PURPOSE: To improve the pressure resistance, reliability and heat exchanging efficiency by connecting the parts of a coiled heat conducting pipe which are closely contacting in upper and lower positions, and providing one or more holes that penetrate the connected part.

CONSTITUTION: The burning gas is, after exchanging heat with the boiler water in a heat transfer pipe 1 in a burning chamber 5, is directed to the peripheral area by a baffle 7 to encounter a heat conducting pipe 2 eventually to be released through a passage 10 and a water drain lid 9. meanwhile, the water is supplied from a feed water inlet 11, and as it moves upward, it exchanges heat with the burning gas to be delivered as hot water from a hot water outlet 12. As the heat pipe 2 is closely contacted and connected, the contact surface with the burning gas is in a convex shape, which enables the heat transfer surface area and the heat exchanging efficiency to be increased and the pressure resistance against the internal pressure load to be increased. In addition, as the burning chamber 5 and a heat exchanging chamber 6 are enclosed by water walls, the reliability against the high-temperature burning flame and burning gas is improved. Further, because of a through-hole 3 which allows air bubbles generating at the time of heat exchange to be released from the through-hole 3 to the hot water outlet 12, the bumping and afterboiling can be reduced.