An instantaneous, automatic gas water heater for a hot water system having a cold water supply conduit connected to a plurality of loosely wound small heat exchange tubes which, in turn are connected to a hot water outlet conduit having faucets. The heat exchange tubes are each of smaller internal diameter than the cold and hot water conduits but their total capacity is substanially the same as said conduits. A gas burner is disposed beneath the heat exchange tubes to subject them to the flame of the burner. There is a cover over the heat exchange tubes which confines the heat from the burner about the tubes. The burner is supplied with gas by a gas supply conduit controlled by a normally closed solenoid valve including a gas control valve and a solenoid controlling the gas control valve. There is an electrical system for the solenoid valve and said electrical system includes a diaphragm controlled switch with the diaphragm subjected to water pressure in one of the conduits such as the cold water inlet conduit.There is also an adjustable temperature control thermostat mechanism subjected to and controlled in accordance with the temperature in the hot water conduit for controlling a second solenoid valve which controls gas to the burner.