PURPOSE: To improve a heating efficiency by a method wherein a chimney is surrounded by a water chamber to recover the waste heat.

CONSTITUTION: A fuel throwing port 2 is provided at the upper part of a water heater main body 1, an ash removing port 4 is provided at the lower part thereof while a combustion chamber as well as a flame path 8 are formed by the wall 9 of water chamber in the heater, a space between the wall 9 of water chamber and the main body 1 is utilized as a water chamber 10, a chimney 11 connecting to the terminal of the flame path 8 is stood and the outer periphery of the chimney 11 is surrounded by the water chamber 12 formed by an outer plate 13. The arrow sign B indicates the flow of water.