enlarged view of the tank provided in the boiler according to the present invention referring to fig3 a main heat exchanger connecting port 111 connected to the second heating water connecting pipe 160 b is formed on one side of an upper portion of the tank 110 and a heating water supplying port 112 connected to the heating water supply pipe 210 a is formed on the other side of the upper portion of the tank 110 a pump connecting port 114 connected to the internal circulation pump 120 installed on the first heating water connecting pipe 160 a is formed on one side of a lower portion of the tank 110 and a heating water returning port 113 connected to the heating water return pipe 210 b and a heating water inlet port 115 connected to the second connecting pipe 192 are formed on the other side of the lower portion of the tank 110 on an upper portion of the tank 110 in addition an air water separator 116 is provided for discharging air contained in heating water in the tank 110 out of the tank 110 and a filter 117 is provided on the lower portion of the tank 110 for filtering foreign substances contained in heating water flowing into the tank 110 via the heating water returning port 113 after passing through the heating equipment 230 and contained in heating water flowing into the tank 110 via the second connecting pipe 192 hereinafter operations for supplying heating water heated in the main heat exchanger 130.