Factors that Affect Geothermal Efficiency
3 pros!
The ground beneath your home dictates the type of geothermal system that will work best for you.
This house was custom built into the side of a hill with a geothermal system. (Photo courtesy of Angie’s List member Suzan E. of West Branch, Iowa)
In part, the performance of your geothermal heat pump (GHP) depends on the condition of the soil where it will be installed. A key component of the vetting process for installing a GHP should include an assessment of your land in general, including the presence of rocks and the thermal conductivity of your soil to choose the best system for your home.
Inside the Soil
Rocky land can greatly decrease the viability of a GHP for your home. Your geothermal contractor will take soil samples from various areas of your yard to determine whether the presence of rocks will interfere with drilling for the looping pipes.
In some states like Texas, rocky land prevents the widespread adoption of GHPs in most areas. If your home is located near a mountain range, for instance, you may be prevented from installing a GHP in your home. However, in most cases, a suitable configuration can accommodate a small amount of rock.
Thermal Transfer
The single, most important factor installers look for in terms of viability involves the thermal transfer capacity of your soil. If the soil is conducive for heat transfer, you'll get a higher return on investment because a GHP depends on the system's ability to transfer heat efficiently during both the heating and cooling process.
• Moisture. So, the soil's ability to store heat energy can make or break the system. Several factors influence heat transfer. Namely, the moisture content of the soil will either increase or decrease transfer. In general, drier soil has a lower heat transfer capacity, whereas moist soil has a higher rate of heat transfer.
• Temperature. Temperature variations under the ground also affect soil conditions. If the temperature variation is great (which is common in shallow, horizontal looping systems), heat transfer decreases, whereas more consistent temperatures commonly found in deep, vertical looping systems offer better heat transfer viability.
• Soil types. The type of soil also contributes to thermal conductivity rates. For instance, saturated sand has the highest rate of thermal transfer per British thermal unit (BTU), 1.44, whereas saturated silt and clay rate at 0.96. Sand, loam and clay range between 0.44 and 0.64.
It All Comes Down to Cost
In general, homeowners with the most conducive soil conditions for a GHP benefit from a lower cost system. An academic study that compared soil types and GHP configuration reveals the difference:
• When the thermal transfer of soil is high, at 1.50 BTU/hr-ft-oF, the installer can design a system that uses fewer looping pipes (10), buried at a more shallow depth (2, 1,200 feet).
• Compared to a much lower thermal transfer of 0.55 BTU/hr-ft-oF, more looping pipes are required (16), with more digging required, which drives up installation costs. In this case, the pipes must be installed more than 1,000 feet deeper into the ground for a vertical configuration.
Testing and evaluating the conditions of the soil on your lot is the first and most important step in determining the viability of and payback for installing a GHP for your home. As a result, it's of the utmost importance that you work with a reputable HVAC contractor.
Have you considered installing a geothermal system? Tell us about your experiences in the comments below.
Editor's note: This is an updated version of an article that was originally posted on Oct. 17, 2013.