Your water heater’s build date is stamped into its serial number — but every brand hides it in a different place, and two serial numbers that look almost identical can mean dates years apart. Pick the brand, type the serial number off the rating plate, and this tool reads the date the way the manufacturer wrote it: month and year for Rheem, year and week for A.O. Smith, and a two-letter code for Bradford White.
Where to find the serial number
Look for the rating plate — a metal tag or printed label on the side of the tank. It lists the model number and the serial number on separate lines; you want the serial. Take a photo of the whole plate before you start: if the code ever needs a second look, you will not have to crawl back in beside the tank. One shortcut first — many newer heaters print a line that says “Mfg Date” or “Date of Manufacture” right on the label. If yours does, that printed date is the answer and it beats any decoding, including ours.
How each brand writes the date
Rheem, Ruud, Richmond, and GE tanks put the date in the first four digits of the serial, after any opening letters. The first two digits are the month and the next two are the year, so a serial starting 1209 was built in December 2009.
A.O. Smith and the brands it builds — State, Reliance, American, U.S. Craftmaster, Whirlpool, and most Kenmore heaters — reverse the order on units from about 2008 on: the first two digits are the year and the next two are the week, so 1210 means the 10th week of 2012. Older A.O. Smith units used a letter for the month followed by a two-digit year, and the tool reads that format too.
Bradford White uses no digits at all for the date. The first letter of the serial is the year and the second letter is the month. The catch: the year letters repeat every 20 years, so the same letter can mean 2004 or 2024. The tool gives you the most recent reading and always shows the older possibility when both are plausible — the look of the tank settles which one is real.
How long do water heaters last?
A standard tank heater typically lasts 8 to 12 years. A tankless unit typically lasts about 20 years, if it gets its yearly flush and descaling. Remember that the serial number gives the date the heater was built, not the day it was installed — a unit can sit in a warehouse or on a store shelf for months first, so the decoded age can run a little ahead of its working life. Once you know the age, the repair-or-replace test weighs it against the repair bill, and the replacement cost tool prices the new one before you call anybody.
Questions
Is the serial number the same as the model number?
No. They sit on the same rating plate but they are different numbers doing different jobs. The model number describes what the heater is; the serial number is unique to your unit and carries the build date. If a code will not decode, check that you copied the serial line and not the model line — that is the most common mix-up.
My brand is not in the list. What now?
The three families in the tool cover most tanks in American homes, because a handful of manufacturers build under many names. Rheem also builds Ruud, Richmond, and GE water heaters. A.O. Smith builds State, Reliance, American, U.S. Craftmaster, and Whirlpool heaters, and most Kenmore units. Lochinvar heaters follow the A.O. Smith year-and-week pattern. If your brand is genuinely none of those, look for a printed manufacture date on the label, or give the manufacturer the model and serial — every maker can date a unit from its own serial.