Posting a message to the forum will remove the above advertisement
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

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

  1. Tigermad

    Tigermad 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 Plumber 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?
    • Agree Agree x 2
  3. Tigermad

    Tigermad 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

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

    Tigermad Member

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

    Craig Watson Plumber 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 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 Active 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 Active Member

    Here is a example of one ABV setting.

    Attached Files:

    • Optimistic Optimistic x 1
  10. Tigermad

    Tigermad Member

    Hi folks me again. Something is not right since the bypass has been fitted. Nearly every night at 6.40pm there a crackling /gushing water sound through the lounge radiator valve. This rad is nearest to the boiler. I have the hot water cylinder programmed on my evohome system to turn off at 6.40pm so I think it’s linked to this. Any ideas why this is happening?
  11. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Air? Valve not quite open enough?
  12. Chuck

    Chuck Well-Known Member

    At 1839 the boiler is blasting away and the pump is pumping lots of hot water around the DHW loop though the cylinder.

    At 1840 and 0 seconds the HW zone valve slams shut but the pump and boiler don't instantly respond so the water flows (fast) round the path of least resistance, which is either the radiator circuit or the ABV depending on how it is set.

    By 1840 and 30 seconds the boiler power has been cut back and the pump has adapted to the new conditions and order is restored again.

    The problem with ABVs is that they detect and act on pressure differences, which are only indirectly related to the flow rate in a system with TRV's and/or a smart pump. ABVs work pretty well with 'dumb' (constant displacement rate) pumps but not so well with 'smart' (constant pressure) pumps.

    To further complicate the story, the purpose of an external ABV has changed over the years. Originally it was an energy-saving refinement to a traditional 'bypass radiator' that was always on ensure a minimum flow rate through the boiler. These days this is not usually needed but ABVs are often still used to to ensure that there is sufficient heat capacity in the circulating circulating water to avoid heat-stress damaging the heat exchanger. Some boilers, typically ones with robust stainless steel heat exchangers don't require an external ABV. Others, particularly ones with aluminium heat exchangers and low water volumes, require not just an ABV but also a minimum loop volume connecting it to the boiler.

    As always, the installer needs to read the manufacturer's installation instructions carefully and follow them to the letter.
    Last edited: Feb 11, 2019
  13. Tigermad

    Tigermad Member

    Thanks. The radiator noise was better before the bypass was fitted. Can I disable it by fully opening it?
  14. Tigermad

    Tigermad Member

    No air in the system because I have tried to bleed all rads a few times now. Although the towel rail upstairs needs constant bleeding and i guess it’s the last rad on the circuit and something weird happens with it. But this thing with the towel rail was happening before the bypass was fitted.

    Do you mean the valve on the rad or bypass? The noise is mainly on the left hand side of the radiator as the water goes in.
  15. John.g

    John.g Active Member

    You disable it by fully closing it, did you ever establish if it is in fact fitted correctly by opening it fully while the boiler is firing normally, you should then feel both flow and return pipes very hot.
Similar Threads - Automatic Bypass Valve Forum Date
Constant flow automatic bypass valves Central Heating Forum Apr 28, 2019
Pump speed/bypass/boiler output/hot water question. Central Heating Forum Apr 20, 2019