Ground Loop Length Calculator

Determine total vertical borehole drilling footage, bore count and grid spacing, and feet-per-ton sizing according to IGSHPA and ASHRAE geothermal standards.

System Heat Pump Tonnage

Primary Design Load

Ground Geology / Soil Type

Grout Conductivity

Undisturbed Deep Earth (°F)

Max Single Borehole Depth

Total Vertical Borehole Footage

950 Feet

190 ft / ton linear borehole depth

Recommended Boreholes

4 Bores

@ 238 ft each

Borehole Grid Spacing

20 Ft O.C.

Min 15-20 ft thermal buffer

Peak Ground Heat Transfer: 72,000 BTU/hr

Total HDPE U-Tube Pipe Length: 1,900 ft (dual leg)

Long-Term Ground Thermal Stability: Balanced Dissipation

Thermally enhanced grout and 20 ft borehole spacing prevent progressive multi-year thermal buildup.

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Tested hardware and components for high reliability

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Geothermal Vertical Closed-Loop Design (IGSHPA & ASHRAE)

Vertical ground heat exchangers consist of continuous loops of high-density polyethylene (HDPE 4710) pipe inserted into drilled vertical boreholes and backfilled with thermal grout. In cooling mode, the heat pump absorbs heat from the building interior plus compressor electrical heat and rejects the total energy into the earth. In heating mode, heat is extracted from the ground and concentrated into the conditioned space.

Borehole Sizing Principles

Thermal Load Calculation:
Cooling rejection load: Q_cooling = Tonnage × 12,000 × [ (EER + 3.412) / EER ] ≈ 14,500 to 15,000 BTU/hr per ton.
Heating extraction load: Q_heating = Tonnage × 12,000 × [ (COP - 1) / COP ] ≈ 8,500 to 9,500 BTU/hr per ton.
Specific Borehole Heat Transfer:
Depending on ground thermal conductivity and grout type, vertical boreholes reject or absorb approximately 35 to 65 Watts per meter (36 to 68 BTU/hr per linear foot).
Rule of Thumb:
High-conductivity wet bedrock requires approximately 150 to 180 ft per ton. Dry, low-conductivity soils or standard bentonite grout require 220 to 280 ft per ton.

Frequently Asked Questions

How many feet of vertical borehole are needed per ton of geothermal heat pump?

On average, vertical closed-loop ground source heat pumps require 150 to 220 feet of vertical borehole per ton of capacity in moist rock, and 220 to 280 feet per ton in dry soil.

Why is borehole spacing important in geothermal ground fields?

Boreholes must be spaced at least 15 to 20 feet apart to prevent thermal interference between adjacent loops, which causes long-term ground temperature degradation over 10 to 20 years.

What is thermally enhanced grout in geothermal drilling?

Thermally enhanced grout is bentonite or cementitious slurry blended with specially graded silica sand or graphite to increase thermal conductivity from 0.40 to over 1.0–1.2 BTU/hr-ft-°F, reducing needed drilling depth by 15% to 25%.