A computer program has been developed for numerical simulation of the dynamic thermal performance of horizontally coupled heat exchangers for ground source heat pumps. The program takes account of dynamic variations of climatic, load and soil conditions. This paper presents the results of the simulation of a horizontally coupled heat exchanger with different installation depths and operating times for the heating season in the UK. It is shown that a heat exchanger installed at a shallower depth can provide a larger heat extraction rate at the early stage of heating operation but this benefit diminishes with operating time. For a given installation depth, the rate of heat extraction decreases with increasing operating time. The heat extraction is also influenced by the start date of operation and the time for assessment. Operating a heat pump with an earlier start date in autumn would give rise to a higher amount of cumulative heat extraction for the same duration of system operation but a lower heat extraction rate than at a time when it is operated with a later start.