AbstractGround source heat pumps are a valid technology for heating and cooling in residential application, since they allow to have a stable temperature of the heat source/sink by means of ground heat exchangers, generally named as Boreholes. In particular, U- pipe heat exchangers are interesting when working with two-phase fluids, especially during the heating season, when natural circulation might occur. This work presents the model of a borehole heat exchanger working with carbon dioxide as secondary fluid in a U-pipe loop. A closed system of equations is introduced with appropriate correlations for heat transfer and pressure drops, accounting for transient heat conduction due to ground temperature variation. Maps of performance varying the main operating parameters are presented and discussed, showing the potential of the model as tool for sensitivity analysis and as sub- model to find the matching between the heat pump and the borehole heat exchanger.