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Heating & gas calculator

Ground source heat pump calculator: borehole depth, loop area, SCOP & cost

Sizes a ground-source heat pump from your heat demand: kW output, borehole metres or horizontal collector area for your soil or rock (VDI 4640 extraction rates), seasonal COP, running cost and installed cost versus an air-source unit and gas.

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Results

A ground-source heat pump of 8 kW needs about 140 m of borehole (2 × 70 m) in “Average soil / rock (λ ≈ 2 W/mK)” at ~47 W/m. SCOP ≈ 4.23, and it costs 141 €/yr less to run than an air-source unit. The length is set by the annual-energy limit (~100 kWh per metre per year) rather than by peak power, to avoid cooling the ground over the years.
Total borehole length
140m
2 boreholes × 70 m · 47 W/m
Heat pump size
8kW
design heat loss 7 kW at -9 °C
Seasonal COP (SCOP)
4.23
air-source equivalent 3.79
Running cost
€1,216
4,342 kWh of electricity per year
Saving vs air source
€141
Saving vs gas boiler
€825
gas 2,041 €/yr at 90% efficiency
Estimated installed cost
€21,818
ground loop 9,818 + unit 12,000
Heat extracted from ground
14,026kWh/yr
100 kWh per m of borehole
Full-load hours
2,296h/yr
Annual heat delivered
18,368kWh
CO₂ per year
1.52t
vs 3.73 t with gas
Ground temperature
11.4°C
Berlin, Germany
Monthly electricity: ground vs air source
Borehole length by ground type

Ground types (VDI 4640 indicative extraction rates)

GroundBorehole W/m (1,800 h)W/m (2,400 h)Horizontal W/m² (1,800 h)Borehole needed (m)Loop area (m²)
Dry gravel / sand252012293642
Moist clay / loam453528166265
Water-saturated gravel / sand706038140234
Limestone (massive)625230140244
Sandstone726030140244
Granite / gneiss756330140244
Basalt524530140244
Average soil / rock (λ ≈ 2 W/mK)554525140293
  • Extraction rates are VDI 4640 indicative values for small systems (< 30 kW) without groundwater flow. For the final design, use a thermal response test and an EED/GLHEPro-type simulation, or the MCS MIS 3005 tables.
  • Borehole length is the larger of the peak-power criterion (W/m) and an annual energy limit of 100 kWh/m (60 kWh/m² for horizontal loops). Boreholes are usually spaced at least 6 m apart. Horizontal loops need roughly 1.5–2× the heated floor area in free land.
  • Climate: Berlin, Germany. In the heating season the brine runs ~4–8 K below the undisturbed ground temperature.

How this ground source heat pump calculator works

A ground source heat pump (GSHP) pulls low-temperature heat out of the earth through a closed loop of brine-filled plastic pipe, either vertical boreholes or horizontal collectors buried in the garden, and raises it to heating-system temperature. Sizing one comes down to three linked questions. How big must the heat pump be? How much heat will it take from the ground? And how long must the loop be so the ground doesn’t cool down over the years? The calculator works through all three:

  1. Heat demand. Floor area and insulation level (or a known annual kWh figure) plus hot water, simulated month by month for your climate with temperature bins. It is the same engine as our air-source heat pump calculator.
  2. Brine temperature and COP. The undisturbed ground temperature is estimated as the annual mean air temperature + 1.5 K, or you can enter it. In the heating season the brine runs 4–8 K below that, more in the coldest months. COP follows from the brine and flow temperatures.
  3. Heat-pump size = design heat loss at the local design outdoor temperature + 10% for hot-water recovery.
  4. Ground extraction. Extraction power = heat output × (1 − 1/SCOP). Divide it by the VDI 4640 specific extraction rate for your ground type (interpolated for your full-load hours) and you get the borehole length or collector area. That length is then checked against an annual-energy limit of 100 kWh per metre per year (60 kWh/m² for horizontal loops), and the larger value wins.
  5. Costs for drilling or trenching and the unit, the running cost, and a comparison with an air-source heat pump and a gas boiler.

The formula

P_extract = P_hp × (1 − 1/SCOP) L_borehole = max( P_extract / q_W/m , E_extract / 100 kWh/m·yr ) A_horizontal = max( P_extract / q_W/m² , E_extract / 60 kWh/m²·yr ) E_extract = heat delivered − electricity used COP = η × T_cond / (T_cond − T_brine + 3 K) T_brine = T_ground − 4 K − 4 K × (monthly load / peak load)

Worked example: 180 m² house in Berlin

The defaults describe a 180 m² house with average insulation in Berlin, underfloor heating (35 °C flow) and hot water for four people. Annual heat demand comes to about 18,370 kWh and the design heat loss is 7.0 kW at −9 °C, so the calculator picks an 8 kW ground source heat pump.

With an estimated ground temperature of 11.4 °C, the seasonal COP is 4.23; an air-source unit in the same house would manage about 3.79. The heat pump uses about 4,340 kWh of electricity, ≈ 1,216 € a year at 0.28 €/kWh. That saves around 141 € a year against air source and about 825 € against a 90% gas boiler (2,041 € at 0.10 €/kWh).

Each year the system draws about 14,030 kWh from the ground. In average ground (λ ≈ 2 W/mK) the peak-power criterion alone would give about 130 m at roughly 47 W/m, but the annual-energy limit of 100 kWh/m pushes it to 140 m, for example two boreholes of 70 m. At 70 €/m for drilling plus 12,000 € for the unit, the estimated installed cost is about 21,800 €. A horizontal collector would need roughly 290 m² of land instead. CO₂ comes to about 1.5 t a year on a 350 g/kWh grid, against 3.7 t with gas.

How to read the results

  • Borehole length / collector area drives the cost. The verdict tells you whether the energy limit or the power criterion sets it.
  • Full-load hours above ~2,400 h mean long running seasons (a cold climate or heavy hot-water use), so lower W/m rates apply.
  • Saving vs air source is what the extra cost of the ground loop buys you. Divide the extra installed cost by it for a rough payback.
  • Ground types table compares lengths for each geology. A site investigation or local drilling records will tell you which one applies.

Typical values (VDI 4640, small systems < 30 kW)

Ground Borehole W/m at 1,800 h Borehole W/m at 2,400 h Horizontal W/m² at 1,800 h
Dry gravel / sand < 25 < 20 10–15
Moist clay / loam 35–50 30–40 25–30
Water-saturated gravel / sand 65–80 55–65 35–40
Limestone (massive) 55–70 45–60 —
Sandstone 65–80 55–65 —
Granite, gneiss 65–85 55–70 —
Basalt 40–65 35–55 —

Tips

  • Get a thermal response test for systems above ~30 kW or with several boreholes. It measures the real ground conductivity and can shorten (or lengthen) the field by 20–30%.
  • Space boreholes at least 6 m apart. Closer than that, they steal heat from each other.
  • Keep the flow temperature low. Underfloor heating at 35 °C is where ground source shines; with 55 °C radiators its lead over air source shrinks.
  • Consider passive cooling. In summer, running brine through the floor gives almost free cooling and partly recharges the ground.
  • Cut demand first with the insulation savings calculator. A smaller heat pump means a shorter, cheaper borehole.

Limitations

Extraction rates are indicative. They assume no significant groundwater flow (which can raise them a lot) and standard borehole construction with single-U or double-U probes. Neighbouring borehole fields, very long running hours, cooling loads and large systems need a proper simulation (EED, GLHEPro or the MCS MIS 3005 tables). Drilling usually needs a permit, especially beyond 100 m or in water-protection zones. Costs vary widely by country and site access, so enter local quotes in the advanced settings.

Method & assumptions

Heat demand and hot water are simulated month by month (temperature bins) with the brine entering the heat pump at Tground − 4 K − 4 K × (monthly load ÷ peak load). The unit is sized to the design heat loss + 10%. Ground extraction power = heat-pump output × (1 − 1/SCOP). Borehole length = extraction power ÷ specific extraction rate (W/m) for the ground type, interpolated between VDI 4640's 1,800 h and 2,400 h full-load values. Horizontal collector area uses W/m² in the same way.

Frequently asked questions

How deep does a borehole for a ground source heat pump need to be?
A common rule of thumb is 15–25 m of borehole per kW of heat-pump output in average ground. For a typical 6–10 kW house that means 100–200 m, usually split into one or two boreholes of 60–150 m. Rock with high thermal conductivity (granite, sandstone) needs less; dry sand or gravel needs two to three times more.
How much land do I need for a horizontal ground loop?
Typically 1.5–2.5 times the heated floor area, buried 1.2–1.5 m deep. A house with an 8 kW heat pump usually needs 250–400 m² of free, unshaded land in moist loamy soil. Dry sandy soil needs considerably more.
Is ground source better than air source?
It is more efficient, with a SCOP of typically 4–5 against 3–3.5, because the ground stays at 8–12 °C all winter while the air can drop below −10 °C. It is also silent outdoors and has no defrost cycles. The ground loop, though, adds 6,000–15,000 to the installed cost, so payback against an air-source unit is often long unless heat demand is large.
How much does a ground source heat pump cost?
In Europe a typical domestic system costs about 18,000–35,000 € installed: 8,000–15,000 € for the heat pump and installation, plus 50–100 € per metre of borehole (or 10–25 € per m² for horizontal loops). Grants such as the UK Boiler Upgrade Scheme (£7,500) and German BEG subsidies can cover a large share.
What is the specific heat extraction rate (W/m)?
It is the heat power one metre of borehole can draw from the ground sustainably. VDI 4640 gives indicative values from about 20 W/m in dry gravel to 70–85 W/m in granite or gneiss, for systems running 1,800 hours a year. More running hours means a lower rate.
Can a ground loop freeze or run out of heat?
If a borehole field is too short, the ground around it cools year after year and the brine temperature drops, which lowers COP. Designers therefore also cap the annual energy extracted, at about 100–120 kWh per metre per year for boreholes. The calculator applies both the power and the energy criterion.

Sources & references

  1. VDI 4640 Part 2 — Thermal use of the underground: ground source heat pump systems
  2. MCS MIS 3005-D — Heat pump design standard (UK Microgeneration Certification Scheme)
  3. IEA Heat Pumping Technologies TCP
  4. US DOE — Geothermal heat pumps

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