PURPOSE: To improve thermal efficiency and to facilitate temperature regulation, by opening a surplus air outlet part in an inlet path of external air so that its opening degree is freely adjusted.

CONSTITUTION: An air chamber is formed by surrounding a combustion chamber 2 into which a burner 1 is protruded, a plurality of headers 3aW3c, and heat exchange pipes 4a and 4b which communicate the headers 3aW3C, by a body 7. A heated air extracting port 8 is opened in one side of the upper part of the back surface of a body 7. The external air inlet path 9 is provided so as to surround the upper part of the body 7 and the outer surface of the lower part of a chimney 5, and an inlet port 10 is provided. The surplus air outlet port 12 is opened in the vicinity of the extracting port 8 of the inlet path 9, and a damper 13 is provided.