A water source heat pump system serving a five-bedroom detached house in Cirencester was fitted and commissioned in December 2022, according to a case study published by Renewables First, the engineering firm that designed it1. The system uses a 13 kW Kensa Evo heat pump drawing heat from a 360,000 m² lake at a former gravel quarry in the Cotswold Water Park1.
Renewables First was commissioned to carry out the feasibility study in 2021, while the project was still in its design phase, and says the study showed increased performance from using the lake rather than an air source heat pump1. The property has an internal floor area of 336 m² over three floors, with underfloor heating on the ground floor and radiators on the first and second floors1. The system was specified to run at the heat pump's lowest flow temperature setting of 35 °C, and is paired with a 305 litre hot water storage tank and a 150 litre buffer tank1.
The ground loop consists of three pond mats, each with a 5 kW collection rating, exceeding the year-round demand of the heat pump1. Each mat is a 250 m length of HDPE pipe coiled onto a frame measuring 2.5 m by 1 m, weighted onto the lake bed with at least 500 mm clearance below the water surface1. The three mats join an underground manifold at the lake edge, which supplies the heat pump through two 40 mm header pipes running under the garden, patio and walkway and into the utility wall1. A passive cooling system was integrated into the pond loop and joined to the property's mechanical ventilation heat recovery system1.
| Item | Figure |
|---|---|
| Heat pump capacity | 13 kW |
| Collector type | 3 x 5 kW pond mats |
| Heated floor area | 360 m² |
| Heating efficiency (SCOP) | 4.2 at 35 °C |
| Hot water efficiency (SCOP) | 3.26 at 55 °C |
| Annual carbon saving | 3,500 kg CO2e per year |
| Grant obtained | £6,000 Boiler Upgrade Scheme |
The case study states that system performance has been estimated at 420 per cent for space heating and 326 per cent for hot water, assuming a constant 0 °C source feed to the heat pump1. During commissioning, when air temperatures regularly dropped below freezing, the system return temperature remained a fairly constant 6 °C, which the firm says sets expectations of better performance than predicted1. The annual carbon saving of 3,500 kg CO2e assumes an alternative gas heating system, calculated using government published data for 20221. A £6,000 Boiler Upgrade Scheme grant was obtained1.
"We selected a 13 kW Evo heat pump from Kensa on the grounds of its intelligent controls, superior efficiency, quiet operational mode and excellent customer support team based in Cornwall."
Why it matters for households
The Cirencester installation is a worked example of a water source heat pump in a domestic setting, where the heat source is a body of water rather than the outside air. The case study reports a heating efficiency of 4.2 at a 35 °C flow temperature and 3.26 for hot water at 55 °C, figures that depend on the source temperature of the collector and the supply temperature required1. The lake is described as generally warmer than the air in winter and cooler in summer, which the firm used both as a heat source and, through passive cooling, as a sink1.
For a household, the practical points are the space and siting requirements rather than the running figures alone. The pond mats sit on the lake bed and the pipework runs underground to the house, so the approach depends on access to a suitable water body; the heat pump and passive cooling unit were installed inside a standard double fronted kitchen cupboard, with the tanks in a first floor cupboard1. The £6,000 grant obtained was under the Boiler Upgrade Scheme, the scheme that replaced the earlier Clean Heat Grant1. The case study does not report the installed cost of the system, the household's annual running costs, or how the figures have performed over a full heating season, and none of these has been reported1.
What happens next
No next steps are set out in the case study. The system was commissioned in December 2022 and the performance figures given are estimates, with the commissioning-phase return temperature described as setting expectations of better performance than predicted1.