Knowing the type of soil or rock is important when designing the ground loop. Different soil and rock types have different heat transfer rates.

The ease with which the soil or rock can transfer heat into and out of the earth is a major factor in the length of the piping.

For example, poor soil requires more pipe surface areas in contact with the ground to get the same heat transfer as a loop with less pipe area in better soil.

A very helpful source is the Natural Resources Conservation Service Soil Survey. This is available from local NRCS offices located in about 3,000 counties across the United States. It gives a detailed layer-by-layer description of the soil down to a depth of five or six feet, along with any rock content, density, soil types and available water capacity.

Ground thermal properties can be determined from soil and rock manuals. One such guide by the International Ground Source Heat Pump Association is titled "Soil & Rock Classification for the Design of Ground-Coupled Heat Pump Systems."

Types of soil

There are three primary groups of soils that need to be identified to distinguish the variations in thermal behavior:

• Course-grained sands and gravels

• Fine-grained silts and clays

• Loam mixtures of sand, silt and clay

The presence of moisture in the soil improves the heat transfer rate, and this element should be considered and taken into account.

The soil/field resistance to heat transfer must be considered in determining the loop pipe length. This resistance varies with the pipe size and type, the soil type and dampness, the run time of the heat pump, and the configuration of the ground loop.

Software is available for this calculation process, and is recommended since the process is tedious.