PURPOSE: To make it possible to absorb the heat of exhaust gases from a boiler to a great degree by a method wherein the titled flue pipe is closely inserted between the outlet of the smoke duct of the boiler and a smoke stack.

CONSTITUTION: A cylindrical water tank 6 is provided around the flue pipe 5 and a cylindrical heat insulating and heat retaining material 7 is provided around the water tank 6. The supply of water to the tank 6 is made through sockets 2 and 3 and the supply of hot water is made through either a socket 1 or a socket 4 according to the purpose for which the hot water is used. Further, a baffle plate 8 is inserted into the flue section 11 of the flue pipe 5 at the outlet 10 of the smoke duct of the boiler so as to make the heat of the exhaust gases standing in the flue section 11. The insertion of a high temperature sensor 13 thereof is required when the tank 6 is connected to a heat accumulation tank through a pipe. In this case, when a low temperature sensor is attached to the heat accumulation tank, it is possible to circulate hot water forcibly while the temperature thereof is elevated automatically. Thus, in case where both of the instant residual heat utilizing tank 6 and the heat accumulation tank are used side by side on a heavy oil boiler or a kerosene oil boiler for the supply of hot water, it is possible to save energy by 75%.