PURPOSE: To improve thermal efficiency greatly, by forming the cylindrical body for circulating heat medium inside by members made by internal chill casting method wherein pipes are supported by fins along the inner surface of the cylindrical body.

CONSTITUTION: Burner 10 is installed inside the lower part of the cylinder body 1, and heats water which is the secondary heat medium circulating within the pipe 2 while combustion gas (which is the heat medium) ascends within the cylindrical body 1. Cylinder body 1 is made of cast material and is formed by the two half bodies 1A and 1B. Pipe 2 is laid along the inner side of the cylinder body 1 and is supported by fins 11(of approx. semicircular disc shape) attached inside of the half bodies 1A, 1B, and are penetrated by the pipe 2, and all the above mentioned members are assembled by internal chill casting method. Under this constitution, surface area of the pipe 2 also acts as heat reception area, and because pipe 2 is connected with the cylinder body 1 by inner chill casting via fins 11, improvement in thermal efficiency and heat resistant property are ensured.