Water Heater Age by Serial Number: How Date Codes Work

Water heater makers hide the build date in the serial number in different ways. Here are the styles we have verified, a worked example of each, and which brands we can and cannot date.

Last updated: 9 November 2026

Most water heater serial numbers contain the date the tank was built, but there is no shared code. Each maker picks its own, and some serials give no date we can check. Below are the two styles we have confirmed against a manufacturer source, an example of each, and a table of which brands we have a rule for.

The date-code styles we have verified

A build date in a serial number is a short code, usually a year and a month or a year and a week, and the codes differ in what they use for each. First find the serial on the rating plate (see where to find the serial number on a water heater), copy it exactly, then see which style it looks like.

Style 1: Bradford White, a year letter and a month letter

Bradford White's own pages say that the first two characters of the serial number represent the year and month of manufacture (Bradford White FAQ). The first character is the year and the second is the month.

The month letters run from A for January to M for December, skipping I (Bradford White date code chart). The year letters skip I, O, Q, R, U and V. That leaves 20 letters, so each letter comes round again 20 years later. Bradford White's serial number reference PDF pairs each letter with two years: A is 1984 or 2004, K is 1993 or 2013 and Z is 2003 or 2023. The letter alone cannot tell you which lap you are on.

The length of the rest of the serial can. The FAQ says the remainder is a sequential production number, seven digits in length before December 2007 (DM) and eight digits in length after. So eight digits means the later lap (after 2007), and seven digits means the earlier lap (before December 2007). The exception is the letters A to D with seven digits, which can fall in either lap: ask Bradford White.

Example with an invented serial, KE12345678: K is the year letter, E is the fifth month letter, May. The number after the two letters has eight digits, so the later lap applies and the heater was built in May 2013. With seven digits, KE1234567 would be May 1993. The same test settles AA12345678: eight digits means January 2024, not 2004.

Limit: from 2028 the digit rule stops working for E to H, because an eight-digit serial with E could be 2008 or 2028. If you need certainty, send the serial to Bradford White.

Style 2: A.O. Smith, year digits, then month or week

A.O. Smith's Canadian FAQ says the date can be found by looking at the first four numbers, ignoring any letter that may start the serial number (A.O. Smith Canada FAQ). The first two numbers represent the year the heater was built. We found this rule on A.O. Smith's Canadian site only. We have not found a US residential A.O. Smith page stating it, though third-party US sources agree.

The year digits tell you how to read the next two:

• Built before 2008: the next two numbers represent the month. The FAQ's example, U0106413635, was manufactured in June 2001.
• Built after 2008: the next two numbers represent the week. The week follows the manufacturer's calendar, so treat it as approximate.
• Built in 2008: the FAQ says "before 2008" and "after 2008" and does not say which applies to 2008 itself, so confirm with A.O. Smith.

The FAQ does not say how to read very old serials.

Example with invented serials: J1237123456 reads as year 12, week 37, so around September 2012. J0306123456 reads as year 03, month 06, so June 2003.

Some 2004 and 2005 A.O. Smith serials start with a month letter and a two-digit year instead, such as G05 for July 2005 on the 75-gallon propane gas water heaters in a 2005 CPSC recall notice. That is a different layout from the one above, so confirm it with the maker.

Which brands we can decode, and which we won't guess at

This table is the status of every brand we have looked at.

If your brand isn't decoded

For Rheem, Whirlpool, Rinnai and other brands not decoded above, work through these steps.

1. Look for a printed date. Some newer rating plates show an MFG date or BUILD DATE next to the serial. If yours does, you do not need to decode anything.
2. Read the rating plate. Copy the model number and serial number exactly, letters and digits, and photograph the plate in case the print fades. The rating plate guide shows where to look.
3. Contact the manufacturer. Brand names are often licensed, so use the company the plate names as manufacturer. If that is A.O. Smith, ask whether its date code applies to your serial. Send the model, the serial and the photo of the plate, and ask for the manufacture date. They are the only party who can state it authoritatively, which matters if a warranty claim or an inspection depends on it.
4. Try the warranty lookup. Rheem's warranty page requires the serial to verify a warranty. We have not confirmed it shows a manufacture date, but it may show the warranty start or status.
5. Check your own paperwork. An invoice, permit or inspection report shows when the heater was installed. It was built on or before that date, so it is at least that old.

If you are looking at a heater in a house you might buy, checking the water heater's age when buying a house covers what to ask for.

After you have a date

Check the date against common sense: a build date in the future, or one after the install paperwork you have, means the style was probably misread.

A build date is not an installation date, since a tank can sit in a warehouse or on a shelf before it is fitted. It is still the best marker of age that the tank itself carries. To see how an age compares with typical lifespans, read how long water heaters last or enter it in the remaining life calculator.

Sources

• My Water Heater, Bradford White. Month letters. Accessed 9 November 2026.
• Serial number data code reference (PDF), Bradford White. Year letters with both laps. Accessed 9 November 2026.
• Frequently asked questions, Bradford White. The serial's first two characters and the seven-digit and eight-digit rule. Accessed 9 November 2026.
• Frequently asked questions, A.O. Smith Canada. Ignore a leading letter; year, then month before 2008 or week from 2008. Accessed 9 November 2026.
• A.O. Smith recall of water heaters, US Consumer Product Safety Commission, 2005. Example of a month-letter serial prefix on 75-gallon propane units. Accessed 9 November 2026.
• Warranty and registration, Rheem. Serial required for warranty verification. Accessed 9 November 2026.

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