Short answer
The build date is coded into the serial number on the rating plate. Rheem uses month then year (1209 means December 2009), A.O. Smith uses year then week (2037 means week 37 of 2020), and Bradford White uses two letters for year and month. Install usually came some months later, so treat the result as the earliest possible age.
Key facts
- Rheem serials have 10 characters: digits 1 to 2 are the month and digits 3 to 4 the year of manufacture.[1]
- A.O. Smith serials start with a 2-digit year and a 2-digit week; a serial starting 2037 was built September 7 to 13, 2020.[2]
- Bradford White year letters repeat every 20 years, so A, B and C now mean 2024, 2025 and 2026 as well as 2004 to 2006.[3]
Find your water heater's age
Pick the brand on the rating label, then type the serial number exactly as printed. It stays in your browser.
Where to find the serial number
Look for the rating plate, a sticker on the side of a tank or the side or front of a tankless unit. It lists the model number, the serial number, and often a "manufactured by" line. Take a photo so you can read it in good light.
How each brand encodes the date
| Brand (and related labels) | How to read it | Example | Confidence |
|---|---|---|---|
| Rheem, Ruud, Richmond | Digits 1 to 2 = month, 3 to 4 = year | 1209D01234 = December 2009[1] | High |
| A.O. Smith, State, American, Reliance | Digits 1 to 2 = year, 3 to 4 = week | 2037... = week 37 of 2020[2] | High |
| Lochinvar tanks, 2011 on | Digits 1 to 2 = year, 3 to 4 = week | 1142N012345 = week 42 of 2011[4] | High |
| Bradford White | Letter 1 = year (20-year cycle), letter 2 = month (A = Jan to M = Dec, no I) | PK12345678 = October 2017[3] | High |
| Whirlpool (built by American) | Digits 1 to 2 = year, 3 to 4 = week | 1023... = week 23 of 2010[10] | Medium |
| Noritz | 4-digit year, then month | 2006.06... = June 2006[8] | Medium |
| Navien | Older: second group is YYYYMMDD. Newer: a date block inside the serial | 98881-20100505-3026 = May 5, 2010[5] | Low: verify with the manufacturer |
| Rinnai | Since about 2009: letter 1 = year, letter 2 = month | BL... = November 2010[6] | Low: verify with the manufacturer |
| Takagi | From about 2010: year, then week | 1709... = week 9 of 2017[9] | Low: verify with the manufacturer |
| GE, Kenmore, Bosch, Stiebel Eltron | Depends on who built the unit | n/a | Low: verify with the manufacturer |
"High" means the maker publishes the rule. "Medium" and "Low" come from inspector and parts-supplier sources; for those brands, call the maker's support line or use its warranty lookup with your serial number.
Two myths to skip
"Rheem serials are week and year." Rheem's own page says month first, then year[1]. Some older field units use a letter plus week and year, which is why the decoder tests the month first.
"Bradford White A means 2004." It can. Bradford White's chart now runs A = 2024, B = 2025 and C = 2026 as well[3]. To break the tie, count the digits after the letters: 7 digits means built before December 2007, 8 means December 2007 or later[3].
What the date tells you
The decoder gives the build date. Install was usually some months later, so the heater is at least that old. Rheem puts typical tank life at 8 to 12 years and tankless at 15 to 20+[7]. Many warranties also start from manufacture; Lochinvar's tank coverage, for example, begins 60 days after the manufacture date[4].
When to call a pro
- Your tank is past 10 years and shows rust, leaks or rumbling.
- The serial does not match any format above.
- You need a warranty decision: the installer or maker confirms coverage from the serial.
Sources
- How to locate and read your Rheem water heating serial numbers, Rheem Manufacturing. Accessed 2026-10-06.
- Manufacturing date of a device, A.O. Smith. Accessed 2026-10-06.
- Determining the age of a water heater, Bradford White. Accessed 2026-10-06.
- Lochinvar limited warranty, determining date of manufacture by serial number (Rev 18), Lochinvar. Accessed 2026-10-06.
- How can I tell the age of a Navien tankless water heater from the serial number?, How To Look At A House (home inspector blog). Accessed 2026-10-06.
- Rinnai serial number lookup guide, Parts Town. Accessed 2026-10-06.
- Water heater lifespan: when to repair vs. replace (Mar 25, 2026), Rheem Manufacturing. Accessed 2026-10-06.
- How do I tell the age of a Noritz water heater from the serial number?, How To Look At A House (home inspector blog). Accessed 2026-10-06.
- How can I tell the age of a Takagi water heater from the serial number?, How To Look At A House (home inspector blog). Accessed 2026-10-06.
- How do I tell the age of a Whirlpool water heater from the serial number?, How To Look At A House (home inspector blog). Accessed 2026-10-06.
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