Ground source heat pumps

Buried loop systems, ground arrays and borehole considerations.

A ground source heat pump uses a buried heat exchanger to collect low-temperature heat from the ground, then raises that heat to a useful temperature for space heating and usually stored hot water. In a closed-loop system, water mixed with a suitable antifreeze circulates through sealed underground pipework. The ground loop fluid and the building’s heating water remain separate circuits.

The heat pump is usually indoors, so there is no outdoor fan and no air-coil defrost cycle. The buried source also changes temperature more slowly than outdoor air. The ground array must support both the peak load and repeated seasonal heat extraction without being driven outside its permitted temperatures.

Horizontal arrays and boreholes

Horizontal arrays place pipe in excavated trenches. They need accessible land, suitable depth and spacing, and a reinstatement plan. The available area alone does not set capacity. Soil type, moisture, ground cover, shading, groundwater movement, loop geometry and the building’s annual heating and cooling demand all affect heat exchange.

Vertical arrays use one or more drilled boreholes containing loop pipe and grout. They need less surface land but require geological information, specialist drilling, borehole construction and access for a rig. Borehole depth, separation and heat extraction per metre cannot be chosen from a national rule of thumb. Geology and groundwater conditions vary, and larger or uncertain schemes may need additional investigation or a thermal response test.

A closed loop can also be submerged in a suitable body of water, where ownership, ecology, access, anchoring and environmental permissions allow it. That is distinct from an open-loop system, which actually abstracts water.

How the array is sized

The heat pump’s output rating is not the ground array size. Array design considers:

the building’s peak heating and cooling loads
annual energy taken from and, where applicable, returned to the ground
entering-fluid temperatures permitted by the heat pump
local ground thermal properties and groundwater movement
loop pipe, spacing, depth, grout and circulating-fluid properties
circulation-pump energy and hydraulic resistance
interactions between adjacent trenches, boreholes or neighbouring schemes
the design life and any future change in building demand

An array can appear to work when first commissioned yet cool progressively over successive winters if annual extraction exceeds long-term replenishment. Symptoms can include falling source temperatures, poorer efficiency, more pumping or compressor energy and eventual low-temperature lockouts. A larger heat pump does not correct an undersized ground heat exchanger.

Where the system also provides cooling, rejected summer heat may help rebalance the ground. That benefit must be modelled; it should not be assumed from the presence of a cooling mode.

The ground-loop fluid

The circulating fluid needs freeze protection appropriate to the lowest design temperature and materials compatible with the pipework, seals and heat exchanger. Antifreeze type and concentration change viscosity, pumping energy and heat transfer. The designer should use the chosen fluid’s properties in the hydraulic calculation rather than calculate with plain water and add glycol later.

Installation includes flushing, pressure testing, purging air, filling with the specified mixture and balancing parallel circuits. The handover record should identify the fluid product and concentration so that future top-ups do not dilute it or mix incompatible chemicals.

Leaks deserve attention even though the array is buried. Pressure loss, air entry or unexplained fluid additions should be investigated. The environmental acceptability of a fluid does not make deliberate loss into the ground acceptable.

Heat distribution still governs efficiency

Ground source does not remove the need for room-by-room heat loss, correctly sized emitters and low-temperature controls. The compressor still works harder as the required water temperature rises. A stable source can improve winter performance, but an inefficient hydraulic system or high flow temperature can waste that advantage.

Circulation pumps are part of the electricity use. Long, restrictive or poorly balanced loops can increase source-pump energy, so a heat pump COP quoted without its auxiliary assumptions may overstate what the complete installation will deliver.

As with an air-to-water system, most domestic installations use a cylinder for hot water. Cylinder design, backup heating, electrical demand, weather compensation and minimum system flow form part of the complete heating system.

Ground risk, permissions and records

Before excavation or drilling, the project needs utility searches, land-ownership checks and information about geology, mining, contamination, groundwater and protected sites. Planning treatment varies by UK nation and property. Drilling may also affect warranties, access rights, insurance and future construction over the array.

Closed-loop systems do not abstract groundwater, but that does not mean they are automatically permit-free. In England, Environment Agency rules introduced for closed-loop heating and cooling schemes include exemption conditions and circumstances where an environmental permit is required. Other UK nations have their own environmental regulators. The designer should check the current rules for the location before ground works begin.

The handover includes an accurate plan of trenches, manifolds, boreholes and buried pipe routes, along with test records, fluid details and restrictions on future excavation. Without that record, later landscaping, extensions or utility work can damage an asset intended to remain in the ground for many years.

Ground-source system design

Ground-array and heat-pump sizing bring together:

the building heat loss and annual energy assumptions
heat-pump performance at the proposed source and water temperatures
the array type, geometry and sizing method
geological and thermal data used, with uncertainty explained
source-flow rate, pressure loss and circulation-pump selection
ground-loop fluid, concentration and commissioning records
permissions, utility searches and drilling or excavation method
a plan for reinstatement, access and permanent as-built records

The higher ground-work cost can be worthwhile where the site and long-term demand suit it. The decision should follow from a defensible array design, not a generic trench length or borehole-depth rule.

Applies to

Heat

Last reviewed

22 Jul 2026