A circulating path 12 is provided for circulating a heat transfer medium
across the cooling section 6A and an exhaust heat exchanger 9. The
circulating path 12 is communicated to an output heat exchanger which is
mounted on a hot water storage. The heat transfer medium is circulated by a pump
10 from the exhaust heat exchanger 9 to the oil heat exchanger 5
and the cooling section 6A. The heat transfer medium to be introduced
into the exhaust heat exchanger 9 is so controlled in the flow rate that
a temperature of the water vapor in the exhaust gas is declined to a temperature
lower than the dew point. The heat transfer medium when having received heat directly
from the exhaust gas and heat generated by condensation of the exhaust heat is
conveyed to the cooling section 6A where its temperature soars up further.
