Case Study: Domestic Water Sourced Heat Pump, Cirencester

Written by: Ian Nock | Engineering Manager |  Renewables First

Background

An exclusive new development on the banks of a 360,000 m² former gravel quarry in the Cotswold Water Park. Our client purchased one of the properties, off-plan, and chose to upgrade the heating system to a water-sourced heat pump (WSHP) to take advantage of the year-round thermal reserves of the lake. Renewables First were commissioned to undertake the feasibility study in 2021 while the project was still in its design phase, showing the increased performance of using the lake reserves over an air-sourced heat pump. The system was fitted and commissioned in December 2022.

Project Summary:

* Alternative heating system assumed to be gas powered with emission rates calculated using government published data for 2022.

System Requirements:

The 5-bedroom detached property has an internal floor area of 336 m² spread over three floors. The ground floor is heated solely by underfloor heating with the first and second floors heated with radiators. With the high insulation specification of all aspects of the design it was our aim to run the heating system at its highest efficiency. To achieve this all aspects of the system were specified to run at the lowest flow temperature setting of the heat pump (35 °C) whilst still attaining the desired room temperature as defined by MCS(Microgeneration Certification Scheme) ¹

The heating system was also to supply the 5 bathrooms, downstairs WC, kitchen and utility room with hot water.
One of our clients’ other requests was cooling the property during the summer months given the large expanse of glazing within the design.

As this was a domestic installation a low maintenance and low impact system was required with the internal system integrated into the fitted units within the utility room.

System Solution:

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. This was teamed with a 305 l hot water storage tank and 150 l buffer tank to meet all the heating and hot water needs.

A low maintenance closed loop design was specified using three pond mat collectors installed within the lake. Each pond mat has a 5 kW collection rating to exceed the year-round demand of the heat pump. Each pond mat consists of a 250 m length of HDPE pipe coiled onto a frame measuring 2.5 m by 1 m – weighted onto the lakes bed and allowing at least 500 mm clearance below the waters surface. The three pond mats joined to an underground manifold on the lakes edge which supplied the heat pump via two 40 mm header pipes running under the landscaped garden, beneath the patio and walkway, alongside the house, and in through the utility wall.

To cool the property a passive cooling system was integrated into the pond loop and joined with the mechanical ventilation heat recovery system serving the house with fresh air year-round. The lake water will generally be warmer than the air temperature during the winter and cooler during the summer offering a temperature source and sink for heating and cooling. A passive cooling system uses this natural low-cost temperature difference to cool the ventilation system’s incoming air providing an affordable and efficient method of comfort cooling an enclosed space.

The heat pump and passive cooling system were installed within a standard double fronted kitchen cupboard and the tanks located in a cupboard on the first floor.

The system performance has been estimated as 420% for space heating and 326% for hot water assuming a source feed to the heat pump of a constant 0°C. During the commissioning phase, when the air temperature regularly dropped below freezing, the system return temperature remained a fairly constant 6°C , setting expectations of a better performance than predicted.

1. MCS certification for Heat Pumps is provided through an umbrella scheme from Kensa Heat Pumps, where we are their installation contractor and are responsible for the design and efficient operation of the system. We can provide plumbing & electrical contractors or we can utilise your own local contractors.

2. Coefficient of Performance (CoP)  will depend on the source temperature of the collector and the desired supply temperature. Please see the following link from the European Heat Pump Association (EHPA) for their verification of the likely CoP White_Paper_Heat_pumps-1.pdf(page 32).

Ian has been working in the renewables sector since 2014 and leads the Net-zero, Heat Pumps and Solar PV teams for Renewables First. He works closely with our engineering team to share practical, evidence-based renewable energy advice.

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