Shallow ground heat exchangers are increasingly studied due to their advantages in cost and long-term energy performance stability when coupled with heat pumps for space heating and cooling. As for borehole heat exchangers, the backfilling material affects significantly the operating efficiency of the whole system, mainly driven by the low thermal diffusivity of the soil. To enhance the heat transfer, the mixing of the backfilling material with phase change materials (PCMs) is a novel strategy still partially investigated, especially with regards of the heat pump on/off cycling. This study presents the results of experimental tests carried out at lab scale to analyse the performance of a shallow Flat-Panel ground heat exchanger (FGHE) coupled with water-sand mixture. Firstly, the comparison between FGHEs coupled with dry sand and water-sand mixture is performed; then, the impact of latent heat resulting from freezing is further studied in three on/off operating modes. A maximum of 31.6% increment in heat transfer efficiency is observed in wet conditions and for the highest on/off frequency. Therefore, coupling FGHE with water-sand mixture enhances the heat transfer, especially in icing interval and when combined with a suitable on/off operating frequency.