Positive displacement rapid water heater and controlling method thereof

A volume style water heater can be used in apartments, offices, hotels and villas. It consists of a sealed water tank, an expanding tank, a flue, a quick heater, a combusting chamber, a blower, a water pump, a safety valve for controlling combustion, a check valve, a controller, a shell, pipelines for cold and hot water, and gas fuel pipelines. A microcomputer as intellectual controller is used to inspect, to determine, to diagnose, and to control circulation. It combines advantages of direct flow style and volume style water heaters together, provides hot water at once as starting with large flow and efficient combustion, saves energy, and works reliably and safely. A method for controlling is disclosed also.