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
Ground types (VDI 4640 indicative extraction rates)
| Ground | Borehole W/m (1,800 h) | W/m (2,400 h) | Horizontal W/m² (1,800 h) | Borehole needed (m) | Loop area (m²) |
|---|---|---|---|---|---|
| Dry gravel / sand | 25 | 20 | 12 | 293 | 642 |
| Moist clay / loam | 45 | 35 | 28 | 166 | 265 |
| Water-saturated gravel / sand | 70 | 60 | 38 | 140 | 234 |
| Limestone (massive) | 62 | 52 | 30 | 140 | 244 |
| Sandstone | 72 | 60 | 30 | 140 | 244 |
| Granite / gneiss | 75 | 63 | 30 | 140 | 244 |
| Basalt | 52 | 45 | 30 | 140 | 244 |
| Average soil / rock (λ ≈ 2 W/mK) | 55 | 45 | 25 | 140 | 293 |
- 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.