Abstract: An instantaneous gas domestic hot water system incorporating a solar heating panel (80) connected to a hot water storage cylinder (81), the outlet from the hot water storage cylinder passing through a heating tube assembly (15) adapted to be heated by a gas burner (13). The outlet from the heating tube assembly is connected to a flow switch (4) and a temperature sensing device (14), the flow switch being connected through an electrical control system (10) to operate a control gas valve (11) and a main gas control (5) to control the flow of gas to a pilot burner (6) which is ignited on sensing of the flow of hot water (3) with the temperature sensing unit (14) controlling the control valve (11) to regulate the flow of gas to the main burner (13) to maintain the water at the water at the water at the outlet (83) at a desired temperature.