How deep do GSHP boreholes need to be?

Borehole depth depends on the heat demand and ground thermal conductivity. Typical domestic installations require 80-150m per borehole. Ground extraction rates vary from 40 W/m in chalk to 70 W/m in granite. A 10 kW system in clay might need 2 x 80m boreholes or 1 x 150m borehole.

How much garden do I need for horizontal trenches?

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.

Is GSHP more efficient than ASHP?

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.

How does ground type affect borehole design?

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.

Do GSHP boreholes need planning permission?

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.

TradeCalculator

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.

Heat Demand

From heat loss calculation

Ground Type

Affects extraction rate (W/m)

Collector Type

Borehole or horizontal trench

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 from ground
Borehole metres: Ground extraction (W) / extraction rate (W/m)
Extraction rates: Clay 50-55, Sand 45-50, Limestone 55-65, Granite 60-70 W/m

Trench alternative

Slinky coils: ~3 kW per slinky loop at 1.2m depth
Extraction rate: ~20 W/m2 of trench area
Depth: 1.2-1.5m below ground surface

Frequently Asked Questions

Related Calculators

Air Source Heat Pump Sizing Calculator

Size air source heat pumps from whole house heat loss with defrost derating.

Heat Pump COP Calculator

Calculate heat pump COP and seasonal COP with running cost comparison vs gas.

Heat Pump Radiator Calculator

Re-size radiators for heat pump 45C flow with de-rating factor.

Solar PV Array Calculator

Calculate solar PV array size, orientation yield factor and annual kWh by UK region.

Solar PV Inverter Calculator

Size solar PV inverters from array kWp with string voltage check.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.