Solar Thermal heating systems use solar panels, called ‘collectors’, fitted to your roof. A collector is generally part of a sealed system, filled with an antifreeze mixture or heat transfer fluid. The heat transfer fluid circulates through the collector, round the system, through a coil in the cylinder and back to the collector once more. Heat is absorbed by the heat transfer fluid in the collector, which is then transported to the cylinder and used to heat the water in the cylinder. The cooled heat transfer fluid is then returned to the collector to be heated again.