a thermostat or controller for controlling water circulation in a solar heating system is disclosed as including a resistant bridge circuit with a solar panel temperature responsive resistance and a storage tank temperature responsive resistance in opposite arms joined at one power input node with a differential comparator connected across the sensing nodes of the bridge . there are also disclosed optional features including additional fixed resistance divider circuits together with respective additional comparators sensing low - limit solar panel temperatures and / or high - limit storage tank temperatures for turning on or off the circulation pump , and / or for running cool water into the storage tank . in still another disclosed feature a sun switch produces periodic operation of the circulation pump to cause a sensor in the outlet of the solar panel to produce a true reading of solar panel temperature .