The initial technical specification for the HIU test regime was produced by FairHeat, under contract to Guru Systems, as part of a Heat Networks Demonstration SBRI research project funded by DECC (now BEIS).

The test regime we developed can be found here (link). The purpose of this test regime was four-fold:

  • To enable the performance of different HIUs to be evaluated within the context of typical UK operating conditions, thereby enabling heat network developers to evaluate the performance of specific HIUs against design requirements.
  • To generate operating data on the expected performance of specific HIUs given “normal” operating parameters, to enable heat network operators to identify anomalous performance.
  • To provide a framework for HIU manufacturers to evaluate the performance of their equipment within the UK context, thereby feeding into their continuous improvement development programmes.
  • To provide data on the impact of different design and installation choices on HIU performance, thereby assisting designers of heat networks to optimise heat network performance.

As part of the DECC funding, FairHeat commissioned SP Technical Research Institute of Sweden (“SP”) to modify the “Swedish Test” test rig, utilised for F:103-7e certification, to meet the test requirements of the UK test specification.

As an output from each test, SP produced a test report which was published on both SP’s website and by FairHeat (see below).

VWART Calculation Methodology

For each test, FairHeat calculated a set of Volume Weighted Average Return Temperature (VWART). VWART provides a figure that represents the average impact that a specific HIU will have on network return temperature, under normal operation.

In total, FairHeat calculated the following six derived measures based on test outputs:

  • DHW Volume Weighted Return Temperature, VWARTDHW
  • Space Heating Volume Weighted Return Temperature, VWARTSH
  • Standby Volume Weighted Return Temperature, VWARTSBY
  • Annual Volume Weighted Return Temperature for Heating Period, VWARTHEAT
  • Total Annual Volume Weighted Return Temperature, VWARTHIU

These derived results are calculated by FairHeat using the methodology set out in [this document]

It should be noted that the reported VWART figures are only applicable to the test operating parameters. Actual VWART performance will depend on the operating parameters for each network. Furthermore, certain functions (such as disabling “Keepwarm”) were not tested as part of this programme.

Test Results: Reports and VWART Calculations

SP tested six HIUs to V1.10 specification, between November 2015 and March 2016.

DECC funding paid for five of the tests, with the sixth test (Thermal Integration), being paid by the manufacturer.

Manufacturers supplied the tested HIU and attended testing in all bar one of the tests. Wilson (KaMo) withdrew from the process, but one of their customers supplied a test unit.

Ener-G Switch2 declined to participate in the programme.

Test results for the different HIUs are set-out below:

Supplier (Manufacturer) SP Report VWART Calculation HIU Documentation
Altecnic (Caleffi)


Altecnic SP Report Altecnic VWART Calculation Altecnic HIU Documentation

Evinox Logo 4-COL

Evinox SP Report Evinox VWART Calculation Evinox HIU Documentation
Pegler Yorkshire (Meibes)


Pegler Yorkshire SP Report Pegler Yorkshire VWART Calculation Pegler Yorkshire HIU Documentation
SAV (Danfoss)

Final SAV Logo Small

SAV SP Report SAV VWART Calculation SAV HIU Documentation
Thermal Integration (HSF)


Thermal Integration SP Report Thermal Integration VWART Calculation Thermal Integration HIU Documentation
Wilson (KaMo) Wilson SP Report Wilson VWART Calculation Wilson HIU Documentation