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Case Study: Domestic Retrofit | 4 Bed Detached House - Air Source Heat Pump & Solar PV

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16kW Dimplex A Class ASHP
at rear of the property
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Control and information panel
for ASHP
Low temperature radiator
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16kW Dimplex A Class ASHP
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16kW Dimplex A Class ASHP
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Solar PV array on stable roof
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As featured in The Sunday Times

The Department of Energy and Climate Change used our installation in their publicity prior to the launch of the Domestic Renewable Heat Incentive. The article was published in The Sunday Times on 6th April and highlighted Roy's heat pump install as the main feature.

  • 16kW Dimplex A Class Air Source Heat Pump
  • 3.65kW Photovoltaic System


Thirty years ago Roy Powel converted this West Sussex barn into a four bedroom home for his growing family. The property is located in a country farm setting, North of the South Downs. It was constructed in brick, block and plain tiles using the 4 bay oak barn as the main frame. The home benefits from cavity wall and ground floor insulation and was built with thermal-brake aluminium double glazing. The annex has recently been fitted with uPVC double glazing.

Located in an 'off the gas grid' area, the property was originally heated by an LPG fuelled conventional boiler for space heating with a LPG fired Aga providing domestic hot water. Some rooms in the home had no radiators and while some benefited from heat "borrowed" from other rooms they were often supplemented by or totally relied on portable electric heaters. It was estimated that the Aga domestic hot water system was only 40% efficient and the 29 year old boiler approximately 65% efficient.

The home suffered from uneven temperatures and lack of comfort in some areas.The house felt warm when the radiators were hot but quickly felt cold once the boiler had turned off.

Roy is a specialist in renewable heating systems and has an in-depth understanding of the heating options available, he spent several years gathering temperature and heat loss data about his property and researching the renewable heating market place for a suitable solution.

Air Source Heat Pump

The property’s calculated winter heat loss of just over 14kW is satisfied by a 16kW Dimplex 'A' Class Air Source Heat Pump. Whilst the first floor radiators were found to be suitable for the new heat source, some adjustments were necessary to the ground floor radiators to take full advantage of the heat pump's higher efficiency at lower supply temperature. This included the fitting of fan assisted Smart Rads in some areas.

The Aga domestic hot water system has been decommissioned, so that the AGA is now used as a classic cooker only. The domestic hot water is now supplied by the Air Source Heat Pump. The high delivery temperature of the Dimplex A Class unit allows the cylinder to be kept at a high temperature. This means that an immersion heater is not required to periodically pasteurise the cylinder to eliminate the risk of legionella bacteria. This further increases the overall system efficiency particulaly whencompared to low temperature heat pumps, further reducing costs and the system's carbon foot print.

Extra monitoring equipment has been fitted to this system so that electrical energy usage and heat energy produced figures are available to accurately measure the systems "real life" performance..

It is the intention to run the Dimplex Air Source Heat Pump constantly to maintain an uniform indoor temperature. The home utilises the 'Economy 7' tariff so the hot water cylinder is topped up at night by the heat pump, taking advantage of the low night time tariff.

Photovoltaic (PV)

The PV system was installed facing 15 degrees east of south on a pitched roof of the stable block. It comprises of 16 off 250watt Suntech panels controlled by a Fronius inverter.

The positioning of the panel array results in shading occurring from early afternoon during the 6 month winter period and mid-afternoon in the summer. However, the overall performance remains substantially ahead of the official Government's "Standard Assessment Procedure" rating.

The householder benefits from the Government's domestic FIT (Feed in Tariff) scheme, and is able to use the "free electricity" in the spring and autumn to help with the costs of running the heat pump..

Air Source Heat Pump

Property Size
220 m2
Heat Pump
Dimplex 16kW ASHP
100% Space and DHW Heating
Calculated Heat Loss (19ºC internal & -3ºC ambient air)
13.83 kW
Replaced Heat Source (LPG boiler)
28 year old Thorn EMI M80/100C
Annual Energy Usage via Gas Boiler for space heating
31,400 kWh
Annual Energy Usage via Aga for domestic hot water
13,300 kWh
Annual comparative Energy Usage via Heat Pump
7,600kWh (inc 20% extra comfort)
Savings in Energy
37,100kWh (78%)
Savings in CO2
3,763.8kg based on data from DEFRA
Money saved annually
Annual RHI Payments

Solar PV

Solar Panels
Suntech STP250S
Number of Panels
Fronius 3.6TL
Module Efficiency
Predicted Annual Output – SAP2009
3,322 kWh
Annual Output - Actual over 1st Year of Installation
3,775 kWh
Annual Output - Actual over 2nd Year of Installation
3,700 kWh