being moved to the position shown in fig1 by a solenoid winding 32 operating on a core 33 when the second mode is sensed a second solenoid winding 34 operates on a second core 35 to transfer both pistons to the right as shown in fig1 the solenoid winding 32 and motor 17 are connected in parallel so that when the valve pistons are moved to the left motor 17 and pump 16 are both energized a similar connection exists between solenoid winding 34 and motor 26 in order to determine the best of two operating modes or none at all four sensors are placed in position adjacent the three heat sources these sensors may be of several forms the most desirable are thermistors since they require minimal current and require no mechanical movement a first thermistor 36 is positioned adjacent to the solar absorber 10 and changes its resistance responsive to the temperature of the absorber a second thermistor 37 is placed in the ground near the ground radiator 12 thermistors 36 and 37 being connected in series and to a buffer amplifier 38 the series connection plus a small current provided by the amplifier 38 impresses a voltage on the amplifier input circuit which is responsive.