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Thread: HR92's Knocking - Water Hammer?

  1. #1
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    Default HR92's Knocking - Water Hammer?

    Hi There

    I have just had a new system boiler installed (swtiched from combi) and installed new radiators

    The HR92's were all working fine prior to the switch.

    The new radiators all have honeywell valencia bi-directional flow TRV's fitted. Some are on return, some are on flow.

    There are still three old valves on the system from radiators that were not that old and did not need to be replaced.

    The issue is that when the heating cools or moves to off we get knocking from one of the HR92's that reverberates throughout the house and other radiators. Upon trying to isolate which radiator is causing the issue, by switching the valve fully off the knocking then moves to another radiator. (so far 3 - all the new TRV's)

    Valves are all new so I cannot imagine more than one of them are faulty, so is this:

    - An issue with valves being randomly fitted on both return and flow (didn't think this was an issue on bi-directional).
    - Or maybe faulty valve somewhere that needs a trial and error to see which one is causing the problem?

    I have read through the similar post here: https://www.automatedhome.co.uk/vbul...ghlight=hammer and it sounds like flow and return should not be an issue as they are bi-directional valves, so struggling to figure what the issue is.

    Any ideas appreciated

    Thanks in advance

  2. #2
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    Default

    That does not sound like a problem with the HR92 but more like air in the system.

  3. #3
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    Quote Originally Posted by G4RHL View Post
    That does not sound like a problem with the HR92 but more like air in the system.

    Thanks. Am working through bleeding all radiators. The bathroom rail is the onyl one that had a lot of air. Going to go through the rest and see if this can sovle the problem.

  4. #4
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    It won't be the HR92's to blame - they can't cause a knocking noise.

    However they do make very fine adjustments compared to regular manual wax pellet TRV's, and also hold their position constant for a minimum of 4 minutes after they do make an adjustment.

    That means if your system is prone to any type of water hammer effect at specific valve openings you're much more likely to experience it with an HR92 than a manual TRV as the manual value is likely to skip right over the critical opening point where water hammer occurs due to the more coarse adjustments it makes.

    Is it possible the differential pressure on your automatic bypass valve is set too high ? Or the pump speed is too high ?

    Did you previously rely on a "bypass radiator" eg one which had no TRV and always ran such as a bathroom or hallway ? Do you still have a bypass radiator or is every radiator controlled now ?

    Do you know whether you have a fixed speed pump or variable speed ? Do you know if there is even an automatic bypass valve installed in the system ?

    Unfortunately it sounds like whoever did the radiator / boiler installation has done something fundamentally wrong with the installation and there is a hydraulic issue such as pump speed/settings, automatic bypass valve setting (or absence of automatic bypass valve altogether) or perhaps even some long poorly supported/unsupported pipe runs which can be easily triggered into a water hammer effect when the pump starts/stops.
    Last edited by DBMandrake; 19th January 2022 at 10:26 AM.

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