Soil & Rock


No single factor is more important to the successful operation of a ground-coupled heat pump system than the efficiency of the heat transfer between the ground-loop and the surrounding soil.

Soil thermal properties vary by a factor of ten and the thermal performance of rocks also varies widely, as can be seen from this table. These variations indicate the importance of an accurate estimate before any ground loop design can be finalized.

A very helpful source is the Soil Conservation Service Soil Survey. This is available from the county SCS offices located in about 3,000 counties in the United States. It gives a detailed layer-by-layer description of the soil down to a 5 or 6 foot depth, 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 manual is the EPRI field manual titled "Soil and Rock Classification for the Design of Ground-Coupled Heat Pump Systems," published by IGSHPA.

Oklahoma State University has also developed a relatively easy method of determining the heat transfer in boreholes. They can be contacted through IGSHPA.

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

  1. Course-grained sands and gravels
  2. Fine-grained silts and clays
  3. Loams mixtures of sand, silt, and clay.

The following table indicates the properties of various soils. 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.