Stately homes exploit water features for heat pumps

A body of water can be a very effective heat source for heat pumps. Neil Otter describes two very different installations.

More than simply an historic house and garden, Bishop’s Palace is a splendid medieval palace, which has been the home of the Bishop of Bath & Wells for 800 years. The Palace is surrounded by a stunning moat, and it is from here that renewable energy is sourced for the newly constructed visitors’ centre. For buildings located near a suitable body of water, a water-source heat pump offers an attractive alternative to ground-source systems. They are virtually silent, maintenance needs and costs are negligible, and there are no visible external units.

My company, Ecovision, won a 2010 British Construction Industry Award for the water-source installation at Castle Howard in Yorkshire, one of Britain’s finest stately homes, where the lake was used to provide the heating and hot water for this ancient building.

The coverage by national newspapers reporting the Hon. Simon Howard’s impressive savings encouraged owners of stately homes to look to our design expertise, including Ascott House, Harrow School, Treago Castle and many more throughout the UK. These older buildings were designed to operate at consistent lower temperatures provided originally by fires, maintaining the thermal mass in the thick stone walls. Heat pumps do something similar, but much more cheaply than an oil- or gas-fired system.

Over the years we have designed and installed an increasing number of closed-loop water-source systems but Bishop’s Palace was a more complex challenge. Unlike the project for Castle Howard, it was not possible to drain the loop area prior to installation. An array of ground loops was designed on a loop support frame, which we lowered into the water using buoys as floatation aids. After the loop array was launched into the moat, it was floated into position using ropes. A diver then guided the array into the final position before lowering it under the water. The array sits on the moat bed but is lifted by weighting blocks which keeps it in position and holds the bottom of the loops 200 mm off the moat bed.

We have installed many closed-loop water-source systems using the same loop layout strategy. In the past we have had the luxury of a dry surface to construct them on. The challenge at Bishop’s Palace was to get the loop set in exactly the right position. Calculations were made to ensure the loops, weighting blocks and frame would float and remain in position when filled and operational. It was a challenging part of the installation. but with accurate planning it was plain sailing.

Closed-loop water-source systems are becoming more and more popular. They reduce the client’s capital outlay and almost eliminate the requirement for the alternative, which would be horizontal trenches or boreholes. In addition they provide a solution for a building that does not have sufficient land space or for clients who want as little disturbance to the grounds as possible. Ecovision is working on several other closed loop systems of varying sizes in large country houses and estates across the UK.

Ecovision, heat pump, ground source
With it not being possible to drain the moat at the Bishop’s Palace, unlike Castle Howard pictured right, the loop array for the heat-pump installation was floated into position and then lowered under the water.

Although the required ground space is often available, the number of horizontal loops required for several of these projects would have been expensive and involved extensive digging through beautiful parts of the grounds.

At Castle Howard there was already a plan to drain and dredge the lake, which offered an opportunity to lay the 56 coils of MDPE pipe on the lake bed before it was re-filled. Each coil was 100 m and filled with a diluted glycol, an environmentally friendly antifreeze, to absorb heat from the lake. All the pipes converge into a chamber on the lakeside, from where the warmed fluid is pumped in buried pipes to the heat pumps in the main house at a temperature of 10°C. It ends up in one of two 100 kW Dimplex heat pumps in the plantroom in the basement of the building.

Closed-loop systems make heat-pump installations more viable; depending on the water temperature and the flow rate, they also provide a more efficient heat source. The Bishop’s Palace closed-loop water-source system comprises six 100 m coils headed into one larger flow and return, which penetrates the moat wall adjacent to the plant room.

The heat pump at the Bishop’s Palace is the Dimplex SIH 20TE. Its output is 22 kW, and it can achieve a maximum flow temperature of 70°C, which will supply all the heating and the hot water for the building.

We estimate that the average temperature of the moat during the heating season to be about 7°C. The underfloor heating has been designed to operate effecti­vely at the lowest possible flow temp­eratures to give an average CoP of about 5.2. This system will be about 20% more efficient over the year than an equivalent ground-source heat-pump system.

The return from the Renewable Heat Incentive will be in the region of £1700 per annum. An alternative conventional oil system would have cost approximately £2900 per annum to heat the building. The heat pump will cost about £1200 per year to run, giving an annual saving on heating costs of £1700 and a combined annual financial benefit of £3400. The project received funding from the heritage lottery fund and Church Commissioners for England.

Neil Otter is operations director with Ecovision.

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