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automatic by-pass differential valve

Discussion in 'Plumbing and Central Heating' started by veto, 10 Nov 2020.

  1. veto

    veto

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    Hoping that an expert will be able to tell me what the correct setting should be for my central heating auto by-pass diff valve should be. I am experiencing some kettling/whistleing noise from the valve and connecting pipes when the heating comes on and switches off. I have already had the valve changed about 10 months ago, so i do not think it is a faulty valve.
    the only helpful info i can provide is;
    Boiler Worcester Bosch 37cdi greenstar combination
    Flow rate 15 litres per minute. Not sure if this means 54 cubic metres per hour
    numbered settings on valve are - open 0.1 closed 0.5 bar. Or have i got these the wrong way round, i.e. open is 0.5 and closed is 0.1.
    I know that the valve must be open to get the flow going round the heating system, but not sure which is open or closed. my gut instinct tells me turning the knob clockwise is closing the valve and turning it anticlockwise is opening it up.
    I would just like someone to tell me if i am right and what should the correct bar number setting should be. I have got it set on the manufactures setting of 0.2 at the moment
     
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  3. oldbutnotdead

    oldbutnotdead

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    The flow rate refers to DHW capacity, nothing to do with the heating so ignore it.
    0.2 bar for a bypass is pretty standard.
    What is the output temperature on the boiler set to (not for hot water, for heating)?
     
  4. veto

    veto

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    Thanks for the quick reply. Not really an expert on central heating systems, but the heating is on and the temperature on the front of the boiler says 63 i presume degrees. With the setting on 0.2 bar am i right in thinking that the valve is nearly fully open.
     
  5. oldbutnotdead

    oldbutnotdead

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    System temp is fine for a combi.
    That valve is set to open when the pressure difference between flow and return reaches 0.2 bar (because all the TRVs have closed but the pump is still running).
    Do you have a room thermostat? Is it in a room where the radiator is fitted with a TRV? (favourite way to cause excessive boiler overrun)
     
  6. veto

    veto

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    I have TRVs on all radiators apart from the one in the hall which has the CH thermostat next to it on the wall. At the moment all of my rad TRVs are fully open on number 5. One thing i forgot to mention is that bypass valve is connected via the out and return flows about 4 inches above the top of the boiler, then the pipes disappear through the ceiling to the ch system. The boiler is fixed to a wall with a head clearance of about 13 inches before the flow and return pipes disappear through the ceiling. My question is " is the bypass valve too close to the boiler.
     
  7. oldbutnotdead

    oldbutnotdead

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    Ah. Yes it probably is, with such a short bypass there's a very small volume of water circulating through it so it will heat up very quickly from the residual heat in the boiler.
    Presumably you have motorised in valves on the system (otherwise the hall rad would act as a bypass), you might have to look at an overrun timer on the valves (so water can circulate through the full system while the boiler is cooling)
     
  8. veto

    veto

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    Yes i have a motorised gate valve on the hot outflow just above the bypass valve for the wet underfloor heating in the conservatory next door. I am contemplating getting a heating engineer in to have a look at the problem for me. Could i ask him to take the bypass valve off altogether ? would the heating system work without one ?. Or could i just leave things as they are ? or would i compromise the effectiveness of the heating system.
     
  9. oldbutnotdead

    oldbutnotdead

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    If thats the only motorised valve on the system (so the rest of the pipework is open between boiler and radiators) you wouldn't have this problem.
    Most modern boilers require a bypass to function properly, there's usually a minimum pipe length specified to avoid your local kettling problem (Intergas require 6 metres).
    Was this a new boiler grafted on to an existing system?
     
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  11. veto

    veto

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    It was a new combo boiler fitted 10 years ago. It replaced a pilot lit boiler that had a water tank and header tank in the loft, plus a hot water tank and pump and diverter valve in the airing cupboard. The only thing that was carried over to the closed circuit combi system was the radiators and pipework. Is the intermittent kettling noise purely down to the bypass valve being too close to the boiler ?.
     
  12. oldbutnotdead

    oldbutnotdead

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    So there's a 3 port valve in the airing cupboard? Thats where the bypass should have been put and yes almost certainly that short bypass is causing the kettling.
    Best fix- Move the bypass to the airing cupboard
    Second best-Rejig the control wiring so the boiler controls the 3 port valve (may not be possible)
    Third best- rejig control wiring so 3 port valve stays open during cooldown running
     
  13. veto

    veto

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    Cannot do any of the best fixes as when the new combi was fitted, the header tank, hot water tank, pump and port valve in the airing cupboard all went. Only the radiators and radiator pipework survived. What old pipework left in the airing cupboard is redundant and not involved in the new closed system. The existing pipework to the radiators is all under floorboards for the upstairs and hidden inside plastered walls downstairs.
     
  14. oldbutnotdead

    oldbutnotdead

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    Ah ok. Bit odd to get your problem with the TRVs all wide open and the bypass rad in the hall. Can you tell if the pump is actually running on when the flame shuts off?
    Has this kettling been going on for 10 years or is it recent? May be a case for getting someone in to have a look...
     
  15. veto

    veto

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    No the problem has only been recent since two months ago. Had the original bypass valve on for 10 years with no problem. then in January this year had problems with the old valve, so got a heating engineer in and fitted a new valve in the same place just above the boiler. Heating was turned off in May for the summer, then in September i got Worcester Bosch into do annual service on boiler. My service plan only covers the boiler only and not valves and ancillary pipework, but the Worcester Bosch engineer said that the bypass valve was incorrectly set and was nearly closed off,i.e. 0.4 bar. So he opened it back to 0.25 bar. So when the heating was turned back in October that is when the intermittent whistling started. I have spoken to the engineer that fitted the new valve but he is not interested in coming back and just said the valve is too close to the boiler. I wii ask another more sympathetic heating engineer to visit and have a look.
     
  16. Madrab

    Madrab

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    As suggested, if you have one radiator in the hall that does not have a TRV then theoretically you don't need a bypass as that radiator will allow the minimum flow the boiler requires. Don't get me wrong it is best practice to fit one, as that radiator could always be manually shut down. Some manufacturers recommend one as a coverall, though a number of boilers do have an internal bypass.

    What make is the ABV, some of the cheaper ones can be noisy especially if they are close to the boiler.

    A bypass is only needed if there is the possibility that the pump will be pushing against a fully closed system (when all TRV's/zone valves are shut) and the boiler can't have a min flow during pump overrun. I assume the 2 port valve you have controlling the UFH is after a tee off of the main flow pipe and there are no other valves on the system?
     
  17. veto

    veto

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    The make of the ABV is a Evolve 22mm CH auto bypass valve at £15.95, don't know if it is a good one or a cheap one. I have noticed that when the boiler stops firing i can hear a pump over run noise for a couple of minutes with a very low hissing noise from the valve area ! could be the pump over run noise ! don't know. Yes the UFH 2 port valve is a tee off of the main flow just above the ABV. Now that the ABV has been reset to 0.2 bar i now may be inclined to leave it alone and not get paranoid about the now low hissing noise. Now i understand how the system works with the TRV's and hall radiator and UFH port valve i feel a lot happier. Many thanks to oldbutnotdead and Madrab for your time and expertise in helping me out.
     
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