This document specifies the essential terms, constructional requirements, tests, energy assessment and marking of indirectly heated water storage tanks for primary water (buffer tanks), with a capacity not exceeding 2,000 l, an operating temperature not exceeding 95 °C, and an operating pressure not exceeding 1,0 MPa. This document covers metallic and plastic made buffer tanks. Although this document does not consider any buffer tanks mainly intended for direct firing, it allows for the provision of electric heating elements for auxiliary purposes. NOTE The energy assessment is performed by EN 15332.