Air source is available to far more homes. Water source can have a stable heat source, but a suitable water body, permissions and a sound design are essential.
Same destination, different heat source
Both technologies use a refrigerant circuit and compressor to raise low-temperature heat to a useful level for a home. An air source heat pump collects heat from outdoor air. A water source heat pump collects it from a pond, lake, river or groundwater through a suitable collector or open-loop arrangement.
The IEA's heat pump report noted that air-source systems made up nearly 85% of building heat pumps sold worldwide at the time of publication. Their appeal is practical: most homes have access to outdoor air, while far fewer have access to an appropriate water source.
Why water can be attractive, and complicated
Water temperatures can be steadier than winter air temperatures, which can help efficiency. But the source must have sufficient capacity, remain accessible and be suitable for the proposed extraction method. Closed-loop systems circulate fluid through a submerged collector. Open-loop systems take in water and return it, so water quality, pumps, filtration and permissions become part of the design.
In England, an open-loop proposal can need environmental permits and, above certain abstraction volumes, an abstraction licence. Government guidance identifies 20 cubic metres a day as an important licensing threshold. These checks should happen before anyone promises a fixed installation price.
Which one should you shortlist?
For a typical home without usable water access, compare well-designed air source proposals first. If you have a pond, lake, river frontage or suitable groundwater, ask a specialist to assess the resource and permissions alongside the home's heat loss. Compare total installed cost, seasonal electricity use, maintenance and responsibilities for water-side equipment.
Do not choose on a headline efficiency rating alone. A property-specific design should explain the source temperature, expected seasonal performance, site works and who maintains the collectors or abstraction system.
The grant does not settle the comparison
Under the current Boiler Upgrade Scheme in England and Wales, qualifying ground source heat pumps include water source systems in the £7,500 grant category. A qualifying air-to-water heat pump also has a £7,500 standard grant. The same headline support does not mean the same final price, because water-side works and permissions can be substantial.
Ask for the gross installed cost of each complete option before the grant is deducted. Include design, collectors or abstraction equipment, controls, commissioning and ongoing maintenance. The right answer is the system you can install, run and maintain reliably at your property.
A note about the numbers
Prices, grants and tariffs can change. The dated figures above come from the linked sources and are examples unless explicitly described as a published average. Your installer should assess your property before giving a final design or quotation.