Open-loop GSHP

An open-loop ground source heat-pump system abstracts groundwater, takes heat from it and then discharges the water. A common arrangement uses one borehole for abstraction and another for reinjection into the same aquifer. The source water may pass through an intermediate heat exchanger rather than through the heat pump itself, protecting the refrigeration equipment from groundwater fouling.

This differs from a closed-loop system, where a fixed volume of heat-transfer fluid circulates inside sealed buried pipe. Open loop can provide a stable source temperature and a large amount of heat from a relatively small surface area, but it depends on a productive aquifer, acceptable water chemistry, pumping energy and regulatory permission.

Feasibility begins with hydrogeology

A map can indicate whether a useful aquifer may be present. It cannot confirm the yield, water level, water quality or the effect of abstraction and reinjection at one address. The British Geological Survey’s screening tool is an early-stage resource, not permission or a substitute for site investigation.

A hydrogeological assessment considers:

the aquifer and expected sustainable yield
static and pumping water levels
seasonal variation and nearby abstractions
water chemistry, suspended solids and dissolved gases
reinjection capacity and the risk of clogging
hydraulic and thermal interaction between abstraction and discharge points
contaminated land, source-protection zones and sensitive receptors

Groundwater investigation and test pumping normally come before final heat-pump and borehole design. A successful abstraction borehole does not guarantee that the reinjection borehole will accept the same flow at a manageable pressure.

Flow and pumping energy

The required water flow follows from the heat transferred and the permitted temperature change across the source. It should be calculated for the selected heat pump and regulator-approved operating range. A fixed litres-per-minute-per-kW rule can be useful only as an early sense check; it is not a final design.

Source-pump electricity can materially reduce system performance. The designer needs the pumping water level, pipe and heat-exchanger resistance, reinjection head and pump efficiency at the operating point. A system with favourable groundwater temperature can still perform poorly if it uses excessive energy to move that water.

Variable-speed pumping and well-designed heat exchangers may reduce auxiliary consumption, but they must remain within minimum flow, fouling and control limits.

Water quality is an operating issue

Groundwater can deposit scale, iron compounds or biological growth, erode components or cause corrosion. The risk depends on chemistry, oxygen exposure, temperature change, materials and operating conditions. Filtration alone does not solve every problem.

The design should specify sampling, material compatibility, heat-exchanger arrangement and a cleaning method that also complies with discharge conditions. Adding treatment chemicals to abstracted water may be prohibited by an exemption or permit. Maintenance access and a way to monitor source and discharge temperature, flow, pressure and pump energy are important from the start.

Declining flow, rising reinjection pressure or worsening temperature difference can indicate borehole or heat-exchanger problems. Waiting for the heat pump to trip may allow a manageable water-side issue to become an expensive well intervention.

Permissions in England

The environmental rules are location-specific and live. As at 22 July 2026, Environment Agency guidance for England says an open-loop groundwater scheme needs groundwater investigation consent before drilling or test pumping. An abstraction licence is required if more than 20 cubic metres per day is taken. The discharge also needs an environmental permit, a registered exemption or to meet a defined low-risk position.

The 2026 English discharge exemption is not simply a small-system allowance. It has conditions concerning volumes, return to the same aquifer, temperature change, contamination, nearby water features, abstractions and source-protection zones. Water within an exempt system must not have substances added or be used for another purpose. A project that cannot meet every condition needs the appropriate permit.

Works near a watercourse or flood defence can require further consent. Wales, Scotland and Northern Ireland have different regulators and rules, so the English threshold should not be carried across the UK.

Applications and investigation should begin early. The final consent conditions may limit abstraction, temperatures, monitoring, discharge location or operating periods, and those limits have to feed back into the heat-pump design.

Heating, cooling and long-term balance

An open-loop system can be designed for heating, cooling or both. Cooling changes which borehole abstracts and where heat is rejected, and may change the permissions needed. “Passive” cooling still uses pumps and heat exchangers and still affects the aquifer.

Balanced annual heating and cooling can reduce net thermal change in some schemes, but it does not remove local plume interaction or regulatory limits. The design model should cover the expected annual load and neighbouring schemes, not only the winter peak.

Using the same water for drinking, irrigation or another purpose is not an automatic side benefit. It changes the abstraction from a non-consumptive heating-and-cooling scheme and may breach the conditions of an exemption. Any combined use needs its own water-quality, licensing and system-separation assessment.

Source and discharge design

An open-loop source calculation needs:

hydrogeological desk study, investigation and test results
abstraction and reinjection design with interference assessment
calculated source-water flow and temperature range
total source-pump energy at the expected operating point
water chemistry, material and fouling assessment
monitoring, cleaning and borehole-maintenance plan
copies of regulator correspondence, consents, licence, permit or registered exemption
a clear legal operator and responsibility for ongoing returns and compliance

Open loop is a specialist water-resources project as well as a heating system. Its success depends as much on the aquifer and discharge arrangement as on the heat-pump unit.