How to size a geothermal ground loop
The inputs that set vertical bore length for a closed-loop ground source heat pump, why soil thermal conductivity has to be measured, and what a rule of thumb per ton leaves out.
What this means
Vertical bore length is calculated from the building's peak block load, the entering water temperature the equipment is selected at, the undisturbed ground temperature, and the soil's thermal conductivity and diffusivity. A rule of thumb expressed as feet per ton skips the soil measurement and assumes a conductivity that may not match the site, which is why two buildings with identical loads can need materially different bore lengths.
Equipment and model context
- Closed-loop vertical ground source heat pump systems
- Worked figures illustrate the method and are not a rating for any product
This explains what the sizing calculation depends on and why each input matters. It does not produce a bore length for a specific site. That requires a completed building load, the selected equipment's entering water temperature range, and site-specific ground data, normally from a thermal response test on a test borehole.
What this covers
- Why feet-per-ton rules of thumb produce inconsistent results between sites.
- What a thermal conductivity test actually measures and why it takes weeks.
- How entering water temperature choice trades bore length against operating efficiency.
- Why annual load imbalance changes ground temperature over years, not just within one season.
What changes the result
- Soil thermal conductivity varies by rock and soil type and by groundwater movement, and assuming a table value in place of a site test can miss the actual figure by a wide margin.
- Undersized loop length produces entering water temperature that drifts outside the equipment's rated range during peak load, cutting capacity when it is needed most.
- A heating-dominated or cooling-dominated building load imbalance shifts the ground temperature around the loop field year over year if the system runs unbalanced without supplemental rejection or extraction.
- Borehole spacing set too close lets adjacent bores interfere thermally, degrading performance that a single-borehole calculation would not predict.
What the length calculation actually solves for
Bore length sizing solves for the loop length that keeps entering water temperature within the equipment's rated operating range at the peak heating and peak cooling hour of the year. Too short a loop and the fluid returning to the heat pump runs colder in winter and warmer in summer than the equipment was selected for, cutting capacity exactly when the building needs it most.
The calculation balances heat extracted or rejected against the ground's ability to conduct that heat away from the pipe wall and into the surrounding formation. That ability is set by thermal conductivity, thermal diffusivity, and the undisturbed ground temperature at depth, which is why those three site properties, not the building load alone, decide how many feet of borehole the design needs.
Why the thermal conductivity has to be measured, not assumed
Soil and rock thermal conductivity ranges widely between formation types, and it can vary meaningfully within a few hundred feet on the same site where groundwater movement or a change in strata is present. A table value picked for a general soil description is a starting estimate, not a design input, because the actual value at the specific bore location is what the calculation needs.
A thermal response test installs a loop in a test borehole to full design depth, circulates a controlled heat load through it, and logs the fluid temperature response over an extended period, on the order of weeks from mobilization to analysis. The logged data is analysed against the heat conduction equation to extract the site's actual thermal conductivity and diffusivity, values the length calculation then uses directly rather than by assumption.
The entering water temperature trade
Equipment selection sets a target entering water temperature range, and the loop is sized to hold fluid temperature inside that range at peak load. A tighter, warmer minimum entering water temperature in heating mode needs more bore length to reach, because more pipe surface area is needed to hold the fluid closer to the undisturbed ground temperature under load.
A design can trade length for efficiency in either direction. Accepting a colder minimum entering water temperature, if the selected equipment's performance data supports it, shortens the loop and lowers drilling cost, at the expense of a lower coefficient of performance during the coldest hours. That is a defensible choice when drilling cost dominates the site economics, and a poor one when it is made without checking the equipment's rated performance at the resulting temperature.
Why the ground itself can drift over years
A closed loop exchanges heat with a finite volume of ground, and a building whose annual heating extraction and cooling rejection are not roughly balanced pulls that volume's temperature in one direction year after year. A heating-dominated building steadily cools its loop field over several seasons; a cooling-dominated one steadily warms it. Either drift erodes the entering water temperature margin the original sizing assumed.
Design responses include increasing bore length beyond the single-season calculation to build in thermal margin, adding a supplemental heat rejecter or extractor to correct the imbalance directly, or accepting a bounded amount of drift where the annual load imbalance is modest. Which one applies depends on the building's actual heating and cooling load ratio, which is why annual load imbalance is a distinct design question from peak-day bore sizing.
For one building's peak block load, this shows how the calculated total bore length changes as the site's measured thermal conductivity varies. Lower conductivity soil conducts heat away from the loop more slowly, so it needs more bore length to reach the same entering water temperature.
- Moving from a conductivity of 1.2 to 0.8 Btu per hour, foot, F, a difference well within the range between two ordinary soil types, raises the required bore length by roughly a third for the same building load.
- A table estimate for a plausible soil type sits in the middle of this range, which is exactly why it can be wrong in either direction for the actual site.
- The curve flattens at higher conductivity, so a design already sized against dense rock or saturated soil is less sensitive to a small measurement error than one sized against loose, dry, low-conductivity ground.
- A thermal response test replaces the horizontal position on this curve with a measured value, removing the assumption entirely.
| Configuration | Land area needed | Where it suits |
|---|---|---|
| Vertical bores | Small footprint, drilled to depth | Limited lot area, or where horizontal trenching would disturb the site |
| Horizontal trenches | Large footprint, shallow trenches | Sites with available open land and lower drilling cost tolerance |
| Slinky coils | Moderate footprint, coiled pipe in shallow trenches | Sites wanting to shorten trench length at the cost of more pipe |
| Pond or lake loop | Minimal land footprint | Sites with adequate water body depth and volume nearby |
Questions people ask about this
Is a feet-per-ton rule ever good enough?
It can produce a rough budget estimate for early project feasibility, but it should not be the figure a drilling contract is written against. The rule embeds an assumed thermal conductivity that may not match the site, and the resulting error compounds with the building's own load calculation error rather than cancelling it.
How long does a thermal response test take?
The test itself, from mobilizing equipment to completing the logged heat injection period, runs on the order of three weeks including the borehole drilling, grouting cure time, and the monitored test period, though the exact duration depends on the driller and the test protocol used.
Does annual load imbalance matter for a well-balanced house?
Less than for a heating- or cooling-dominated one, but few buildings are perfectly balanced across a full year. The calculation still deserves a check, because even a moderate imbalance compounds over the loop field's service life, and correcting it after installation is far more disruptive than sizing for it up front.
Can two adjacent boreholes be sized independently?
Not accurately. Bores spaced too closely interfere thermally with each other over a heating or cooling season, each one raising or lowering the ground temperature the other draws from. Field layout and spacing are part of the sizing calculation, not a separate site-planning step done afterward.
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This page is awaiting source verification against the documentation in its evidence record: United States Department of Energy and ASHRAE technical literature. Its documentation class and intended scope are shown here while that check is pending.
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