An extreme condensing boiler includes cold and hot water headers, burner, and multi-tubing heat exchanger, so that cold water absorbs heat from exhaust gas of the burner being heated to medium-hot, which absorbs heat generated by the burner being heated to hot water through absorbing heat from both sides of tube portion of the multi-tubing heat exchanger, and so that the hot water is delivered to the hot water header, and a combustion air heat exchanger is provided around the multi-tubing heat exchanger. The condensing boiler uses dual/triple tubes for fire and water tubes, increasing efficiency by absorbing heat from both sides. The fire tube is surrounded by the cold water tube so as to minimize exhaust gas temperature and maximize condensing by absorbing waste heat. The condensing boiler realizes a direct fire boiler through the fire and water tubes, and an energy-saving tankless boiler including headers and tubes only.