reactance and is hence inexpensive the voltages and currents disclosed herein are by way of example only and any desired range of voltages and currents could be used and additional resistors can be inserted in shunt or in series with element 60 to set the level of current portions 74 and 76 at any desired values in addition by changing the value of the resistor 43 the location of curve 80 may be shifted to increase or decrease the time following energization of the transformer 38 at which the contact 44 opens to shut down the fuel valve a shutdown sensing signal may be fed by means of a connection 53 from the junction between delay 40 and the solenoid 46 to the control system to deenergize the blower motor 26 and or other elements of the system following a period of time such as a minute or so from the burner start the fluid in the heat exchanger 10 will have been raised to a sufficient temperature to actuate the thermal sensor 22 to circulate the fluid through the heat exchanger 16 by actuation of pump 14 an additional feature of safety may be introduced by utilizing the pressure regulator 50 as a zero pressure regulator such that the gas pressure at the output of the regulator is supplied to the blower slightly below atmospheric pressure thus if the blower 26 motor is not actuated due for example to motor failure or a malfunction of control circuit 20 no gas will be.