Determining the Appropriate Length of a Geothermal Borehole

Geothermal heat pumps owe their unparalleled efficiency to the fact that the earth maintains a more or less stable temperature profile regardless of the season or weather happening up at the surface. As you would imagine, the deeper the depth, the less impacted the earth’s temperature is by the weather conditions above. In a closed loop geothermal system, water flows through a ground loop circuit, traveling through the stable underground environment, absorbing heat in the winter, and dispersing heat in the summer as it goes. As the ground loop adds or subtracts more and more heat to or from the ground throughout a given season, the ground loop heats or cools the ground around it. 

Because of this, the coldest time period for the ground loop and the ground around it is early spring, because the system has been pulling heat out of the ground all winter. Similarly, the warmest time period for the ground loop and the ground around it is late fall, because heat has been transferred into the ground all summer. The equation that is used to find the appropriate length of a geothermal ground loop (and therefore the required length of borehole to drill to contain it) solves for the length of ground loop required such that the water traveling through it stays within an acceptable temperature band. In other words, the equation solves for the length of borehole required so the water within it doesn’t get too hot or too cold, in order that the heat pump within the home maintains its rated performance. 

The acceptable temperature range, the definition of what exactly is ‘too hot’ and ‘too cold’, is specified by the geothermal heat pump manufacturer, because the ground loop itself doesn’t “care” what the water temperature is – it’s the heat pump that “cares.” For many geothermal heat pumps, the manufacturers’ target range is 30F to 90F (note that even with this target range, the system will still run well a few degrees colder or warmer – these temperatures are targets). The equation used to find the appropriate length of a geothermal borehole solves for the length of borehole required so that the water running through the ground loop within it is unlikely to get much colder than 30F or warmer than 90F. 

There are many factors that influence how much ground loop is needed to stay within this temperature range. Those factors include:

• The amount of heating and cooling the home will need. This is a function of the weather in the area (in colder climates, more heating will be required, for example), how large the home is, how well insulated it is, what temperature the homeowner plans to heat or cool to (the “set points”), what materials the home is made out of, etc. This is formally referred to as the “heating load” and the “cooling load” of the home.
• How readily the ground around the borehole gives up or accepts heat. Some materials are very good at transmitting heat. Think of a steel frying pan, for example. Some materials do not transmit heat as readily. Think of the rubber handle on that frying pan, for example. While not as extreme as steel vs rubber, the ground also has some materials within it, like bedrock, that are highly conductive, and other materials, like clay, that are less so. This is formally referred to as the “thermal conductivity” of the ground. The thermal conductivity of the grout surrounding the ground loop, and the thermal conductivity of the ground loop itself are also taken into consideration for the same reason.
• The size and geometry of the ground loop within the borehole. A given length of ground loop with a larger diameter will transmit more heat than the same length of a smaller diameter ground loop because a larger diameter ground loop has more surface area that is in physical contact with the ground surrounding it.
• The relative spacing of ground loops if there is more than one. If you have two ground loops and they are close to each other, they might impact each other’s heat exchange, because they are both cooling or heating the same area of ground. Therefore, the closer multiple ground loops are placed to one and other, the more length is needed, to account for any interference.

With those factors explained, this is what the equation looks like:

Where:

L_H,T is the total borehole design length for heating. This is what we are solving for (how long the borehole should be so that it provides enough heat to the house). There is a similar equation to determine how long the borehole needs to be to provide enough cooling, and when Dandelion designs geothermal ground loops, we solve both equations. That said, in the northeast US, if a borehole is long enough for heating it will be long enough for cooling (because so much more heating is required), so it is sufficient for the purposes of this explanation to focus on the equation for design length for heating, as this equation will typically govern the ground loop length.

HC_D is the heat pump heating capacity at design heating conditions in Btu/hr. This is a property of the heat pump and defined by the manufacturer.

COP_D is the coefficient of performance at design heating conditions. This is also a property of the heat pump and defined by the manufacturer.

R_B is the borehole thermal resistance, in hr ft F/Btu. This is calculated using the dimensions and the thermal resistance of the material the ground loop itself is made out of, as well as the thermal resistance of the grout used.

R_G is the steady-state thermal resistance of the ground surrounding the borehole. Dandelion calculates this by using data on ground thermal resistance from thousands of installations throughout the U.S. as well as publicly available geological data.

F_H is the run fraction in heating mode during the heating design month (January). This metric refers to the proportion of time the heat pump operates in heating mode during the coldest month of the year.

T_G is the average ground temperature along the borehole length. Dandelion uses publicly available geological data to determine this value (temperature is relatively consistent throughout the entire depth of the borehole).

EWT_min is the minimum entering water temperature at heating design conditions, in degrees F. We use 30F for EWT_min.

LWT_min is the minimum leaving water temperature at heating design conditions, in degrees F. This value is calculated using the performance tables provided by the heat pump manufacturer.

This equation was not developed by Dandelion; it is the gold standard of the geothermal industry and endorsed as the loop design best practice by IGSHPA, the International Ground Source Heat Pump Association. It has been used by engineers to design geothermal loops across the United States for decades.

Because weather and geology vary tremendously across the Northeast, some areas require substantially longer boreholes than others. For example, Westchester County, NY tends to have lots of bedrock, which is highly conductive, whereas Southern Vermont has many areas with less conductive geology. Because Southern Vermont has less conductive geology and colder weather than Westchester County, we typically see longer ground loop requirements for a given heating load in Southern Vermont than we do in Westchester County. 

Dandelion uses industry-accepted best practices for loop design instead of a “rule of thumb” because we are committed to delivering long lasting, high quality geothermal systems to the homeowners we serve. If a rule of thumb is used, like a common one that calls for “150 feet of borehole per ton of capacity”, there is no adjusting for factors like: what is the diameter of the piping used in the ground loop? Is it 1.25” or 1.5”? Is a single loop or a double loop being used? How conductive is the grout? Is that 150 feet based on heating tons or cooling tons? Is the geology in the area primarily clay or bedrock? How efficient is the heat pump being installed? etc. All of these factors impact how much ground loop is required, and they are all ignored by a rule of thumb method. In the end, the homeowner pays for this oversimplified approach, either by paying to install longer (and therefore more expensive) ground loops than their system requires, or by not getting enough ground loop installed. 

Related to the point above, one question we hear from customers from time to time is: “if we install a longer ground loop than is required for the 30F to 90F temperature range, will my heat pump be more efficient?” The answer to this is technically “yes”; the longer you make your ground loop, the more efficient the heat pump will be, but there are severely diminishing returns. To illustrate this with numbers, let’s say a homeowner decides to install 50% more ground loop than what would typically be prescribed to achieve a 30F minimum water temperature. With 50% more loop, the minimum water temperature would be 36 degrees Fahrenheit versus the 30 degrees Fahrenheit typically targeted. The Coefficient of Performance (COP) of one of the heat pump models Dandelion typically installs when it’s operating at 30F is 3.92, whereas the COP of this unit operating at 36 degrees F is 4.17. Assuming electricity costs 25 cents per kWh, a homeowner in the northeast with a 5T heat pump and a 50% longer-than-standard ground loop would pay, on average, about $15 less per month on their electricity bill. However, the cost premium for a 50% longer ground loop is around $10,000. As you can see, the payback on this longer loop does not make financial sense, and it can be the difference between a geothermal system that saves a homeowner money and one that does not. So it is important to install the correct amount of loop, and therefore not to overcharge the homeowner.

Dandelion is invested in installing the correct amount of ground loop for our customers. Each system Dandelion installs comes with a 3-year Workmanship Warranty, so we contractually commit to our customers that if there is an error in the design or installation of their geothermal system, we will fix it at no cost to the customer. In order to achieve our mission of delivering high-quality and cost-effective geothermal systems, which will be required to bring this technology to scale, Dandelion is committed to adhering to professional, engineering and science-based design standards and to thus guaranteeing homeowners the highest degree of quality.