It has been suggested that heat-transfer from a solar collector to water can be accomplished by use of a phase-change system in which the operative liquid is boiled in the collector and condensed in heat-exchange with the water to release the heat of vaporization.  The pressure in the system .[.would be maintained.].  .Iadd.may be limited .Iaddend.at a suitable .[.high.].  level by the use of an accumulator.  It has been found in practice, however, that although the principle is satisfactory, certain practical considerations prevents such a system for operating in the optimum manner.  For instance, it has been found that there is a tendency for the vapor to be converted to liquid in the tubes connecting the collector to the condenser, thus causing slugs of the liquid to pass into the condenser.  Since it is the return of the heat of vaporization in the condenser that serves to heat the water in the optimum manner, the presence of any amount of liquid in the condenser tubes causes a less-than-effective operation.  Also, the presence of large bubbles in the liquid sets up an oscillatory condition that can cause flow of liquid into the condenser, as well as causing a vibration of the mechanical elements that can lead to their destruction or to the failure of joints.  The phase-change solar heater lends itself to installation in remote locations, because it is a self-regulating system that needs no electricity or supplemental fuel.  However, the installation in such areas is difficult because of the necessity for assembling it in the field and providing suitable start-up pressure for the accumulator.  These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention. 
It is, therefore, an outstanding object of the invention to provide a solar water heater in which very little liquid finds its way into the condenser. 
Another object of this invention is the provision of a solar water heater which is free of large liquid level oscillations. 
A further object of the present invention is the provision of a solar water heater in which the danger of failure of the elements due to fluid oscillation and mechanical vibration is substantially reduced. 
It is another object of the instant invention to provide a solar water heater which can be installed in the field and in remote locations less need for skilled workmanship or electrical power at such a location. 
A still further object of the invention is the provision of a solar water heater which is simple in construction, inexpensive to manufacture, and which is capable of a long life of useful service with a minimum of maintenance. 
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.