Structural foundation piles are being used increasingly as heat exchangers to provide renewable heat for new buildings. To design such energy systems a steady state is assumed within the pile, which is conventionally characterised by constant thermal resistance. However, there has been little research regarding pile resistance and there are few published case studies. Numerical modelling results are presented here to provide typical values of pile resistance, depending on the details of the heat exchange pipes. Analysis suggests large diameter piles may take several days to reach steady state; in these cases a transient design approach may be more appropriate.