This part of ISO 22391 specifies the characteristics of fittings for piping systems made of ⎯ polyethylene of raised temperature resistance (PE-RT), Type I, and ⎯ polyethylene of raised temperature resistance (PE-RT), Type II, intended to be used for hot and cold water installations within buildings for the conveyance of water, whether or not the water is intended for human consumption (domestic systems) and for heating systems, under the design pressures and temperatures appropriate to the class of application according to ISO 22391-1. This part of ISO 22391 covers a range of service conditions (classes of application), design pressures and pipe dimension classes, and also specifies test parameters and test methods. In conjunction with the other parts of ISO 22391, it is applicable to fittings made of PE-RT, as well as to those made of other materials, intended to be fitted to pipes conforming to ISO 22391-2 for hot and cold water installations, the joints of which are in accordance with ISO 22391-5.