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Will A Differential Bypass Valve Stop My TRVs Hissing?

Discussion in 'Plumbing and Central Heating' started by holdyerplums, 16 May 2009.

  1. holdyerplums

    holdyerplums

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    SYSTEM = Glow Worm 30 cxi combi & 6 rads.

    The TRVs in the house that we have recently moved in hiss loudly at various settings. We have resorted to turning all TRVs to max (which obviously defeats the object of having them), but some still hiss, especially when the heating has been on a while.

    Will a differential bypass valve solve this? Or, would it be allowable to fully open both valves on the hall rad that doesn't have a TRV? Would this work? Why are the TRVs hissing? Any other suggestions welcome.

    Thanks
     
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  3. holdyerplums

    holdyerplums

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    Can anyone help?
     
  4. doitall

    doitall

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    Open the hall rad fully and if the noise stops, you know an DPV will work.

    If its adjustable you could turn the pump speed down a notch as well.

    Is it microbore pipes, because they are noisy anyway.
     
  5. raydar

    raydar

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    Got a ultracom cxi found radiators and pipework to be noisy, suspected a bypass would cure. Built in auto bypass in boiler but not sure how to set up so left alone. Hall radiator has no trv,s and fully open. Played with pump speed made no real difference on my system.
    Anyway finally got round to fitting hydronic plinth heater to the new kitchen I am fitting (this has no trv,s) and now as suspected the noise has all but gone in all rads.
    If its a ultracom im sure someone that knows what they are doing could adjust the auto bypass or you could just fit a external bypass to solve your prob.

    Im not a heating engineer.
     
  6. holdyerplums

    holdyerplums

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    Thanks for the replies.

    DOITALL - Will have a play with the hall rad valves. Its not microbore, however it is a reasonably powerful boiler (I think!) with only 6 smallish rads.
    How do I turn the pump down on a 30cxi?

    RAYDAR - Seems like the extra rad/plinth heater without TRVs did the trick.
    Does anyone know how to adjust the cxi's built in bypass?
     
  7. mogget

    mogget

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    It should be in the manufacturers instructions, as supplied with the boiler.
     
  8. bengasman

    bengasman

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    An external bypass is not a good idea, to put it mildly.

    Unless this cx is not he cx I have in mind, it has a bypass built in.

    The bypass would disturb the balance of the system and reduce the flow/return differential by possibly as much as 70%.
    That would leave the boiler running continuously outside the manufacturer's specification.
    It would also mean that the condensing mode would be almost completely negated, resulting in noticeably higher gasbills.

    What you are proposing is similar to pulling the handbrake to bring the speed back from 80 to 70 mph. No doubt it will work, just not a very clever way of doing it.
     
  9. holdyerplums

    holdyerplums

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    Thanks again for the replies.

    BENGASMAN - If I were to try opening both the valves fully on the hall rad (no TRVs fitted on this one), would this give me the same problems with the boiler that you described as fitting an external bypass?
     
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  11. Agile

    Agile

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    A differential bypass valve is normally only used on commercial sized installations.

    Domestic installations use an auto bypass valve. These open a little when the pressure across them exceeds the set value. When correctly set they are closed until the TRVs are mostly closed. This means that the condensing operation of the boiler is not affected most of the time.

    A fixed bypass, using a gate valve, which is so often used by cheapscate installers, does reduce the efficiency of the boiler which is why they should not be used.

    Tony
     
  12. doitall

    doitall

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    It says in the manual, bypass if required, an hall rad with no valves acts as a crude bypass, so yes if playing with the rad helps, fit one.

    Don't know what pump you have on them but there may be a speed control on the side of the motor housing.
     
  13. Agile

    Agile

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    Pump is in combi and should always be on max ( but has no adjustment anyway! ).

    Tony
     
  14. bengasman

    bengasman

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    Similar enough.

    A heating system is, or should be, balanced during which process every rad gets the flow adjusted so its heat consumption/emission is relative to its size.

    The flow is dependent on a combination of pressure and restriction; if you suddenly create a large flow increase somewhere in the system, the pressure at the other rads will drop, and they will give off less heat.

    Visualise an adjustable shower head; on the setting with few little holes, the spray will reach much further than with more or bigger holes.
     
  15. bengasman

    bengasman

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    Just looked at a ferroli manual, only biflow I had at hand.

    Pump is adjustable, and the manual states the flow should be kept above 6l/min, and nothing about keeping it always at 3.
    I would interpret that as: "set it on whatever is needed, as long as it stays above minimum flow.

    I have set new pumps in adjustable biflow boilers below 3, if only limited size system, and it has always worked flawlessly.
    Why would you need to set the pump at 3, if there are only 5 rads with a combined dissipation of 6 kw, and the ch is range rated at minimum?
     
  16. Agile

    Agile

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    The OP says its a 30 cxi.

    If this is an Ultracom 30cxi then the manufacturer's specs say that it has a plate secondary HE.

    All the combis I have ever seen with a secondary HE have said the pump should always be on maximum. Almost all pumps in combis with secondary HEs have no external speed control.

    Tony
     
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