Formation Thermal Conductivity | Frequently Asked Questions

Q: Thermally-enhanced grout is specified for the final loop field design.  The test bore was grouted with a low conductivity, 20% solids, bentonite grout.  How do I adjust the thermal conductivity value to account for this?

A: While the conductivity of the grout is important for the loop field design, it is not important for determining formation thermal conductivity.  We use the “line source” method to analyze data, which assumes an infinitely thin line rejecting heat at a constant rate into an infinite medium. The initial ten hours, which is influenced by the bore dimensions and grout conductivity, is ignored in the analysis.  However, once the heat has penetrated into the formation, the temperature rise of the formation approaches steady-state.  It is the slope of the temperature rise that is used in the analysis. Hence, no adjustment to the reported formation thermal conductivity is required.

Q: The software I use to design the loop field requires that I input a value for “soil conductivity”.  Is this the same as formation thermal conductivity?

A: Absolutely.  Formation, soil, and ground are all used interchangeably to describe the conditions in which the u-bends will be installed.  The use of the word “formation” simply implies that the installation conditions may be soil, rock, or some combination of the two.

Q: I’ve just received your report. I have a formation conductivity of 1.54 Btu/hr ft °F. How do I translate that into a loop length requirement, in terms of bore depth (in feet) per ton?

A: The formation thermal conductivity test provides values for three key parameters required for the ground loop design. These are the “Undisturbed Soil Temperature, Formation Thermal Conductivity, and Formation Thermal Diffusivity.“ These parameters, along with many others, are inputs to commercially available loop design software (e.g. GchpCalc, available at The software uses all of the inputs to determine the required loop length in bore depth per ton.

Q: Is the “Undisturbed Soil Temperature” value listed in the report the temperature that I enter into my loop design software where it calls for the “Deep-Earth Temperature”?

A: Generally, yes. The “Undisturbed Soil Temperature” is the constant temperature of the formation. We attempt to determine this value by measuring the temperature of the water entering the test unit at the beginning of the test. However, the value we measure and report may be inaccurate if the test is initiated too quickly after the installation of the test bore, or if the testing operator failed to activate the data acquisition unit prior to energizing the heating elements. If you suspect the temperature we are reporting to be too high or too low, we recommend that you investigate further through other sources.

Q: What is thermal diffusivity and what does its unit of measurement mean?

A: Thermal diffusivity is just the ratio of thermal conductivity to heat capacity, and the units are (Btu/hr-ft-°F / Btu/ft3-°F = ft2/hr x 24 hr/day = ft2/day). It is a measure of how fast a thermal disturbance at one point in the heat transfer domain can be "felt" at another point. Soils with higher thermal diffusivity generally have less thermal storage capability than low thermal diffusivity soils and the temperatures will change faster over time with the addition or removal of heat. The ideal soil for ground loops is a soil of high thermal conductivity and low thermal diffusivity (high heat capacity) so that heat transfers quickly and the soil has a lot of thermal storage capability.

Q: How do you determine the thermal diffusivity value you report?

A: Thermal diffusivity is calculated by taking the measured thermal conductivity and dividing by the estimated heat capacity of the soil or rock (average based on the drill log). Heat capacity is a function of soil or rock type, density and moisture content and we have to make our best estimated of what exists along the borehole. We use the drill log and tables for that.

Q: We have been told that in order to accurately measure thermal diffusivity, you would need to drill two test bores side-by-side, where one bore is use for the generation of heat and the other bore used to measure temperature change versus time versus distance from the heat generation bore, so why do you use the above method?

A: You can measure thermal diffusivity this way but the accuracy of placement of the temperature sensors along with controlling power, etc. makes this method no better than what we do. It is important to note that the method you described would require that you install both test bores exactly parallel to each other, which is virtually impossible.