Free serial decoder
A.O. Smith water heater age
Type the serial number from the label. We read the year and week, or the older letter code, and show when it was made.
Same code: State, American, Reliance, U.S. Craftmaster, Whirlpool.
Decode your A.O. Smith serial
It's on the label, after "Serial" or "S/N". Not the model number.
How to read A.O. Smith serial numbers
Digits 1 and 2 are the year, digits 3 and 4 are the week of the year.
- 15Year
- 12Week
- A123456Unit number
1512A123456: 15 = 2015, 12 = week 12. Made around March 2015.
- SFactory
- 13Year
- 18Week
- F123456Unit number
S1318F123456: One letter first is the factory. 13 = 2013, week 18. Made around May 2013.
- LMonth
- 07Year
- A123456Unit number
L07A123456: Older letter code: L = November, 07 = 2007. Made November 2007.
Example serials, made up to show the format.
A printed date wins. If the label prints a manufacture date, use it instead of the serial.
Month letters on older serials
- AJan
- BFeb
- CMar
- DApr
- EMay
- FJun
- GJul
- HAug
- JSep
- KOct
- LNov
- MDec
No letter I.
Week to month
- Jan1 to 5
- Feb6 to 9
- Mar10 to 13
- Apr14 to 17
- May18 to 22
- Jun23 to 26
- Jul27 to 30
- Aug31 to 35
- Sep36 to 39
- Oct40 to 44
- Nov45 to 48
- Dec49 to 53
Weeks shift by a day or two from year to year.
Where's the serial number?
On the rating label on the side of the tank. On gas models it's next to the gas control valve. Copy the serial with any letters at the start, since older serials put the date after them.
A.O. Smith warranty
When it starts
Starts on the install date. Without proof, it depends on the model: newer heat pump models start 90 days after they were made, some tankless models 60 days, and older tanks from the date they were made.
New owner?
Usually the original owner only. Some tankless models stay with the home.
How long
ProLine tanks are usually 6 years, ProLine Master 8, and the 10 year lines include the Voltex heat pump.
General terms only. Your model's warranty, and A.O. Smith's lookup, have the final word.
How long it lasts
10 to 15 yrs
Tank water heaters
20+ yrs
Tankless water heaters
Planning ranges from the U.S. Department of Energy, not deadlines. LA and Orange County's hard water can shorten them, so yearly flushing matters. Which kind do you have?
Questions
How do I tell how old my A.O. Smith water heater is?
Find the serial number on the rating label. On most A.O. Smith tanks the first 2 digits are the year and the next 2 are the week of the year, so 1512 is week 12 of 2015, which is March. If there's one letter first, skip it.
Which brands use the same code?
State, American, Reliance, U.S. Craftmaster, and Whirlpool water heaters made by A.O. Smith use the same year and week code.
What about older serials that start with letters?
Older units used a month letter and a 2-digit year, sometimes after a factory letter. L07 is November 2007, and MD91 is April 1991. Guides disagree by a month on a few letters, so treat the month as close and the year as solid.
When does the A.O. Smith warranty start?
Starts on the install date. Without proof, it depends on the model: newer heat pump models start 90 days after they were made, some tankless models 60 days, and older tanks from the date they were made.
Does the A.O. Smith warranty transfer when I buy a house?
Usually not. A.O. Smith tank warranties cover the original owner. Some tankless models stay with the home.
How long do A.O. Smith water heaters last?
The U.S. Department of Energy says tank water heaters usually last 10 to 15 years and tankless heaters more than 20. Hard water and skipped flushing can shorten that.
Rather snap a photo?
The full Water Heater Check reads the label, asks a few taps, and tells you what it needs.
Open the Water Heater CheckOther serial guides
Sources
Checked November 4, 2026. We aren't affiliated with A.O. Smith.