HOT|COOL NO. 1/2017 - "System Integration"


The Test Procedure includes a test which is conducted with radiators without flow control to clearly demonstrate the impacts. This also demonstrates the importance of correct commissioning of the substation and the secondary heating systems. In the author’s experience, surveying of operating UK DH networks show, time and again, that better commissioning will lead to more efficient DH operation. When the system performance is poor, the tendency is to blame the installed equipment, but experience shows that the design or the commissioning are more frequently the source of problems. In the UK, design calculations and drawings only state return temperatures for peak load, and nothing is defined for other operating conditions. It is therefore difficult during commissioning to challenge high return temperatures from the system under all but peak loads. The VWART measurement indicates what the primary return temperature should be in operation; which gives a benchmark against which high return temperatures can be questioned. With the VWART representing real operation, the ambition is that maximum acceptable return temperatures at full and no load can be written into a specification and then demonstrated at commissioning.

The main substation suppliers in the UK were asked if they would like to offer a substation for testing free of charge. Five suppliers accepted and had one of their substations tested. The results show the range of performance of the different substations. Figure 1 shows the VWART for each of the substations’ functions. Figure 2 presents the calculated annual primary flow volumes. The three main observations are: 1) The variation in results – and yet they would all comply with a typical UK specification for a substation. 2) The high primary return temperatures during space heating – this is due to poor heat exchanger performance resulting from the heat exchangers being too large for the small heat loads used in the test. 3) The significance of standby operation and variation in standby performance between substations. The UK HIU Testing Procedure has promoted awareness of a range of DH performance issues and led to improvements in specifications and products. All the manufacturers attended the tests and all but one have made modifications to their substations as a result. The research funding has now ended and the UK HIU Testing Procedure has recently been adopted by the Building Engineering Services Association. A steering committee has been established which includes the representatives from the larger DH utilities. The steering committee seeks to ensure that the UK HIU Testing Procedure is maintained and developed such that it can help to deliver safe, efficient and cost effective substations that benefit operators and customers. One of the first steering committee tasks has been to develop Version 2 of the UK HIU Testing Procedure. Version 2 includes an option to test at typical underfloor heating temperatures and some of the pure research tests have been removed to keep the costs of the test down. There are a number of manufacturers planning to test their substations as soon as Version 2 is launched.

Results of research testing

The UK HIU Testing Procedure and results can be found at

Figure 1. Components of VWART and overall VWART

Thanks to the 4DH Research Centre

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Figure 2. Annual component primary DH flow volumes for an individual substations

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