the blower 33 a proportional directional valve control circuit 63 is provided to control the amount of fuel gas by adjusting the opening degree of the proportional directional valve 45 in response to the outputs from the revolution detecting circuit 62 and that from the thermocouple 52 the revolution detecting circuit 62 is comprised as shown in fig3 by way of illustration in the revolution detecting circuit 62 a brushless electric motor not shown is incorporated into the blower 33 in which a permanent magnet 33b is secured to a rotary shaft 33a of the motor in addition to the magnet 33b the revolution detecting circuit 62 has a hall device 33c a signal generator 62a which includes coils co1 co4 an analogue switch means 62b a decoder 62c and a voltage transducer 62d the transducer 62d changes its voltage v in proportion with the revolution n of the blower 33 within the range of a certain voltage as seen in fig4 the proportional directional valve circuit 63 determines the amount of energization a toward the proportional directional valve 45 according to the output voltage v from the revolution detecting circuit 62 as seen in fig5 the valve circuit 63 makes up for the amount of the energization toward the valve 45 so as to keep the ratio of air flow to gas fuel at the appropriate value with the structure thus far described the electronic control circuit 5 works as follows operation of the knob 54 changes the reference voltage in response to the output from the water temperature sensor 53 so that the blower control circuit 61 instantaneously changes its output.