the highest energy i e highest temparature point of the storage chamber in this case at the opening adjacent duct 29 where thermostat sensor t3 is located the fourth location is at the highest energy i e highest temperature point in the solar collector in this case near the top outlet where thermostat sensors t1 t1 39 and t1 34 are located sensor t1 is part of a differential thermostat in combination with sensor t2 it operates relay cr1 which shifts switch 42 between two positions to complete a circuit for low speed blower windings or a circuit for high speed blower windings for motor a of blower a differential thermostates are conventional items marketed for example under the trademarks 34 honeywell 34 and 34 rho sigma 34 the operation of which is explained hereinafter in fig1 a 2a 3a 4a and 5a is set forth an electrical diagram depicting a control circuit each figure showing the control circuit in a different mode of operation the circuit is powered from bus lines l1 and l2 and includes relays cr1 cr2 cr3 cr4 cr5 cr6 cr6a cr6b and cr7 as well as temperature sensors t1 t1 39 t1 34 t2 t3 and t4 it includes an electrical motor a for blower a and an electrical motor b for blower b a motor operated damper c and a motor operated damper e a differential thermostat 40 employs temperature sensor t1 in the solar collector and.