It is well known that great effort is being made to conserve power, and part of the conservation is accomplished by using more solar energy.  Accordingly, many buildings which have been previously heated entirely by consumable fuels are now being converted so that solar sources may be used whenever their energy is adequate for the purpose.  Efficient use of this energy requires achieving a degree of automatic control, and also achieving compatible operation between, on the one hand the main heating and cooling system of a building and its already existing thermostats, and on the other hand an auxiliary solar heating source newly added to the building. 
The prior art shows various circuits for controlling operation of an auxiliary solar heating system together with a conventional building heating and/or cooling system. 
U.S. Pat.  No.  4,034,912 to Thomas Edward Hayes teaches a method and apparatus for combining a solar heating system with a fuel fired heating system for the purpose of substituting the solar source for the fuel burning source whenever the energy in the solar source appears adequate to supply the required heat.  This system provides a microprocessor control circuit which integrates the two systems together, as distinguished from an interface according to the present invention which introduces an auxiliary solar source into an existing system without requiring extensive rewiring of the latter. 
U.S. Pat.  No.  4,034,738 to Everett M. Barber, Jr. provides a heating system in which a control system seeks first to heat the building with solar energy, but failing that effort changes over to a fuel burning system if the thermostat continues to fall.  No interface system is provided for adapting an auxiliary solar system to an existing building heating and air conditioning system without rewiring the latter.  U.S. Pat.  No.  3,977,601 to Vittorio Bearzi is of a similar type showing, however, the use of a fuel burning system to supplement the solar source. 
U.S. Pat.  No.  3,028,093 to Richard D. Miller shows a system which controls several separate fuel burning heating systems from a common thermostat while at the same time compensating the current through the anticipator of the thermostat so that the thermostat sees substantially the same current regardless of whether one, or several of the heating systems, is at the moment being operated.  The patent does not show an auxiliary solar system in combination. 
U.S. Pat.  No.  4,052,000 to Terrence C. Honikman shows interfacing of a solar water heater with an existing electrical water heater by interposing a control logic circuit which is responsive to the temperature in the solar system as one criterion for substitution.