Our resources reflect our commitment to the industry and the overall accuracy and experience our customers receive. These PDFs can be completed on a laptop onsite and submitted by email or can be filled out by hand and returned by fax.

The flow and power rates in my test specifications are different than the GRTI FTC Example Specification. How do I address these differences?

While we recommend following the GRTI FTC Example Specification as closely as possible, we will work with you (and/or the specifying engineer) to find a reasonable solution whenever possible. Minor equipment adjustments are possible for an additional fee.

My loopfield design software requires a value for “soil conductivity.” Can I use formation thermal conductivity data for soil conductivity values?

Absolutely! Different softwares use different terminology for the loopfield substrate interchangeably: formation, soil, and ground are some examples. We use “formation” to indicate a comprehensive term for all materials in the loopfield installation site.

What is thermal diffusivity?

Thermal diffusivity is a measure of how quickly a thermal disturbance at one point can be “felt” at another within the heat transfer domain. Formations with high thermal diffusivity generally have less thermal storage capability than formations with low thermal diffusivity, which means the temperatures will change faster over time as heat is added or removed. An ideal formation will have high thermal conductivity and low thermal diffusivity, as it has a high thermal storage capability that transfers heat rapidly. Thermal diffusivity is expressed as a ratio between thermal conductivity and heat capacity.

Can I use the “Undisturbed Soil Temperature” value for “Deep-Earth Temperature” in my design software?

Generally, yes. We determine the Undisturbed Soil Temperature by measuring the water temperature entering the test unit at the beginning of the test. If the value in your GRTI report seems inaccurate, other resources are available to determine the undisturbed soil temperature.

My GRTI test report indicates FTC at 1.54 BTU/hr ft. °F. How does that translate into loop length requirements, in terms of bore depth (in feet) per ton?

Most softwares require three key parameters to determine bore depth (ft/ton): Undisturbed Soil Temperature, Formation Thermal Conductivity, and Formation Thermal Diffusivity. All three of these parameters are included in your GRTI report. Use these values as inputs in your software to determine bore depth (ft/ton).

My test bore was grouted with a low-conductivity 20% solids bentonite grout. How do I adjust for the thermal conductivity value to account for this?

The short answer: you don’t have to. Grout conductivity is important for the loopfield design, but it does not play a factor in FTC tests and analysis. We use the “line source” method, which assumes an infinitely thin line rejecting heat at a constant rate into an infinite medium. The initial 10 hours of the test—influenced by bore dimensions and grout conductivity—does not factor into the analysis. Instead, we use the slope of the formation’s temperature rise, so no adjustment to the reported FTC is required.

Does the report include bore thermal resistance?

This value is not typically listed but can be included in the report if desired. We recommend that loopfield designers utilize bore resistance values calculated by a loop sizing software package for their designs. However, bore resistance can be a helpful as a quality control measure on production bores, contact GRTI for a more in-depth discussion on this matter.