How old is my A.O. Smith water heater?

The short version

Find the rating plate on the side of the tank and read the line marked Serial No. Take the first four digits, ignoring any letters in front of them.

InspectApedia's worked example: serial 1604 123456 decodes to year 2016 and week 4. Fast Water Heater's independent example for the same family: serial 1220 345678 decodes to 2012, week 20, so mid-May.

Both sources agree, and InspectApedia adds a confirmation from inside the factory, a reader comment from 2012 by someone identifying themselves as an A.O. Smith employee: "To tell the age of any AO Smith water is the first 4 numbers. The first 2 are the year and the second 2 are what week it is. I work in the Q.C. Department there and make the serial plates all the time."

Turning a week number into a month

Fast Water Heater publishes this conversion and calls it approximate, because a week that straddles a month boundary can land either side depending on the year.

For an age calculation the month rarely matters. If you are within a few months of a warranty expiry it does, and then the safer move is to have the serial checked against the manufacturer's own record. Whether the extended cover was worth buying in the first place is a separate question, on the warranty page.

The two-digit year trap

Two digits are ambiguous. Fast Water Heater's rule: 80 to 99 means the 1980s or 1990s, and anything from 00 up to the current year is the 2000s. A serial starting 97 is 1997, not 2097.

Our note, marked as ours: in practice this is self-resolving. A tank from 1997 would be nearly thirty years old, and almost none are still in service, so if the arithmetic gives you a 1990s date on a tank that looks serviceable, re-read the digits before you trust it.

The older formats, and how to tell which one you have

This is where most online decoders stop and where the plate in front of you often does not fit. InspectApedia documents five formats. Ours is the only page we know of that lists them together.

A sixth, from the 1970s, reads N69-C-YY-12345, where the date sits in the YY pair.

How to pick: if the serial starts with four digits, use the year-week rule. If it starts with a letter, the letter is either the month or a factory code, and the next characters carry the month and year. InspectApedia's moderator worked one of these through for a reader: serial MB00-0070742 reads factory M, month B for February, year 00 for 2000.

The month letters

A contradiction in the source, resolved. InspectApedia's prose describes these letters twice, once as "A - N for January - December (the letter I is not used)" and once as "A - M". Only one can be right, and the arithmetic settles it: A to M with I removed leaves exactly twelve letters for twelve months, while A to N would leave thirteen. The table on their own page ends at M for December, so A to M without I is correct. We checked this rather than copying one of the two sentences.

Why the order matters so much

Rheem, Ruud, Richmond and GE put the month first. A.O. Smith puts the year first. The same four digits therefore mean two completely different dates.

Over seven years apart. Fast Water Heater calls this "the single most common mix-up we see". Check the brand on the plate before you decode anything, and if the heater is a rebadge, see the Rheem page or run the age decoder, which applies the right rule once you pick the brand.

Which brands this rule covers

InspectApedia's brand index files all of these under A.O. Smith or State: A.O. Smith, State, American Water Heater, Reliance, U.S. Craftmaster, Whirlpool and Kenmore. Fast Water Heater lists the same family. The reason is corporate: InspectApedia records that A.O. Smith was founded in 1915, registered its first US patent in 1936 for the glass-lined tank, and "by 2006 the company had purchased both Canadian GSW Incorporated and the American Water Heater Company."

A Whirlpool plate decodes by the rule on this page: InspectApedia records that Whirlpool units are built under licence by American Water Heater Company, which A.O. Smith now owns.

State is the exception, and we flag it rather than wave it through. InspectApedia's own State page gives a different rule, a month letter or a month-year pair in the first three or four positions, and notes that published guides disagree on the order. If your plate says State, read the State page instead of this one.

What the age tells you

A tank water heater lasts 8 to 12 years on average. Once you have the year, the repair or replace page and the expected life page turn it into a decision, and the replacement cost calculator gives you the number to compare a repair quote against. If you want the whole picture rather than just the next bill, the lifetime cost page adds running cost to purchase price.