The Solar Controller - Brains of the operation
Solar Thermal hot water systems are controlled by a Differential Temperature Controller, or DTC. The way that a solar controller works, is with the use of accurate sensors in the collector (the solar panel) and the cylinder.
These sensors are wired back to the controller. When there is sufficient difference between the cylinder temperature and the collector temperature, the controller switches on the pump, which transfers the useful, heated fluid in the panel down into the bottom coil of the cylinder, which in turn heats the stored water for use at the taps.
Once the desired cylinder temperature is reached, the control stops the pump. Simple!
There are a large number of controllers on the market, each with it's own pros and cons. Some give the user simplicity, and need rare attention by the end user, as the primary function is to regulate the system fully automatically (eg the Resol control, below)
Other controllers can be complex, allowing full integration of the boiler system. For instance, allowing 3 programmed 'on' periods for the boiler to fire daily, but holding off the boiler during timed 'on' periods if the controller suspects the cylinder will reach temperature through solar gain alone (eg the Vaillant 620 controller, below)
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