There are two types of Ground Source Heat Pump boreholes, known as “closed-loop” and “open-loop.” Closed-Loop boreholes are designed to harness heat from the ground by circulating thermal transfer fluid within plastic pipes which are installed in boreholes. The plastic pipes, known as ‘collector loops’, are fitted with a u-bend. The thermal transfer fluid will be relatively cold as it enters the borehole but is warmed by the ground temperature as it circulates to the u-bend at depth and back up towards the surface. When the fluid exits the borehole it is circulated towards the heat pump, which effectively extracts the heat from the fluid. The fluid, now cooler, is circulated back towards the borehole again where it repeats the cycle. Hence the term “closed-loop”! Our example demonstrates the typical design of a closed-loop borehole. Open-Loop boreholes are designed to harness heat through the abstraction of groundwater from aquifers or underground mine workings. The differences between closed-loop and open-loop boreholes are explored in a dedicated section on this page. Closed-loop boreholes are the more common solution but Open-loop systems can be especially viable on developments with an especially high demand for heat. This can include district heating schemes or industrial warehouses and factories.