HIU Testing Standards

For too long have we lived with a complete lack of accountability for the use of badly designed or old fashioned HIUs. Until now there has been no standards against which to check an HIUs performance, and the importance of certain features to the efficiency of an entire scheme has been ignored. As a result, we live with a plethora of poorly performing district heating schemes that are so inefficient they cost the end user significantly more.

Imagine a gas boiler industry without standards or any way of gauging efficiency - completely unthinkable, but currently the case with HIUs.

This is set to change, with the first set of HIU tests now completed using a new UK standard derived from the Swedish HIU standards, but flavoured to our own climate. A significant portion of the well known manufacturers have undergone independent testing and the results are close to publication.

The tests have been funded by DECC and managed by a body independent from HIU manufacturers.

It should be noted that as yet this standard is not a document of compliance, and there is no legal requirement for manufacturers to submit HIUs for testing, to publish results, or for specifiers and installers to follow the requirements.

We feel however, that clients should be wary of those who do not, as they are the ones left to live with the results of poor performance.

Test Regime, Version 1.9

The following document outlines the test regime that has been put in place.

{{#l:Modified Swedish Test UK HIU Test Regime_V1-Rev-009.pdf|Modified Swedish Test UK HIU Test Regime_V1-Rev-009.pdf}}

{{#l:160330_VWART Calculation Methodology.pdf|VWART Calculation Methodology.pdf}}

About the Tests

The tests cover a large range of criteria, all important. The documents themselves provide the best explanation of each test and its reasoning.

The VWART figures are also important, and the key to an efficient system. The lower the better

Volume-Weighed Average Return Temperature, is a figure that represents the average return temperature you can expect under normal operation. Its a simple fact that heat networks and sources work more efficiently with low return temperatures. Furthermore, the higher temperature differences allows networks to deliver more heat.

The published figure takes into account keep warm modes in constant operation. It does not allow for economy modes on electronic HIUs, or for a low temperature approach (see below) and as yet no figures have been published as part of the report.

Test Results for Data HIU

We are please to publish the test results for The DATA HIU.

{{#l:5P07886D-rev2 -Thermal Integration 160405.pdf|5P07886D-rev2 -Thermal Integration 160405.pdf}}

In summary, the unit passed on all but one of the tests.

The failure, an overshoot on DHW temperature, was incurred under maximum DP conditions of 2.5 bar, with the unit commissioned for a DP of 0.6 bar, and would not be encountered under site conditions where the units would self-learn the system DP, be setup for appropriate DP ranges, or fitted with the optional DP valve.

Improvements

Heat Meters

HIUs were to be submitted with a choice of a heat meter,in which case approval only holds if similar model of heat meter is used, or a spool piece that would be replaced with a 25kPa restriction but is then approved for used with any heat meter.

We feel the test should be using a spool piece with a Kv equivalent to an average of the 1.5m3 heat meters available. If the HIU has the option for a 2.5m3 heat meter, then a DP loss test should be run including loss at 30kW, and loss at peak output.

Keep Warm Function

The tests currently give no weight to the ability of an HIU to let primary pipework go cold when there is no demand. This already been seen to improve performance significantly, and as such the tests should provide a clear indication of the savings resulting from use of this feature within an HIU.

Low Temperature VWART Figure

A VWART figure should be provided for a DHW setpoint of 45C, allowing for sterilisation functions and use of economy keep warm modes.

A sterilisation function test should be carried out.

The aim is to provide a lower limit on the potential return temperature performance of an HIU if that is the desired specification criteria.

Max DP Test 5b

The maximum DP (Differential Pressure) test (on which we failed) does not give HIUs any time to pickup on a high DP value and alter its operation accordingly. In the tests, our HIU was simply hot with a DP value 2.5 times that it had been working on previously. As such the true operation of the unit was not tested, as on site, units do have time to learn the DP characteristics.

It can be seen on the test graphs for 5b that following the first two valve opening movements, the system adjusts for the higher than expected DP value and adjusts. This is most clearly seen on the DH flow line (dotted red), that spikes for the first two movements, but is then corrected for in following draw-offs. Arguable, the addition of a few cycles prior to test start would enable self-learning HIUs to adjust for the rising DP as they would on site.

It is also unclear what the test is for, in that there is firstly the ability of the unit to perform at the extremities of its range, and secondly, the ability to react to unexpected rises in DP. We would suggest that a DP swing test is a separate test to that of testing the unit at extremities - which would allow for the unit to be commissioned accordingly.

Self-Learning of HIUs

Allowance needs to be made for the fact that the latest HIUs learn about their environment, and as such any test should allow for a bedding in period or number of function cycles.

Additional Features

The test should include a report on additional functionality provided but not yet tested for in current regime, including:

Software features How tamperproof systems are (from both public and installer) Commissioning procedures Quality of installation documentation Security valve options Network connectivity (Serial, M-Bus, etc) and features

HIU mounting on test rig

The current rig layout mounts HIUs to a metal back-plate using metal screws creating a heat bridge. With HIUs designed to avoid heat bridging to the environment, and with heat losses now so low, we believe the test rig could introduce an additional heat loss that can be easily avoided if HIUs are screwed to a wooden back-plate more representative of real world installation conditions.

Higher DHW Outputs

It is a fact that specifiers are asking for outputs over 50kW DHW from a single unit. Even though our HIU can achieve 75kW, the test rig was unable to go this high. The righ should be updated to take a minimum of 100kW, and preferably 150kW, to allow results to accurately reflect the full output range of the HIUs.

Allowance should be made to allow for a further test with settings adjusted to accommodate high than average DHW outputs (>50kW) by including additional cycles in the high DP overheat test 5b.

Test Facilities

We would like to see an approved test facility within the UK that can cope with the demands of all manufacturers submitting all their ranges.

Results Repository

Test results for all HIUs should be made available to the latest versions on a web site, as with other recognised standards. This is to ensure that no specifier is left unaware of the performance of equipment they specify, and provides a result against which field units can be held to account.