HIU Testing Standards v2

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The following are a list of amendments we would like to see made in the second version of the HIU testing standard.

UK HIU Test Regime_V2-Rev-001.pdf
UK HIU Test Regime_V2-Rev-001_RequestforFeedback_THERMAL.docx


Maximum DP Test

As some HIUs provide on-board DP regulation by means of self-learning, the test procedure for maximum DP needs to allow such functions to perform, rather than be left to test operator. If test is to see performance under a sudden large step change in DP, where only a DP valve can provide immediate effect, this needs to be stated, and treated a separate test to maximum DP under normal operating conditions.

Low Temperature Keep-Warm

We have demonstrated that a reduced keep warm temperature of 25C results in primaries been kept hot enough that when used in conjunction with an initial boost flow delivers hot water within acceptable limits (faster than most).

We are now looking at a proposed keep-warm temperature that is elevated to 40C (primary flow maintained above this) as a means of assigning a comparable VWART to all makes.

This however renders the VWART less valuable. It no longer stands as a mark of how efficient the system can be made within performance limits.

To counter this a test should provide a VWART figure that is indicative of best performance, without a keep-warm turned on if possible. This is also to determine if an HIU can be used without keep-warm, by demonstrating the delay in delivery times to taps.

It is not simply enough to state VWART without keep warm as is it is know DHW delivery times become unacceptable with certain makes under these conditions. The tests therefore need to demonstrate performance under these conditions.

The volume of the primary pipework left cold is to be stated but should be at least 10 litres. DHW draw-off to be at shower flow rates, and if a manufacturer deems fit, a keep-warm can be turned on to a temperature determined by the HIU manufacturer that they are confident will deliver hot water in within acceptable limits.

Tests 1e and 1f for unbalanced heating

These tests need to remain. They were put in for good technical reasons and removed for no clear technical reason. The removal of the test does however prevent it becoming possible to determine if an HIU will protect system efficiency with unbalanced radiators - a known cause of system inefficiency that can be protected against.

The argument of cost is redundant as the test need only be optional. We as an HIU manufacturer would be happy to commission SP to do the tests ourselves.

Test 1g

An additional test to demonstrate how the HIU performs when primary flow temperatures are reduced to below initial central heating target temperatures. This is again a known cause of system inefficiency that should be highlighted in tests.

As 1e and 1f, it is effectively a test of the return temperature limiting functionality of the HIU.

Again, an optional test.

Legionella Assessment

Conversations with water authorities, Legionella experts, and WRAS suggest that permitting reduced keep warm temperatures without a Legionella protection cycle introduce a potential risk, that is still to be determined.

This would imply that an HIU should not be installed with a keep-warm below 45C-50C, the threshold below which Legionella starts to grow, and above which it starts to die, unless any growth can be periodically killed by exposing to temperatures of 60C.

While water authorities and approval bodies are just waking up to the implications, the HIU testing standards should make it possible to determine that an HIU does indeed provide suitable protection. WRAS themselves do not perform running tests to demonstrate effectiveness.

We can see two forms of protection, however the steering committee should take independent advice from Legionella experts:

  • A periodic Legionella cycle to 60C
  • Keep-warm mode turn off after length of inactivity (3-5 days)