A.O. Smith has changed its water-heater serial-number format more times than any other brand covered on this site, and independent references converge on four distinct eras rather than one rule. On current production since 2008, the first two digits are the two-digit year and the next two are the production week, from 01 through 52 — a serial beginning 1604 was built in week 4 of 2016. Before 2008, A.O. Smith used a two-digit month followed by a two-digit year in the first four digits instead of year-then-week.
Older still, a single letter carried the month, A through M skipping I — this page corrects the cited reference's own stated range, which named a 13th letter for only 12 months — followed by a two-digit year. The oldest documented format adds a leading factory-code letter before that same month-letter-and-year pattern. Because the eras use opposite digit orders, matching the serial's actual length and character pattern to the right era matters more than memorizing any single rule.
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Decode your own serial number
What this covers
- Current A.O. Smith residential water heaters (2008–present) with an all-numeric serial: year then week in the first four digits
- Pre-2008 A.O. Smith water heaters with an all-numeric serial: month then year in the first four digits
- Older A.O. Smith water heaters opening with a single month letter (A–M, skipping I) or a factory-code letter followed by a month letter, both followed by a two-digit year
What it does not cover
- ×Any A.O. Smith serial that does not cleanly match one of these four patterns
- ×A.O. Smith's own consumer-facing site, which this review could not confirm publishes this table directly
- ×Commercial-only A.O. Smith product lines not separately confirmed
Try in this order
0 of 4 completedLocate the data tag
ReversibleCheck the exterior jacket of the tank, commonly on the upper side near the gas valve or thermostat cover.
Match the serial's shape to an era
ReversibleAn all-numeric serial with no leading letter is either current (2008+, year-then-week) or older (pre-2008, month-then-year). A leading letter points to one of the two oldest eras.
Read the current all-numeric format as year then week
ReversibleFirst two digits are the year, next two are the week (01–52). 1604 means week 4 of 2016.
Read the pre-2008 all-numeric format as month then year
ReversibleFirst two digits are the month (01–12), next two are the year. A serial reading 0399 means March 1999.
Evidence ledger
Sources behind this answer
Where the manufacturer's article stops. This reading comes from a detailed independent inspection reference documenting all four eras with worked examples; A.O. Smith's own site was not found to publish the digit table in consumer-facing text, which is why the evidence grade is B rather than A.
Exact questions
Questions about Decode an A.O. Smith water heater serial number
How do I tell the current format from the pre-2008 format if both are four digits of numbers?+
Both are ambiguous from digits alone in some cases; the appliance's overall condition, an owner's-manual date, or a plausible-range check (a "month" over 12, or a "week" over 52) can indicate which order applies.
Why does A.O. Smith have so many more documented formats than other brands?+
A.O. Smith has manufactured water heaters under its own name and for other brands across many decades; the independent reference used here documents four distinct schemes across that span rather than one continuous system.
Is this confirmed by A.O. Smith directly?+
No. This review could not locate an A.O. Smith consumer-facing page stating the digit table directly, so this page relies on a detailed independent reference and is graded B rather than A.
How do I read the serial?+
Compare against the maker's format.
Where is the label?+
On the tank.
Is the date exact?+
It shows the approximate age.