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Geothermal Conductivity Test Pros and Cons

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When deciding whether or not to use geothermal on your home or commercial building, a very common litmus test is the results of what is called a “thermal conductivity test.” Often referred to as a “TC” test, or just simply a conductivity test, it is essentially a single well test on an already drilled geothermal loop. The purpose is to find out how conductive the earth’s surface is at a specific location. The test well will be a permanent installation, and can be used as a geothermal well in your project if you choose geothermal.

Specifics of a Conductivity Test Machine

A conductivity tester is a small machine that is simply a water pump with an electric heating element. It is the size of a small generator, and is mounted very close to the top of a test well. The conductivity test machine has hoses that connect to the geothermal loop, and will circulate water through the loop. After connected, an insulation blanket is attached around the device and loop to minimize the amount of heat loss to the air. After installed and insulated, the machine will pump water through the loop, and the heating element(s) will turn on. The device monitors the amount of heat put in the water before it goes into the ground loop and measures the water temperature of the water as it returns. The difference of these temperatures is the amount of heat that was absorbed by the earth.

How Long Does a Conductivity Test Take?

A geothermal conductivity test takes about 48 hours from set up to teardown, and takes anywhere from 24 to 48 hours of actual run time. This allows the machine to get repetitive sample measurements, and also allows the machine to cycle on and off through day and night. It is important that the appropriate time be given so the machine can vary the heat loading scenarios. The on board computer measures and compares several things, and has three key outputs:

  1. Undisturbed Deep Earth Temperature
  2. Thermal Conductivity
  3. Thermal Diffusivity
Your contractor will use these three outputs to help decide how suitable the ground is for geothermal. Ideally, the deep earth temperature will be in line with the predicted gradient (Expected ground temperatures Appendix 1), the conductivity will be greater than 1.5 Btu/(hr*ft*degree F) and the diffusivity will be greater than 0.8 ft^2/hr. In the case of larger commercial projects, this is actually very useful information so that the appropriate number of geothermal wells can be designed. Too few will not provide adequate heating and cooling, and too many will provide enough performance but will be very costly.
Now that you know the basics of Geothermal Conductivity Tests, let’s move on what you should expect on your geothermal application.

Residential Geothermal Conductivity Tests

Pros:

  • Have exact data as to what to expect from your installation.
  • Will help size your system exactly.

Cons:

  • Very costly for a system that will only have a few wells.
  • Locks you in to a vertical bore system type, data not good for horizontal or slinky systems.
In our opinion, conductivity tests for residential applications simply isn’t worth it. It is too costly ($6,000 and up) per well, and there is very little return on investment comparably. Vertical bore systems for residential applications are going to have very few wells in the first place, so super fine tuned calculations don’t make as much of a difference. As an insider tip, you can typically get comparable information for free from geothermal contractors if you simply ask. If you are within 30 – 50 miles of a commercial geothermal application, you can inquire their conductivity numbers, and assume yours are similar.  There is always a risk that your plot of land will be over a cave or have other voids in the subsurface, but in most cases if that exists, it will be unavoidable.

Commercial Geothermal Conductivity Tests

Pros:

  • Can verify geothermal as a possible HVAC system type early in design.
  • Helps engineer design a better, more balanced, and better performing well field.
  • All subsurface conditions and depths are documented.
  • Typically lowers contractor’s bid price because there is less risk.

Cons:

  • It is expensive ($6,000 and up per bore).
  • Often times you need a surveyor to stake test bore location.
  • Must be compatible with project location (accessible for drill rig, no extensive grade work to be done afterward)

In our opinion, a test bore and thermal conductivity test is an absolute MUST when it comes to designing a commercially sized geothermal system. The return on the initial investment is worth it in many different scenarios. If the site is acceptable, the design engineer will have adequate information to size the entire well field appropriately, which saves you money from over-design. Also, the well used in the conductivity test can easily be tied into a final well field circuit if protected and capped after the test. Finally, when planning a multimillion dollar commercial project, it is more important to know early on whether or not a project site has the subsurface conditions that support geothermal. If you happen to be on a project site that sits over a cave, or the subsurface conditions have voids, it is better to know this at the beginning of the design process so another HVAC system can be budgeted and designed.

Put simply, never design a commercial project without installing a test well and having a thermal conductivity test performed (do 2 or more if your site and budget allows). This makes design much more accurate to give you an affordable system with adequate capacity. However, for a residential project, you can do a test if you’d like, but we don’t see it being worth it.