A transient coupled model for a variable speed CO2 direct expansion ground source heat pump (DX-GSHP) is developed. An annual simulation for integrated space heating and cooling of a small residential building located in a Canadian climate is conducted, during which numerous operation parameters are evaluated. The model includes a control strategy that regulates pressures of the fluid, thereby allowing the compression cycle to go from subcritical to transcritical for optimal heat rejection. The model can be used to simulate DX-GSHP systems under various loads and evaluate the effect of dynamic thermal behaviour of the ground on the system performance. The results show that there is a strong thermal short-circuiting within BHE when the system is operating at low mass flow, which hinders the system performance. Crown Copyright (C) 2019 Published by Elsevier Ltd. All rights reserved.