Ground heat exchanger design tools have become increasingly important for the sizing of energy-efficient heating and cooling systems. Most such heat exchanger design tools incorporate a simulation that uses both long and short timesteps (a "hybrid timestep" procedure). Current tools typically expect engineers to exercise judgment to determine the magnitude and duration of the shorter timestep. This paper proposes an accurate and efficient methodology for developing this hybrid timestep formulation, which is validated against hourly simulations for a set of three building types. Overall, the method performs well for design purposes, with the error in ground heat exchanger sizing averaging less than 1% and always less than 8%. (C) 2011 Elsevier Ltd. All rights reserved.