Soil or sub-soil (ground source) systems are used for residential and commercial applications and have similar advantages to water-source systems, due to the relatively high and constant annual temperatures resulting in high performance. Generally the heat can be extracted from pipes laid horizontally or sunk vertically in the soil. The latter system appears to be suitable for larger heat pump systems. In the former case, adequate spacing between the coils is necessary, and the availability of suitably large areas (about double the area to be heated) may restrict the number of applications. For the vertical systems, variable or unknown geological structures and soil thermal properties can cause considerable difficulties. Due to the removal of heat from the soil, the soil temperature may fall during the heating season. Depending on the depth of the coils, recharging may be necessary during the warm months to raise the ground temperature to its normal levels. This can be achieved by passive (e.g. solar irradiation) or active means. In the later case, this can increase the overall cost of the system. Leakage from the coils may also pose problems. Both the horizontal and vertical systems tend to be expensive to design and install and, moreover, involve different types of experts (one for heating and cooling and the other for laying the pipe work).
Rock (geothermal heat) can be used in regions with no or negligible occurrence of ground water. Typical bore hole depth ranges from 100 to 200 m. When large thermal capacity is needed the drilled holes are inclined to reach a large rock volume. This type of heat pump is always connected to a brine system with welded plastic pipes extracting heat from the rock. Some rock-coupled systems in commercial buildings use the rock for heat and cold storage. Because of the relatively high cost of the drilling operation, rock is seldom economically attractive for domestic use.
The ground constitutes a suitable heat source for a heat pump in many countries. At small depth temperatures remain above freezing. Furthermore the seasonal temperature fluctuations are much smaller than those of the air. Heat is extracted from the soil by means of a glycol solution flowing through tubing embedded in the ground. If a horizontal grid of tubing is utilized, several hundred square meters of surface area are needed to heat a single family building. In urban areas such space is rarely available. In addition considerable costs are involved. For these reasons vertical ground heat exchangers are more preferred presently.
Geothermal heat sources for heat pumps are currently utilized in various countries, particularly in the USA, Canada and France. These resources are generally localized and do not usually coincide with areas of high-density population. In addition, the water often has a high salt component which leads to difficulties with the heat exchangers. Due to the high and constant temperatures of these resources, the performance is generally high.