Several solutions are currently being tested to improve the thermal efficiency of ground heat exchangers (GHEs) employed in ground source heat pumps. For shallow exchangers, the main effort is towards maximizing the surface available for heat exchange, while reducing the interference among exchangers; popular solutions towards this end are the slinky coil and the radiator shape. Recently, the flat panel has been proposed as novel alternative for horizontal exchangers. In this paper, the performance and impact of the radiator and flat panel installations are compared by solving the transient flow and heat transport problem within the surrounding ground via a numerical model. Adopting the same computational domain, boundary and initial conditions (the latter derived via a preliminary model run in absence of the GHE), and identical specific power outputs, the two installations yield different resulting domain thermal fields. The flat panel showed a higher capability to affect larger volumes of surrounding ground, so the soil temperatures reached values less extreme than in the radiator case. Since the temperatures remain 2÷3 degrees warmer, a higher COP is expected for the flat panel.