A.O. Smith Water Heater Age: How to Read the Serial Number

Last updated: July 2026 · Based on A.O. Smith serial number dating conventions

To find the age of an A.O. Smith water heater, read the first four digits of the serial number: the first two are the year and the next two are the week of manufacture. A serial number starting with 1542 means the unit was built in the 42nd week of 2015. Knowing the age tells you whether the tank is near end of life and whether it is still under warranty — here is how to decode it.

Where to Find the Serial Number

The serial number is on the rating plate (a sticker or metal label) on the side of the tank, usually near the top or middle. It is a longer string of numbers and letters; you only need the first four digits to date the heater.

How to Decode the Age

Modern A.O. Smith water heaters use a simple YYWW format at the start of the serial number:

• First two digits = year. “15” = 2015, “22” = 2022.
• Next two digits = week. “42” = the 42nd week of that year (around mid-October).

Note: some older or commercial A.O. Smith units, and units sold under brands A.O. Smith manufactures (like some State, Reliance, and American models), may use a different code — check the rating plate or the manufacturer if the YYWW format does not seem to fit.

Why the Age Matters

Most tank water heaters last 8–12 years. Knowing the age helps you decide whether a problem is worth repairing or whether replacement makes more sense:

• Under 6 years: likely still under the tank warranty (commonly 6–12 years) — check before paying for a new tank.
• 8–12 years: approaching end of life; weigh repair costs against replacement.
• Over 12 years: living on borrowed time; a major failure usually means replace, not repair.

If your unit is leaking, its age is the deciding factor: a leaking water heater that is 10+ years old is usually a replacement, while a younger one is often worth repairing.

Other Brands

Decoding a different brand? See our guide for Whirlpool water heater age, which uses the same YYWW format.

Checking Your Warranty

A.O. Smith tank warranties are typically 6, 10, or 12 years depending on the model. If your heater failed and the serial number shows it is within that window, you may get a free or discounted replacement tank — register the serial number on the manufacturer’s site or call with it before buying a new unit.

FAQ

How do I tell the age of my A.O. Smith water heater?

Read the first four digits of the serial number on the rating plate. The first two are the year and the next two are the week of manufacture — so “1642” means the 42nd week of 2016.

How long do A.O. Smith water heaters last?

Like most tank water heaters, 8–12 years on average, sometimes longer with regular flushing and anode rod maintenance. Tankless models last longer, often 15–20 years.

Is my A.O. Smith water heater still under warranty?

Decode the age from the serial number and compare it to your model’s warranty length (commonly 6–12 years on the tank). If it failed within that window, contact A.O. Smith with the serial number — you may qualify for a replacement tank.