A hot water heating system for use in a boat or vehicle which utilises the engine of the vehicle or boat, an AC power source or a fuel powered heater to heat a living area and to heat potable water. Each of the power sources can be utilised independently or in combination depending on the location and operation of the vehicle or boat. A valve is automatically controlled by a thermostat to allow heated fluid to circulate through the living area or to bypass the area. Aquastats are provided to automatically allow potable water to be heated. A display is provided for displaying textual and numerical information in user friendly format relating to the operation of the hot water heating system.