Where to find the serial number
The rating plate is a metal or foil sticker on the side of the tank, usually near the top on gas units and near the control panel on electric units. It lists the model number, serial number, capacity and energy ratings. The serial number is the one that encodes the date. The model number tells you the size and type but not the age, with one exception: Kenmore, where the model prefix tells you which factory built the heater.
How each brand codes the date
A.O. Smith, State, American Water Heater, Reliance, U.S. Craftmaster, Whirlpool. All built by A.O. Smith. The first two digits are the year, the next two are the production week. A serial starting with 1510 was built in week 10 of 2015, so early March. A serial starting with 2318 is week 18 of 2023, so early May.
Rheem, Ruud, Richmond, GE, Hotpoint. Built by Rheem. After an optional letter prefix (R, RN, RHLN and similar) the first two digits are the month and the next two the year. RHLN0722F12345 is July 2022. Some plates use a week code instead, so if the first pair is above 12, read it as the week of the year. The decoder handles both.
Watch the trap: 1219 on a Rheem plate is December 2019, but 1219 on an A.O. Smith plate is week 19 of 2012. Picking the right brand matters more than anything else here.
Bradford White. Two letters, then digits. The first letter is the year, the second the month. Bradford White skips I, O, Q, R, U and V in the year column, which leaves 20 letters that repeat every 20 years: A is 1984, 2004 and 2024; W is 2000 and 2020; Z is 2003 and 2023; B is 2025 and C is 2026. The month letter runs A (January) to M (December) without I. A plate starting WC was built in March 2020 or March 2000. The decoder shows the most recent possibility and names the earlier one so you can judge from the tank's condition.
Kenmore. Sears never built water heaters; Kenmore is a rebadge. Both references we use place Kenmore in the A.O. Smith family: InspectApedia files Kenmore under A.O. Smith and State, and Fast Water Heater states that Kenmore tanks came from the A.O. Smith side of the industry and should be read year first, then week. So select Kenmore in the decoder and it applies the year-week rule.
Do not read anything into the model number. Every Kenmore water heater model begins with 153, because that is Sears' category code for the product group, not a code for the factory. Kenmore water heaters were discontinued around 2018, so nothing newer exists.
Correction, 8 October 2026: until this date, this page and the decoder claimed that a model number starting with 153 identified a Rheem-built unit. That was wrong, and it was contradicted by the sources listed below this page. Since every Kenmore model carries the 153 prefix, the old rule sent every Kenmore down the Rheem month-year path. On a plate reading 1219 that produced December 2019 instead of week 19 of 2012, an error of more than seven years. If you used this decoder for a Kenmore heater before this date, please check it again.
One page per brand, with the full detail
The summaries above are enough for most plates. Where a brand has several formats across the eras, or where the published references disagree, the detail is on its own page:
| Brand page | What it adds |
|---|---|
| A.O. Smith | five serial formats across the eras, the month letter table, and a contradiction in the source resolved |
| Rheem | the nine formats InspectApedia documents, the week-code exception, and the limit of Rheem's own checker |
| State | two published rules that contradict each other, both shown, with a test for which applies |
| Whirlpool | who actually builds the unit, and what that means for warranty and parts |
| American | three formats, a photographed example, and the documented case where none of them work |
| GE | four formats, two of which read the same digits differently, with a self-confirming example |
| Kenmore | why the 153 model prefix tells you nothing, and the error we published and corrected |
| Rinnai | two eras, and the skipped letter that puts units from 2017 a year out |
Ruud and Richmond follow the Rheem page. Reliance and U.S. Craftmaster follow the A.O. Smith page. Hotpoint follows the GE page.
Brands not covered yet
Navien, Noritz, Bosch, Lochinvar, Giant and John Wood use their own formats, and the published references we checked either disagree with each other or cover only one of them. Rather than guess, we left them out of the decoder. Rinnai now has its own page, where we set out which part of the rule rests on a single source. Most tankless units from these brands print the manufacture date in plain text on the rating plate. If you find the manufacturer's own documentation for one of these codes, tell us and we will add it with the source.
What the age means
Tank water heaters last 8 to 12 years on average. Tankless units last 15 to 20 years if they are descaled regularly. Past year 10, a tank that needs a repair costing more than a third of a replacement is usually not worth fixing, because the tank itself is next to go and a leaking tank causes water damage that costs more than the heater. The replacement cost calculator gives you the number to compare against.
Sources
- Bradford White For The Pro, Determining the Age of a Water Heater (official year and month letter tables)
- InspectApedia, Rheem Water Heater Age Decoder (serial formats and examples)
- Fast Water Heater Company, How Old Is My Water Heater? (A.O. Smith family week code, Rheem, Kenmore)
- InspectApedia, Water Heater Age & Manuals (brand index, Kenmore filed under A.O. Smith and State)
- InspectApedia, A.O. Smith Water Heater Age (four serial formats with worked examples)
- Rheem, How Old Is My Equipment (official checker, for verifying a Rheem-family result)