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EON Retrofit Test House: Heat pumps

Solid-wall, ‘hard to heat, hard to treat’.


This case study is part of the CALEBRE project, 'Consumer Appealing Low Energy technologies for Building REtrofitting', which focused on solid wall 'hard to heat, hard to treat' properties.

The project considered a range of current, medium and longer-term technologies for domestic energy efficiency refurbishment, with the perspective of the householder at the heart of all thinking and decision making.


The majority of heat pumps require the removal of the existing radiator system and installation of under flood heating to offer the best energy efficient system. In a refurbishment project, this can be expensive and disruptive as it requires the removal and potential replacement of floor and wall coverings.

The CALEBRE research team designed and tested three air source heat pumps for use in domestic refurbishment - one gas fired and two electric. They have been designed and manufactured with consideration for householder appeal.

The three heat pumps are:

  • An affordable gas-fired heat pump which can replace a conventional gas-fired boiler. Further research is continuing to develop an indoor unit which is no bigger than a standard boiler.
  • An EVI (Economised Vapour Injection) compressor heat pump was developed into a compact unit suitable for the domestic market.
  • An expansion turbine heat pump was also developed.


Gas Fired Heat Pump

The unit was tested in an environmental chamber to EU standards and was found capable of producing output water at 60°C, as would be required for domestic radiators.

Economised Vapour Injection (EVI)

This heat pump was also tested to EU standards in an environmental chamber. A limited field trial realised a seasonal COP of 3.7.


Initial results were promising, but internal refrigerant leakages and internal heat transfer negated gains. A new unit, with an improved design was then produced and initial testing results were promising.


This research work has identified that gas-fired heat pumps can feasibly replace gas boilers when undertaking domestic refurbishment. The gas-fired heat pump can deliver 33% savings in gas use when compared to conventional boilers, resulting in pay-back of less than 3 years.

Both EVI and Expander (i.e. advanced electric) heat pumps can overcome the drawbacks of many conventional systems and will be able to provide low cost heating in homes which may have otherwise required major refurbishment work to adopt heat pump technology. Continuing laboratory testing and field trials will lead to greater reliability of these systems and improved social acceptability.

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