How spacing controls geothermal borehole interference
Ground-source heat pumps count on the Earth’s thermal mass to stay efficient in both winter and summer. Each vertical borehole acts like a heat exchanger: fluid descends, trades heat with the surrounding soil or rock, and returns to the heat pump at a temperature that ideally sits within a narrow band. When boreholes are spaced too closely, their thermal plumes overlap. Heat that should disperse into the ground instead migrates to a neighboring borehole, raising summer loop temperatures and dropping winter temperatures. The result is compressor stress, higher pumping power, and in worst cases tripped safeties. This calculator estimates those interference penalties so designers, installers, and owners can vet field layouts before drilling hundreds of feet into the subsurface.
Most residential projects rely on empirical tables or software that embed complex g-functions describing transient heat conduction. We built a transparent approximation that highlights the key drivers without hiding them behind proprietary curves. The inputs align with data you already collect: peak heating and cooling loads, equivalent full-load hours, borehole depth and count, material conductivities, and the allowable entering fluid temperature window specified by the heat pump manufacturer. The output reveals how far the predicted temperatures drift from your limits and suggests a spacing adjustment that reins the system back in.
At its core, the model computes the heat per unit length carried by each borehole. That value feeds a thermal resistance network comprising pipe, grout, and ground segments. Pipe resistance reflects the U-bend configuration and circulation rate; grout conductivity shapes how quickly heat leaves the borehole wall; and the ground term accounts for both steady-state conduction and the logarithmic time dependence captured by g-functions. We approximate the time component with a logarithmic relationship between annual run hours and total hours in a year, which captures how sustained loads pull more heat from the ground than shoulder-season pulses.
The simplified resistance chain looks like this:
In this expression, Rpipe is the user-supplied pipe resistance, kg represents grout conductivity, ksoil captures bulk ground conductivity, s is the borehole spacing, r0 is an effective borehole radius we approximate internally, tyear is 8,760 hours, tload is the annual full-load hours, and F is an interference factor derived from spacing geometry. The logarithms echo the physics of cylindrical heat flow, where doubling the radius only adds a modest resistance. Though simplified, this relationship mirrors what detailed design tools deliver and captures the intuition that tighter spacing raises loop temperatures in summer and lowers them in winter.
Consider a 12-borehole array serving a 9-ton heat pump. Each borehole is 220 feet deep, spaced 18 feet apart. The peak heating extraction load is 110 kBTU/hr and the cooling rejection load is 120 kBTU/hr. Annual equivalent full-load hours tally 1,900 for heating and 1,200 for cooling. Ground conductivity is 1.2 Btu/hr-ft-°F, grout conductivity is 0.8, the pipe resistance is 0.12 °F-hr-ft/Btu, and the undisturbed ground temperature is 55°F. The heat pump manual requires entering fluid temperatures to stay between 30°F and 100°F.
Our calculator first divides the seasonal loads among the boreholes to find the per-borehole line loads: 46.6 Btu/hr-ft in heating and 45.5 Btu/hr-ft in cooling. It then computes the composite resistance, yielding 0.241 °F per (Btu/hr-ft) in heating mode and 0.229 in cooling mode. Multiplying those resistances by the loads and adding them to the ground temperature predicts a minimum entering fluid temperature of 43°F in winter and a maximum of 92°F in summer—comfortably inside the manufacturer’s recommendations. The interference index, a dimensionless ratio of temperature swing to allowable swing, lands at 0.62, signaling ample safety margin.
What if the site is constrained and spacing drops to 12 feet? Rerunning the calculation shows resistances climbing by roughly 30 percent, pushing the minimum temperature toward 35°F. That is still safe but sacrifices resilience against a harsher winter. The calculator flags this by elevating the interference index to 0.9 and suggesting a revised spacing of 15.2 feet to restore the original buffer. That recommendation comes from iteratively increasing spacing until the predicted temperatures fall back to the midpoint of the allowable window.
The table in the results area summarizes both heating and cooling entering fluid temperatures, along with pass/fail indicators. The CSV output includes every intermediate value—per-borehole loads, thermal resistances, interference index, and recommended spacing—so designers can paste the numbers into proposals or commissioning reports. For installers, the tool offers a quick check before mobilizing a drill rig, ensuring that the borehole plan aligns with the manufacturer’s warranty requirements.
Long-form narratives matter because geothermal projects involve multiple stakeholders. Homeowners seek ROI and quiet comfort; designers juggle regulatory setbacks and subsurface data; drillers want production rates that keep crews busy. The text accompanying the calculator walks through the physics, financial stakes, and coordination tips so that each party sees the project through the same lens. That clarity reduces change orders and fosters trust in a technology that still feels novel in many markets.
A few limitations deserve emphasis. The calculator assumes uniform soil and grout properties, yet layered geology can yield asymmetric conduction paths. Highly anisotropic rock or groundwater flow may transport heat faster than the cylindrical model suggests. Seasonal thermal imbalance—where cooling loads exceed heating for years on end—can ratchet summer temperatures upward beyond our logarithmic estimate. Designers should cross-check the results with software that accepts detailed borefield geometry and load profiles if the project is large or mission critical.
We also ignore pump power penalties and antifreeze selection, both of which influence operating cost. Lower entering fluid temperatures force higher compressor lift, raising energy use even if the system remains within the allowable range. Users should interpret the interference index as a signal to check those secondary effects rather than a guarantee of peak performance.
Despite those caveats, the calculator empowers teams to have informed conversations early in design. By adjusting spacing, adding boreholes, or tweaking run hours, you can see how the loop field responds. That experimentation is essential when working on tight urban lots or retrofits where driveway sections, septic systems, or tree protection zones limit drilling locations. It also enables grant writers and policymakers to understand how incentives for deeper boreholes or higher-conductivity grout translate into measurable thermal resilience.
When coupled with actual borehole thermal conductivity tests, the tool becomes even stronger. Replace the default conductivity with measured values, rerun the analysis, and refine the spacing recommendation before final drawings. Many projects skip this step due to time, yet the combination of a quick field test and this calculator can avert costly redesigns and provide documentation for utility rebate programs.
Ultimately, geothermal success hinges on respecting subsurface physics. Spacing is more than a surveying challenge; it is the lever that determines how comfortably your loop field navigates extreme seasons. With this calculator, you have a transparent method to quantify interference, communicate risk, and optimize layouts before the drill rig arrives. Use it as a collaborative tool with clients and contractors, iterating until the predicted temperatures land squarely within the limits that keep compressors happy and homeowners comfortable.
As the market accelerates due to electrification incentives, teams that can explain their design math in plain language will win bids. Share the CSV report, walk stakeholders through the MathML equation, and use the worked example to demystify the process. In doing so, you elevate geothermal from a niche curiosity to a mainstream, data-backed option for decarbonizing home comfort.
Remember that thermal plumes evolve slowly; decisions made today affect performance decades from now. The calculator encourages that long view by tying full-load hours to temperature drift. Lean on it to defend spacing decisions in dense developments or to justify the cost of high-conductivity grout. With a few minutes of input, you can turn a hand-drawn borefield sketch into a resilient design narrative ready for permitting, financing, and construction.