Walk around your water heater and visually inspect the upper half of the tank. You are looking for a large sticker, often about the size of an index card or larger, which is the manufacturer's data plate. It contains a lot of printed information, including warnings, technical specs, the model number, and the serial number. You may need a flashlight to see it clearly, especially if the water heater is in a dimly lit area like a basement or closet.

Once you've found the data plate, scan the text for the serial number. It will be clearly labeled, typically as 'Serial No.', 'S/N', or 'Ser. No.'. It's a long string of letters and numbers. The best practice is to use your smartphone to take a clear, well-lit photograph of the entire label. This ensures you capture it accurately and can refer to it later without having to go back to the water heater. Make sure the photo is not blurry and the numbers are legible.

This is the crucial step. The decoding method depends entirely on the manufacturer. Here are the most common formats: * **A.O. Smith, State, Kenmore, Reliance (and others made by A.O. Smith):** The first two numbers are the year of manufacture, and the next two numbers are the week. For example, a serial number starting with `1805...` was made in the 5th week of 2018. * **Rheem, Ruud, Richmond:** The serial number format is often `MMYYP...`. The first two digits are the month, and the next two are the year. For example, `0419...` means it was made in April 2019. * **Bradford White:** This brand uses a letter code for the year and a letter code for the month. The first letter represents the year (A=2004, B=2005, etc., skipping some letters). The second letter represents the month (A=Jan, B=Feb, etc.). You will need to look up the manufacturer's specific year code chart online. Since we assumed the brand is **A.O. Smith**, we will use that format. Look at the first four digits of the serial number you recorded. The first two digits represent the year, and the second two digits represent the week of that year. For example, a serial number `1732J123456` means the heater was manufactured in the 32nd week of 2017.

To be extra certain, look for other date indicators. Plumbers often write the installation date on the side of the tank with a permanent marker. This date should be close to the manufacturing date you decoded (usually within a few months). You can also check the ANSI (American National Standards Institute) date on the label, which is a standards certification date. While this is not the manufacturing date, a unit cannot have been made before this date, providing a helpful baseline.