoder.
- Tankless heaters sold as A. O. Smith or Takagi since 2009 use the same year-and-week serial as the tank heaters.
- The manufacturer can confirm the date from the serial: A. O. Smith registration, warranty or serial lookup.
What the age tells you
| Equipment | Typical service life |
|---|---|
| Tank water heater | 10 to 15 years |
| Tankless water heater | about 20 years |
About A. O. Smith
A. O. Smith Corporation of Milwaukee, Wisconsin bought State Industries in 2001 and the American Water Heater Company in 2006. A. O. Smith, State, Reliance, American and Whirlpool-brand water heaters now come from the same company and share its current serial layout.
Brands that share this serial system: State, Reliance, American, Whirlpool, Kenmore.
Questions and answers
Which characters in an A. O. Smith serial number show the date?
Skip a leading letter if there is one. The first two digits are the year and the next two are the week of that year. This applies to units built since 2008.
Why does my A. O. Smith serial number look different?
A. O. Smith has used 3 serial formats: year and week, first four digits (built since 2008); month letter, then two-digit year (built until 2007); factory letter, month letter, two-digit year (built until mid-2004). The decoder tries each one.
Sources
- A. O. Smith Canada: frequently asked questions, reading the serial number (manufacturer): https://www.hotwatercanada.ca/faqs/
- A. O. Smith International: manufacturing date of a device (manufacturer): https://aosmithinternational.com/manufacturing-date-device/
- CPSC recall 19-029: A. O. Smith ultra-low NOx water heaters, serial ranges by build date (US Consumer Product Safety Commission): https://www.cpsc.gov/Recalls/2019/A-O-Smith-Recalls-Ultra-Low-NOx-Water-Heaters-Due-to-Fire-Hazard
- Building Intelligence Center: A. O. Smith water heater age (independent reference): https://www.building-center.org/a-o-smith-water-heater-age/
- CPSC recall: A. O. Smith, State, Reliance, Maytag and Apollo water heaters, serial prefixes by month (US Consumer Product Safety Commission): https://www.cpsc.gov/Recalls/2005/cpsc-ao-smith-recall-of-water-heaters
- CPSC recall: A. O. Smith gas water heaters, serial ranges for units sold November 2007 to January 2008 (US Consumer Product Safety Commission): https://www.cpsc.gov/Recalls/2008/ao-smith-recalls-gas-water-heaters-due-to-fire-and-carbon-monoxide-hazards
- Building Intelligence Center: State water heater age (independent reference): https://www.building-center.org/state-water-heater-age/
- CPSC recall: A. O. Smith gas water heaters built April to July 1991, serial prefixes MD91 to MG91 (US Consumer Product Safety Commission): https://www.cpsc.gov/Recalls/2012/certain-30-and-40-gallon-ao-smith-lp-and-natural-gas-water-heaters-voluntarily-recalled
- InspectAPedia: A. O. Smith water heater age (independent reference): https://inspectapedia.com/plumbing/AO-Smith-Water-Heater-Age-Manuals.php
Formats last checked against these sources on October 5, 2026. Found a serial that does not fit? Send a correction. How formats are verified: method and sources.