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Discuss Automatic Bypass Valve Query in the Central Heating Forum area at

  1. Tigermad

    Tigermad New Member

    I had an automatic bypass valve fitted to my unvented system yesterday and it was left at 0.4. Goes from 0.1-0.5. Anyhow I thought I would set it today according to the instructions. So I closed it off completely to th 0.5 mark and then turned all the trvs on. I then began turning it until the pipe started to get warm. Problem is the abv is in the airing cupboard and it is fitted to the pipes marked flow and return. The flow pipe is hot but the return pipe remains stone cold. Even after leaving the heating on for 10 minutes. So turning the valve makes no difference. Am I doing something wrong? Why is the return pipe stone cold?
    I just tried turning the heating off and the pipe from the ABV got red hot even though the ABV is turned off

    Ignore the non copper pipes. These are from the old system and are redundant.

  2. Craig Watson

    Craig Watson GSR Top Contributor!!

    I presume the pipe to the zone valve is the flow? If it is then the abv looks to fitted the wrong way.

    Is the abv fitted before or after the zone valve?
  3. Tigermad

    Tigermad New Member

    Do you mean the horizontal one? The horizontal one is coming downhill from the flow to return. Is this wrong? The return is on the left of the bypass and the flow is on the top (horizontal), the one with the arrow. The zone valve is attached to the return.
    The unvented hot water cylinder is in the loft so the abv is before the zone valve from the boiler but after the zone valve coming down from the cylinder.

    Last edited: Jan 13, 2019
  4. SimonG

    SimonG Trusted Plumber Top Contributor!!

    Glad it wasn't just me. Thought I'd had too much beer last night.
    • Like Like x 1
  5. Tigermad

    Tigermad New Member

    So is it right or wrong after my reply?
    • Like Like x 1
  6. Craig Watson

    Craig Watson GSR Top Contributor!!

    Sounds like it's fitted ok, unless the valve is faulty or blocked then you shouldn't have any issues setting it up.
  7. Tigermad

    Tigermad New Member

    So which pipe do I have to feel getting warmer. The very small piece on the left going to the return?
  8. John.g

    John.g Member

    The 0.1 to 0.5 settings are 0.1 bar to 0.5 bar or 1 M (meter) to 5 m.
    The purpose of the ABV is to to maintain a minimum flow rate through the boiler on pump over run and also to maintain a minimum flow rate in the event of a very low heat demand where some/all of the TRVs are throttled in.
    I presume that you were just testing its functionality with the above test with all TRVs ON (open??) and a setting of 1 M (0.1). Now the circ pump, depending on its speed setting may or may not be producing 1 M head with a large flow rate (all TRVs open) so you may not have got any flow rate due to this (pump head < 1 M) and maybe a circ pump speed setting of only 1?.
    You could look at the pump make/model & current speed setting, then assuming that you need say 0.3 M3 per hour flowrate, by looking at the pump speed curves you can see what pump head is required, for example if the speed setting is 2 then the pump head at this speed required might be 3M so you would set the ABV to 0.3, and so on, you can then test it by shutting off the TRVs and ensure it is then opening.
  9. John.g

    John.g Member

    Here is a example of one ABV setting.

    Attached Files:

    • Optimistic Optimistic x 1
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