Size the vertical borehole heat exchanger (BHE) field of a ground-source heat pump using the ASHRAE/IGSHPA simplified method. Enter the building heating and cooling loads, ground thermal conductivity, undisturbed ground temperature, annual run hours and per-borehole depth, and the required total bore length, number of boreholes, land footprint, COP and annual CO₂ savings update in real time.
Parameters
Heating load Q_h
kW
Peak heating output required from the building
Cooling load Q_c
kW
Peak cooling output (heat to reject)
Heating COP
Rated COP at design EWT/LWT
Cooling COP (≈ EER/3.412)
Cooling COP at rated condition
Ground conductivity k_g
W/(m·K)
Dry sand 1.5 / wet clay 2.0 / granite 3.0
Undisturbed ground temp T_g
°C
Annual mean temperature below 10 m
Heating run hours
h/y
Equivalent full-load hours per year
Per-borehole depth
m
Typical 100-200 m, residential ~100 m
U-tube / double-U factor R_b
m·K/W
Single-U 0.11-0.13, double-U 0.08-0.10
Results
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Design ground load (kW)
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Required bore length (m)
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Number of boreholes
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Field footprint (m²)
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Electrical input, heating (kW)
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Annual CO₂ savings (kg)
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Borehole field cross-section — ground-loop animation
Building at the surface, vertical U-tube boreholes below. In winter the loop extracts heat (blue→red); in summer it rejects heat (red→blue). Deep ground temperature is almost constant year-round.
L: total borehole length [m], Q: ground-side load [W], R_b: borehole resistance [m·K/W], R_g: ground resistance [m·K/W], ΔT: design fluid-to-ground temperature difference (typically 8 K). R_g is a function of ground conductivity k and load factor. SPF is ≈ 85 % of the steady-state COP.
Number of boreholes N_bh, field footprint A_field, spacing s (typically 6 m). Residential 4-6 m, commercial 6-8 m to avoid long-term thermal imbalance.
Ground-Source Heat Pump (GSHP) — Borehole Design and COP
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"Ground-source heat pump" — does that mean you dig down to hot water, like a hot spring?
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Different idea. A GSHP uses the "mild" temperature of the shallow ground. Below 10 m depth, the soil sits at roughly the local annual mean air temperature all year — about 13-16°C in Japan, 8-13°C in central Europe and the US Midwest. So in winter the ground is warmer than the outdoor air, and in summer it is cooler. The heat pump uses this stable temperature as both its winter heat source and its summer heat sink. Compared to an air-source heat pump (ASHP), the big win is that performance does not collapse in a hard winter or a hot summer.
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So what do you actually put in the ground? A tank?
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Much simpler: a vertical hole, 100-200 m deep, with a PE (polyethylene) U-tube inside. That is the "borehole heat exchanger" (BHE). The annulus between the tube and the borehole wall is filled with a thermally enhanced bentonite grout. A propylene-glycol/water antifreeze circulates inside the U-tube to swap heat with the ground. One borehole handles roughly 5-10 kW; a single house needs 1-2 boreholes, a commercial building a "borehole field" of 30-100. Move the load sliders on the left and you will see the required count and land footprint update immediately.
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How do you decide the total bore length — "30 kW of heating means X metres"?
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The ASHRAE/IGSHPA simplified method gives the rule of thumb. For a ground-side heat load Q (kW), the required length scales with the sum of borehole resistance R_b and ground resistance R_g, divided by the allowed temperature difference ΔT: L = Q·(R_b+R_g)/ΔT, with ΔT around 8 K. R_b is 0.08-0.13 from the tube and grout; R_g comes from ground conductivity k and the annual load factor. With the defaults (Q_h=30 kW, Q_c=25 kW, k=2.4, 2000 h/y, 150 m/hole) you get 430 m total, 3 boreholes and a 108 m² field — small enough for a typical residential lot (1-2 parking spaces).
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How do you measure ground conductivity k on a real site? I can only tell "sandy" or "rocky" by eye.
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For a real project you drill one test borehole and run a 48-hour Thermal Response Test (TRT). You inject constant electrical heat into the loop and read the inlet-outlet temperature; the log-time slope gives k and the intercept gives R_b. The standard procedure is in ANSI/CSA C448 and IEA Annex 21. For a house, people skip the TRT and use a value from geological maps (1.8-2.5 W/(m·K) covers most cases). One catch: groundwater flow can apparently raise k by 2-3×, while dry volcanic rock can fall below 1. Use a conservative (lower) k for design.
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A COP of 4 is shown. Does that translate directly into the electricity bill?
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That is where COP and the Seasonal Performance Factor (SPF) diverge. COP is the instantaneous "heat output ÷ compressor electricity" at a rated condition. SPF is the season-averaged ratio that includes circulator pump power, part-load losses and temperature drift. Real-world SPF is typically 75-90 % of COP. This tool uses SPF ≈ 0.85·COP to estimate annual electricity and CO₂. Even with that derating, the default case saves about 5,250 kg-CO₂/year against a gas-heating reference — which is why GSHP is a workhorse technology for ZEB (Net Zero Energy Building) targets.
Frequently Asked Questions
A GSHP exchanges heat with the ground, which below ~10 m depth stays at almost the local annual mean temperature (about 10-16°C in Japan, 8-16°C in North America). Evaporator and condenser temperatures therefore stay stable in both peak winter and peak summer, so seasonal COP typically reaches 4-5. An ASHP follows outdoor air temperature, so its COP swings from about 2 to 3 and can fall below 1.5 at −10°C, and it loses capacity to defrost cycles. GSHP capital cost is 2-3× ASHP because of drilling, so this tool estimates required borehole length and annual electricity / CO₂ savings to support the investment decision.
The ASHRAE/IGSHPA simplified method writes L = q·(R_b+R_g)/ΔT, where q is the ground-side heat exchange [W], R_b is the borehole resistance [m·K/W], R_g is the ground resistance and ΔT is the allowable fluid-to-ground temperature difference. R_b is 0.08-0.13 for one or two U-tubes with thermally enhanced grout, and R_g depends on ground conductivity k and the annual load factor. Dividing the total length by the per-borehole depth (typically 100-200 m) gives the count. With the defaults here (ΔT=8 K), you get about 430 m total and 3 boreholes.
For mid- and large-scale projects, drill one test borehole and run a 48-hour Thermal Response Test (TRT) per ANSI/CSA C448 or IEA Annex 21. This measures the effective conductivity k_eff and R_b directly. For residential systems, take a representative value from geological maps: dry sand 1.5, wet clay 2.0, granite 3.0 W/(m·K). Undisturbed ground temperature is roughly the annual mean air temperature + 1-2°C, so Hokkaido ~10°C, Tokyo ~16-17°C, central Europe 9-12°C, US Midwest 10-13°C. Use this tool to see how each parameter shifts the required borehole length.
COP is the instantaneous efficiency at a rated condition (e.g. EWT 0°C / LWT 35°C). SPF is the season-integrated ratio of delivered heat to total electricity, including circulator pump and controls. Real SPF is usually 75-90% of the rated COP because of part-load operation, ancillary power and operating-condition drift. This tool uses SPF ≈ 0.85·COP for a quick CO₂ estimate; detailed evaluation should follow VDI 4650 or ISO 17742.
Real-World Applications
Residential GSHP in cold regions (Hokkaido, Tohoku, Scandinavia, US Midwest): Detached houses with 8-15 kW heating loads are typically served by 1-2 boreholes of 80-120 m. In Hokkaido and Sweden, annual running-cost savings of 30-50% vs oil or electric resistance heating are common, with similar reductions in CO₂. Because COP is preserved in cold weather, GSHP wins decisively over ASHP in these climates. Many municipalities (Sapporo, Obihiro, several US states) offer rebates, bringing simple payback into the 8-15 year range.
Commercial buildings and district heating/cooling (DHC): In redevelopment projects in Tokyo, Yokohama, Stockholm and Toronto, office and government buildings of 5,000-50,000 m² gross floor area host 30-200-borehole fields under their footprint or parking lots. Designers verify that the annual heating-to-cooling balance is between 0.6-1.4 to avoid long-term ground-temperature drift, with spacing of 6-8 m. The high COP directly supports CASBEE, LEED and BREEAM ratings.
Greenhouse agriculture and aquaculture: Year-round production of strawberries, tomatoes and cucumbers uses GSHP for root-zone and air heating, cutting heavy-oil use by 50-70% in Hokkaido, Niigata and Nagano. In aquaculture, trout, sturgeon and eel farms use ground heat for summer cooling and winter warming with a single system, which is much harder with air-based equipment.
Net-Zero Energy Buildings (ZEB / nZEB): PV + high-performance envelope + GSHP is the standard "stack" for reaching ZEB / nZEB targets. An SPF above 4 is the threshold that pushes a project from "ZEB-ready" toward "nZEB" or full "ZEB". The bore length and CO₂ savings produced by this tool line up with the basic indicators required in ZEB documentation.
Common Misconceptions and Pitfalls
The first big misconception is "deeper boreholes are always better". Deep ground is indeed more thermally stable, but beyond 200 m the per-metre drilling cost rises sharply (it is the practical limit of residential rigs), and at greater depths the borehole may cross a productive aquifer with strict groundwater regulations. The rule of thumb is "keep the total length but increase the number of holes". For 600 m of bore, 4 holes × 150 m is preferred over 2 × 300 m. Move the per-borehole depth slider here to see the trade-off between hole count and field footprint, and pick what fits the site.
The second pitfall is thermal imbalance over the years. In cooling-dominated climates (Singapore, the southern US) the ground absorbs net heat every year, so over 10-20 years the mean ground temperature rises 2-5°C and cooling COP slips. In heating-dominated cold climates the opposite happens. At design time, check that "annual heat extraction / annual heat rejection" sits roughly between 0.6 and 1.4, and add a balancing source (cooling tower, solar-thermal recharge, hybrid ASHP) if it does not. This single-year tool cannot see that drift — back it up with g-function tools such as EED, GLHEPRO or pygfunction for long-term simulation.
The third pitfall: "high COP automatically means a low electricity bill". GSHP electricity is the heat pump plus the ground-loop pump, the building-side fan-coils and the controls. Ancillaries can be 15-25% of total consumption, and in a poorly designed large building the ground-loop pump alone can push SPF below 3. Pick generous pipe diameters (flow velocity 0.7-1.2 m/s), use a variable-speed inverter pump and a variable-speed compressor, and the design COP of 4 will actually appear on the meter as an SPF of 3.5-4.0. Turning that design number into a real operating number is where the engineering happens.
How to Use
Enter heating and cooling capacity requirements in kW (e.g., 50 kW heating for a 400 m² office building)
Input coefficient of performance (COP) values—typically 4.5–5.2 for heating, 3.8–4.5 for cooling in temperate climates
The simulator calculates design ground load using ASHRAE methodology, then determines total bore length, borehole count (standard 150 mm diameter), field footprint spacing, electrical input power, and annual CO₂ offset versus conventional systems
Worked Example
A 6,000 m² hospital requires 120 kW heating (winter peak) and 85 kW cooling (summer peak). COP heating = 4.8, COP cooling = 4.2. Ground thermal conductivity = 2.1 W/m·K, 40-year lifespan. Simulator outputs: design ground load ≈ 28 kW, required bore length ≈ 3,200 m, approximately 22 boreholes at 145 m depth each, field footprint ≈ 880 m² (6 m spacing), electrical input heating ≈ 25 kW, annual CO₂ savings ≈ 185 tonnes versus natural-gas baseline.