This specification covers the general requirements for materials used in preformed rubber seals that contact the circulating liquid in solar energy systems. Rubber seal can be classified into types, grades and classes. It has two kinds according to what climate they should be used: Type C which is intended for use in cold climates and Type W which is intended for use in warm climates. Its grade designations represent differing degrees of hardness such as Grade 3, Grade 4, Grade 5, Grade 6, Grade 7 and Grade 8. As for the classes, it should be determined into 3 kinds. Class A for seals used with aqueous liquid at a certain maximum service temperature. Class AT for seals used with aqueous liquids at an above maximum service temperature and class N for use with nonaqueous liquids. Different tests shall be conducted in order to determine the following properties of rubber seals in liquid heat-transport systems: compression, resistance to heating, hardness, ultimate elongation, volume, resistance to ozone, resistance to low temperature, and resistance to liquid.