The Annex investigated the testing of the most common combined heat pump systems with heating of domestic hot water by

  • desuperheating
  • subcooling of the condensate
  • cascade hot water heat pump
  • using the heat from the supply line of the heating system

The aim of the testing was to generate necessary data in order to calculate the overall seasonal performance factor of such heat pump systems. The system investigated included the heat pump, the hot water storage (or a once-through water heater) and an op-tional supplemental back-up heating. This Annex applied to heat pumps with hydronic heat distribution systems and alternative or simultaneous domestic hot water production.

  • Start date: 25 October 2005 E
  • End date: 30 April 2005, extended until October 1, 2005


  • The first objective of the Annex was to work out a test procedure, which provided the necessary data in order to calculate the overall seasonal performance factor of combined heat pump systems with a minimal require-ment of testing equipment and testing time.
  • The second objective was to work out an easy to use method for calculating the seasonal performance factor of the systems.

Project manager and participating countries

Switzerland was the Operating Agent acting through the Swiss Federal Office of Energy. The participating countries were: Austria, Canada, France, Germany, Japan, Norway, Sweden and USA.  UK withdrawn their participation at the end of 2004.


The following tasks and activities was planned:

Task 1 – Systems to be investigated
  • Analyze the most common systems for combined space and domestic water heating.
  • Find out the required parameters for the relevant systems to be measured by the test proce-dure in order to characterize the system and calculate the overall seasonal performance factor with an easy to use calculation method, supported by computer simulation.
  • Gather the typical temperature required in the different countries for domestic hot water, the daily hot water consumption pattern as de-scribed in existing standards and regulations, and the impact of disrupted electricity supply on heat pump operation and efficiency.
  • Assess the existing standards for testing of heat pumps for separate space and domestic water heating.
Task 2 – Developing a test procedure
  • Assess existing test procedures of heat pump sys-tems for space and domestic water heating, for instance according to EN255, ASHRAE 124 or ARI 470.
  • Point out what is missing for the combined space heating and domestic water heating.
  • Develop and document in detail a comprehensive test procedure for heat pump systems with alterna-tive and simultaneous space and domestic wa-ter heating.
Task 3 – Developing a calculation method
  • Assess, develop and propose methods to calculate the overall seasonal performance factor of heat pump systems for the combined space and domestic water heating.


More information can be found on the Annex 28 website.

Publication and results dissemination

  • IEA HPP Annex 28 – Test Procedures and Seasonal Performance Calculation Methods – Workshop
    Workshop in connection with the 8th International Energy Agency Heat Pump Conference, Las Vegas, May 2005.
  • Annex 28 – Comprehensive and uniform testing and calculation for heat pump systems – Newsletter Article
    An article on the Workshop and Annex 28 was published in issue 3/2005 of the HPC Newsletter.
  • Test procedure and seasonal performance calculation for residential heat pumps with combined space and domestic hot water heating (final report)
    Order No. HPP-AN28-1; Published September 2006. Full price € 60 and reduced price € 20. Can be ordered as CD rom only.

    The final report comprises two parts. The first part contains proposals for calculation method and test procedure, which can be integrated in CEN standards. The second part comprises the background on the project, the national contributions, and introduction to the present situation in standardisation and systems on the markets, performed evaluations and the discussion and decision of the approaches.

Reports are available at Annex 28 Publications.