The thermal loads of commercial and institutional buildings are generally cooling-dominated. When such buildings use ground source heat pump systems (GSHP) they reject more heat to the ground-loop heat exchanger than they extract over the annual cycle. In these situations, supplemental heat rejecters can be used to reduce the required size of the ground-loop heat exchanger, thereby reducing the first cost of the system. This paper describes the development, validation, and use of a design and simulation tool for modeling the performance of a hydronic pavement heating system as a supplemental heat rejecter in ground-source heat pump systems. The model uses a finite difference method to solve the transient two-dimensional heat conduction equation and has been formulated for use in component based system simulation. Full-scale experiments were conducted concurrently with the development of the model, the results of which have been used for validation purposes. An example application simulation is presented to demonstrate the use of the model as well as the viability of the use of pavement heating systems as supplemental heat rejecters in GSHP systems. [S0199-6231(00)00404-4].