This paper presents an iterative spectral approach for the determination of the maximum heat pump capacity that ensures the longevity of a specified ground heat exchanger field in hybrid ground source heat pump systems. Fast fluid calculation is obtained by spectral convolution of the incremental ground load signal and the ground heat exchanger transfer function. A correction function is used in an iterative scheme to determine the maximum heat pump capacity that maintains fluid temperature within the operation limits. Results show that the maximum capacity is case dependent, and that long-term simulation is important for its determination. The proposed approach is fast, convergent, general and can manage different building load profiles, either balanced or imbalanced, and different heat pump controls. Crown Copyright (C) 2021 Published by Elsevier Ltd. All rights reserved.