A number of websites exist to help you interpret or decode your water heater's model number. The water heater's model number often contains coded information that indicates physical features of the heater, as well as the date of manufacture.
Your water heater's model number is listed on an information plate on the side of the unit. Many company websites provide interpretation guides to the model numbers of their own units. Other websites provide guides to a variety of brands. Building Intelligence Center provides a fairly comprehensive guide to a large number of brands. The first few digits often refer to the month and date of manufacture. In some brands, this information is spelled out, while other brands use an alphanumeric code.