Water Heater Model Numbers and Age
All water heaters have a Rating Plate that lists important data such as the model and serial numbers, manufacturing company's name, length of warranty, wattage of elements installed (on electric models), gallon capacity, input BTU rating (gas models), and more.
You can determine the age of the water heater by de-coding the serial number. Some manufacturers make it easy by incorporating the month and year built into the first 4 digits of the serial number. For instance, the picture below shows that this Rheem water heater was manufactured in December of 1991 (1291). Other manufacturers use different formats, but in a similar fashion.
Let's decode the above rating plate: This is a model 21V50-2, serial number 1291A39968, manufactured in December of 1991, 50 gallons, 36000 BTU's per hour, and it set to work with Natural Gas only. The other info you see relates more to Service Technicians, and shouldn't be needed by the average homeowner.
The picture below shows the entire Rheem Rating plate. This 50 gallon electric tank was built in August of 1984, and came equipped with 4000 watt elements
AO SMITH puts the year first (1991), and the month second (07, or JULY). It's a little harder to find the info on the AO SMITH plate, but it's all there!
American's format puts the year first, then the week the tank was built. In this example, the tank was made in the 17th week of 1998.
The example rating plates is on a tank made in June 2002.
Below is a close up of the Bradford White rating plate. This is a 40 gallon electric water heater, model number MI40L6DS13, manufactured in June of 2002. It comes equipped with 4000 watt elements. Note the "Maximum Watts" at 4000, this means this tank was built to operate in the "Non-simultaneous" mode, meaning only one element will energize at a time (standard for residential water heaters). It is designed to operate with a maximum of 150 PSI water pressure, but was tested to 300 PSI at the factory for your safety (a Federal Standard!).