A very useful bit of kit for solar thermal systems is the differential temperature controller (DTC). This ingenious device has a built-in microprocessor which monitors the temperature at two points in your installation via sensors. It also controls a relay which can switch the pump on or off and, therefore, controls the flow of solar fluid.
DTC sensors are electronic, so they don't need any special planning when you're plumbing your solar thermal system. The controller watches the temperatures and, in most models, compares them to settings you choose: when the temperature difference reaches a certain point, the pump is switched on to heat your domestic supply; once the temperature has equalized, the pump is switched off to save electricity.
Some DTCs come with a "high limit" feature, which switches off the pump when the domestic storage temperature reaches a specific level, or a "recirculation" feature which monitors the collector temperatures and pushes warm solar fluid around the system if they're close to freezing. These two features are not useful in a closed-loop system using an antifreeze fluid, so in that case you can switch them off.
As you would expect, newer DTC models come with all sorts of bells and whistles: multiple settings for summer cooling and vacation times, Btu calculations, variable-speed pump controls, irradiance measurements and Wi-Fi compatibility for logging and remote monitoring are all among the extras on offer.
One innovation which deserves special mention is a dual-relay model created especially for pressurized systems. This DTC controls a second relay which operates a secondary pump or a valve in the solar thermal system: when your domestic water storage tank reaches a defined "high limit" temperature, the second relay engages and diverts the solar fluid to a shunt load, without switching off the main circuit. The practical result of this is that the solar fluid's excess heat is passed to the shunt load (thus cooling it) but circulation continues, so the fluid doesn't lie stagnant. This extends the life of the solar fluid very effectively.