Accurate modeling of heat transfer in the ground and inside the borehole is crucial to correctly size and assess performance of ground coupled heat pump systems. The model proposed here uses a hybrid approach combining two techniques. First, the rapid transient behavior inside the borehole is handled numerically with a fine grid in combination with a model size reduction technique to reduce computation time. Secondly, the surrounding ground is modeled using modified g-functions. The resulting hybrid reduced (HR) model is implemented as a TRNSYS type using a load aggregation algorithm. Results show that differences between the proposed model and a well-known non-capacity model are within an acceptable range of the order of ± 0.8°C. The differences are partly attributed to the simplification methods. However, they are mainly due to the fact that the HR accounts for the thermal capacity in the borehole. In simulations over a heating season, the inclusion of borehole thermal capacity results in outlet fluid temperatures that can be up to 2 °C higher than when thermal capacity is not accounted for. In terms of computation time, the HR model is about 37 times faster than a complete hybrid model, but with almost no loss in accuracy.