When designing the number and depth of boreholes, usually on commercial applications it is critical to understand the thermal properties of the ground. To assist in this it is universally regarded as necessary to perform Thermal Response Testing (TRT's) as part of the initial Site Investigation. Initially, the design of a borehole field for a given Ground Source Heat Pump system relies on published Geotechnical data. This data provides approximate information relating to how the boreholes would be expected to perform given the local ground conditions. This approach accommodates its inaccuracies by either unnecessarily increasing the number of boreholes, the most expensive cost element per kW for a closed loop GSHP system, or worse still underestimating the true requirement, resulting in an unsustainable solution. Thermal Response Testing significantly reduces the potential for this inaccuracy, allowing us to design a truly bespoke GSHP solution for any given project and site.