1. Scope 1.1 This standard cover the requirements for instantaneous domestic water heater of nominal useful heat output less than 25 kW (based on gross calorific value), for use with Piped Natural Gases (PNG) at a working pressure of 21 mbar (21.41 gf/cm2). It is an inlet water controlled appliance. It specifies the main constructional and performance characteristics for the instantaneous domestic water heater. It also sets the appropriate test methods for evaluating test characteristics. Appliances for boiling water are not covered in this standard. NOTE — For calculation 1 Kcal/h = 1.16278 watt. 1.1.1 For convenience, this standard has been divided into three sections as follows: a) Section 1 — Construction, b) Section 2 — Performance requirements, and c) Section 3 — General