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Ground Source Heat Pump (GSHP) Calculator
Calculate borehole depth from heat demand and ground type, with horizontal trench alternative. Includes running costs and gas boiler comparison.
From heat loss calculation
35°C for underfloor heating, 45°C for radiators
Ofgem cap Q3 2026: 26.11p/kWh
Ofgem cap Q3 2026: 7.33p/kWh
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How We Calculate This
This calculator sizes a ground source heat pump system by calculating the required borehole depth or horizontal trench length based on your heat demand and local ground conditions.
Borehole calculation
- Ground extraction: total heat output − compressor electrical input = heat drawn from the ground
- Borehole metres: ground extraction (W) ÷ specific extraction rate (W/m)
- Extraction rates (VDI 4640 Blatt 2, ~1800-2400 h): moist clay/loam 35-50, dry sand/gravel under 25, water-bearing sand/gravel 65-80, limestone 55-70, sandstone 60-70, granite 60-80 W/m
These are indicative design values. A Thermal Response Test (TRT) measures the actual ground thermal conductivity and is the authoritative basis for sizing larger systems.
Trench alternative
- Slinky coils: ~2-3 kW per slinky loop at 1.2-1.5 m depth (indicative)
- Extraction rate: ~10-40 W/m² of ground (≈20 W/m² typical, varies with soil moisture)
- Depth: 1.2-1.5 m below ground surface, trenches at least 3 m apart
Horizontal-loop figures are common installer rules of thumb (MCS MIS 3005-D / VDI 4640); confirm against the manufacturer’s collector design.
Frequently Asked Questions
Borehole depth depends on the heat demand and ground thermal conductivity. Typical domestic installations use 80-150m per borehole, with deeper bores split into two or more shafts. Specific extraction rates (VDI 4640) range from under 25 W/m in dry sand to 80 W/m in water-bearing gravel, with moist clay around 35-50 W/m. A 10 kW system in clay extracts roughly 7 kW from the ground, needing about 150-200m total — typically one deep borehole or two shorter ones.
Horizontal (slinky) collectors need approximately 100-200 m² of garden per 10 kW of heat demand, laid at 1.2-1.5m depth. Trenches should be at least 3m apart. The ground above cannot be covered by buildings or hard surfaces as rainfall replenishment is needed.
Yes, GSHP typically achieves a higher seasonal COP (3.5-4.5) compared to ASHP (2.8-3.5) because ground temperature remains stable at 10-12C year-round, while air temperature varies. The higher efficiency offsets the greater installation cost over time.
Ground thermal conductivity varies significantly: clay 1.5-1.8 W/mK, sandstone 2.0-2.5 W/mK, granite 2.5-3.5 W/mK. Higher conductivity means shorter boreholes. A thermal response test (TRT) is recommended for systems over 15 kW to measure actual ground properties.
Most domestic GSHP installations are permitted development in England and Wales. However, if you are in a conservation area, listed building, or if the borehole is near a Source Protection Zone for drinking water, additional permissions may be required. Environment Agency consent may be needed for deep boreholes.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.