terminal 82 to one terminal of a solenoid actuated switch 85 the other terminal of switch 85 is connected to conductor 67 leading to terminal 86 to which conductors 68 and 69 are connected fan motor 58 is energized to operate through conductors 57 and 69 as before when solenoid switch 85 of switch 10a is closed and switch contact 53 is closed at terminal 49 of switch 46 primary coil 90 of transformer 91 is connected between conductors 57 and 83 as shown secondary coils 92 93 of the transformer are connected to full wave bridge rectifier 95 by conductors 96 99 the direct current rectifier output is delivered to the remainder of the circuit by conductors 101 and 102 sensors 104 and 105 each a diode are positioned to detect and respond to temperature differences sensor 104 is placed at an exterior location to sense outdoor temperatures and sensor 105 is placed at an indoor location for example in an attic to sense temperatures at that location cable assemblies 107 108 have conductors 109 110 respectively which through plug in connectors 109a 110a are connected together and to conductor 102 through resistor 112 in conductor 114 conductors 116 117 of cable assemblies 107 108 respectively are connected through plug in connectors 106a 117a and respective conductors 118 119 to a balance assembly including resistors 120 121 and variable resistor 122 variable resistor 122 is adjusted.