Air gap may easily emerge in the progress of backfill in ground heat exchange due to field operation problems. The single U tube three dimensional numerical heat transfer model is set up and there are 3 kinds of backfill air gap assumptions for computational analysis

which are the most common situations in practical projects. Different air gap situations will have considerable effects on soil heat transfer

especially the factor of soil temperature recovery performance

which is investigated in this research

and there is practical project operating data for the comparison confirmation with the theoretical calculation results in inlet and outlet water temperatures. It can be concluded that the backfill air gap will increase the soil average excess temperature in the period of recovery inside the place where the air gap is

and degrade the heat transfer performance of the ground heat exchange.