One of the challenges of a solar water heater is that you can't turn the sun off. If you don't use the water it just gets hotter and hotter, and if you stop the circulating pump then the fluid in the solar collector boils. So we don't have to have compulsory baths after every sunny day there is a "heat dump" in the loft that is switched on when the solar cylinder water exceeds 80°C. This is an old radiator and a two way valve controlled by the solar controller. With the valve energised, water from the collector passes thru the radiator before going to the cylinder, thus all the heat is dumped into the loft and the water in the cylinder doesn't get too hot. Sadly this does mean that electrical energy for running the pump and energising the valve is wasted, but the system only needs to kick in during the second day of full sun when no hot water has been consumed. In return it means that the collector can be generously sized so we should get some useful heat during the spring and autumn. If going away from a summer holiday it is possible to partially drain the system and switch it off.
The valve has a thermic actuator which expands thermally when energised. This takes a few tens of seconds to respond but uses less energy than a coil type actuator. Obviously when the system is finished the pipes will all be lagged.