Foundation Heat Exchangers (FHX) are a novel form of ground heat exchanger for residential applications and, by virtue of lower costs, could increase the up-take of efficient heat pump technology. This work has aimed to develop a new efficient model of such heat exchangers for system simulation. The recently developed Dynamic Thermal Network approach has been applied to formulate a model of the FHX that includes the basement, pipes and adjacent ground. This response factor approach allows complex three-dimensional geometries such as this to be represented. The formulation of the method and its application to the FHX is described along with a numerical procedure to calculate the required weighting factor series. An improved method of calculating this data and reducing it to a compact form is presented. Experimental data has been used to verify the results.