River House, Cambridge

Ground Source Review River House ǀ Heat Pump - Keith Clarke in front of River House

Ground Source Review: River House.

Mr Clarke replaced his gas boiler with a Kensa 24kW Single Phase Twin Compressor to harness heat energy from a nearby lake to provide space heating and hot water to his Grade II listed farmhouse.


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Ground Source Review: River House

Keith Clarke and his family recently moved into a spacious Grade II listed farmhouse in Cambridge. They discovered that their new home was heated by a 20 year old mains gas boiler which struggled to keep the house up to temperature and was expensive to run.

Watch the video case study here:


Keith was familiar with ground source heat pump technology and knew it was an environmentally friendly choice that would eliminate reliance on fossil fuels. He approached Cambridge-based renewables installer Cernunnos to talk about designing a system that would meet the family’s heating and hot water needs.

Compact Twin 2In the initial consultation Cernunnos recommended Kensa Heat Pumps because its models are manufactured in the UK and also because the Kensa 24kW Twin Compact, the largest single phase ground source heat pump on the market, was best placed to handle the property’s high heat load.

There is a large lake on the grounds which is fed by a natural spring. Cernunnos measured its flow rate along with its area and depth, and provided this information to Kensa who recommended that the Clarke’s use the heat energy in the lake for their heating and hot water. As water is an excellent conductor and has a higher heat transfer rate than the ground, this made it an especially efficient system.

The team from Cernunnos sunk specially created Kensa pond mats consisting of slinky pipes attached to corrosion resistant stainless steel frames into the lake on a closed loop system. They connected the system up to the heat pump which is housed in an annex building via pipes running under the lawn up to the main house. This method removed the need for large amounts of drilling or digging, thereby reducing the cost and duration of the installation.

Mr Clarke said:

I knew that ground source was a great technology for extracting the natural heat that’s available from the environment to provide a completely green heating system. The good thing about the pipes being in the lake is that water has great conductivity meaning we are able to extract heat sustainably throughout the winter to keep the house perfectly warm.

He continues:

I know that there’s a big investment involved in getting a ground source heat pump, however we realised that we could get the whole thing paid for with the Renewable Heat Incentive (RHI: this scheme has now closed – please visit this page to explore other funding), including the installation. Plus, we’ve ended up with a system where the yearly cost of electricity is less than we would have been paying for gas.”

Due to the fact that the building was a Grade II listed farmhouse Cernunnos needed to ensure that the energy efficiency of the property was considered, and improved where necessary; the less efficient the property, the greater the heat loss, which would result in a bigger and more expensive ground source heat pump and associated running costs to maintain temperature in the home.

Cernunnos undertook a detailed heat loss study at the start of the project which showed that the house had previously installed basic insulation measures to improve its heat loss, however the annex building where the heat pump is housed was poorly insulated. Cernunnos therefore installed extra insulation underneath a suspended timber floor using 100 mm rigid insulation between and 25mm below the rafters, as well as adding loft insulation.

The study also showed that many of the existing radiators were already oversized and simply needed upgrading from single to double panels. Oversizing radiators ensures that they will work at a lower flow temperature, thus reducing the amount of work the heat pump is required to do to ‘uplift’ the renewable heat energy from the ground to the required temperature for the home. Cernunnos also installed underfloor heating in some rooms downstairs, which works well with the ambient temperature delivered by the heat pump, which together with the new radiators ensure an efficient installation all round.

In the vast majority of cases planning permission is not required when installing a ground source heat pump. However, as River House is a Grade II listed building, the Clarke’s had to seek permission for the installation and the upgrades to the insulation and heat distribution system. In this case the local authority planning department was very supportive throughout the process and permission was granted without a problem.


Key Facts

  • 24kW Single Phase Twin Compressor
  • Replaced mains gas boiler
  • Large, Grade II listed farmhouse
  • Highly efficient water source system
  • Pond mats sunk into lake
  • Costs covered by Renewable Heat Incentive (RHI: this scheme has now closed)