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How old is my GSW water heater?

Serial number age decoder for GSW tank water heaters: 7 format eras, the decode rule in plain English, transcribed tables, and the official lookup.

Serial number decoder for GSW water heater

Type the serial from the data plate, not the model number. Nothing is sent anywhere; the tables are in this page.

Format example: 1210A002243

Format eras

How to read the serial

before 2010 formatVERIFIED

Shape A##A###### · example E07A135491 · confidence medium

Year
digits 2-3
Week or month
month = letter 1 (A=Jan..)
Month codeMonth
AJanuary
BFebruary
CMarch
DApril
EMay
FJune
GJuly
HAugust
ISeptember
JOctober
KNovember
LDecember

Example: serial E07A135491 decodes to May 2007.

Trap: Discontinued 2008-2010

Listed under A.O. Smith in the source; GSW is named in that format's family.

2008 to present formatVERIFIED

Shape ####A###### · example 1210A002243 / S1318 F000046 / 1836111786352 · confidence high

Year
digits 1-2 (YY) after optional leading S
Week or month
week = digits 3-4
Plant
letter after the 4 date digits (unpublished)

Example: serial 1210A002243 decodes to week 10 of 2012.

Trap: Sub-brand applicability (State/American/Reliance/Whirlpool/Kenmore) is ESTIMATE: building-center a-o-smith page covers AO Smith; each sub-brand has its own page to fetch in fan-out. Century: 20YY only for this style.

Listed under A.O. Smith in the source; GSW is named in that format's family.

2008 to present formatVERIFIED

Shape A#### A###### · example U1117 F700633 / S1001 F701883 · confidence high

Year
digits 1-2 after any leading letter (20YY)
Week or month
week = digits 3-4 (month if built before 2008 per GSW FAQ)
Plant
trailing letter group (F7...) unpublished

Example: serial U1117 F700633 decodes to week 17 of 2011.

Trap: GSW FAQ: month before 2008, week after. But building-center style 1 0331263882 (2003) decodes as week 31, so the month rule was not universal; value >12 => week.

2008 to present formatVERIFIED

Shape ############# · example 1924115263020 / 1104T403904 · confidence high

Year
digits 1-2 (YY, 20YY) after any leading letter
Week or month
week = digits 3-4
Plant
letter after the date digits when present (unpublished)

Example: serial 1924115263020 decodes to week 24 of 2019.

Trap: American family generic YYWW shape (American, Whirlpool WH, U.S. Craftmaster, Craftmaster, GSW, John Wood, Kenmore-by-State). building-center mislabels American style 2 as 'begins with two letters / discontinued' (copy-paste error). Same regex as A.O. Smith current style; GSW/John Wood FAQ says digits 3-4 = MONTH before 2008, but Craftmaster 043812019 (2004, '38') proves US-built units already used weeks; treat value >12 as week.

Listed under American Water Heater in the source; GSW is named in that format's family.

before 2008 formatVERIFIED

Shape AA##A###### · example AF04A093001 / GG03-1495366-S29 · confidence medium

Year
digits 3-4
Week or month
month = letter 2 (A=Jan..)
Plant
letter 1
Month codeMonth
AJanuary
BFebruary
CMarch
DApril
EMay
FJune
GJuly
HAugust
ISeptember
JOctober
KNovember
LDecember

Example: serial AF04A093001 decodes to June 2004.

Trap: Two discontinued 2-letter-prefix styles; month letter table (I skipped?) not stated in text

Listed under A.O. Smith in the source; GSW is named in that format's family.

before 2007 formatVERIFIED

Shape ##### ##### · example 05035 35368 / U0106413635 · confidence medium

Year
digits 1-2 (YY)
Week or month
month = digits 3-4 (building-center American style 1; GSW/John Wood FAQ: month before 2008)

Example: serial 05035 35368 decodes to March 2005.

Trap: Pre-2008 month vs week is brand/plant dependent (Canadian GSW/John Wood used month; Craftmaster used week in 2004). Value >12 => week. Collides with the Rheem MMYY shape (0804B10488) - brand must be known.

Listed under American Water Heater in the source; GSW is named in that format's family.

1970 to 1979 formatVERIFIED

Shape A##-A-##-###### · example A20-H-78-015482 · confidence medium

Year
2 digits between hyphens (78)
Week or month
month = letter between hyphens (H=Aug)
Month codeMonth
AJanuary
BFebruary
CMarch
DApril
EMay
FJune
GJuly
HAugust
ISeptember
JOctober
KNovember
LDecember

Example: serial A20-H-78-015482 decodes to August 1978.

Trap: Confirm era via ANSI revision year on plate

Listed under A.O. Smith in the source; GSW is named in that format's family.

Where the data plate is on a water heater

The rating plate is a large label on the side of the tank near the top or beside the control. The serial is printed on it, often beside the model number and the ANSI standard line.

General guidance, not brand-specific: the owner's manual or the installer can point to the exact spot. See the data plate guide.

For scale only: the InterNACHI Standard Estimated Life Expectancy Chart for Homes lists Water Heater (conventional) at 6 to 12 years of expected service (nachi.org). That chart is a guideline, not a prediction for your unit.

Official lookup: the authority

Open the official American Water Heater lookup takes serial; returns warranty status; URL verified live

This site is a reference built from published tables. The manufacturer's own lookup wins whenever the two disagree.

What we could not confirm

Sources

A.O. Smith family All water heater brands Guides