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Really stupid question about automatic bypass valve

Discussion in 'Plumbing and Central Heating' started by tobers, 24 Oct 2019.

  1. tobers

    tobers

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    I’m not a plumber! I want to understand how the turning of an automatic bypass valve knob is meant to work to determine if mine is weird.

    Our actual plumber recently installed an auto bypass valve to our central heating system. Previously it had a gate valve.

    He set up the bypass valve to setting 4.5 and said that, should the boiler overheat, I should open it by half a setting.

    So, lo and behold the boiler overheated. It’s a Baxi Solo 3 PFL 80. Since the auto bypass valve was fitted, it overheats maybe once a day, when the pump is running after the thermostat has reached temp and turned the boiler flame off.

    The bypass valve is just like one of these: https://www.toolstation.com/auto-bypass-valve/p28336

    So I loosened the screw on the end of the bypass valve a bit, and turned the knob anti-clockwise a quarter of a turn. But the indicator didn’t move. I figured out that the black indicator numbers move about half unit every turn. OK.

    To get a reference, I turned it anti-clockwise until it reached a stop. This was 7 full turns.

    Question 1: I assume this is “fully open” - would that be right?

    Question 2: I now turn it clockwise. Each full clockwise turn, there is a “click” (a spring-loaded twang). Is this normal?

    Question 3 etc: I turn it clockwise until I get the indicator to 4.5. This is quite a lot of turns, say 8. But if I want to now turn it back anticlockwise, it hits a stop before it gets a full turn. Is this normal? Does it only “ratchet” one way? Why did it take 6-7 full turns anti-clockwise to get to a stop originally, and now won’t even turn anti-clockwise a single turn?

    Thanks for help on this I’m sure it’s something really obvious.
     
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  3. ReJect

    ReJect

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    I can’t understand why your plumber told you to tamper with a bypass valve when boiler overheats?
    The bypass valve functions are to allow pump circulation when circuits close down entirely, or mostly.
    Also to allow the boiler to lose heat in the overrun cycle.
    And prevents excess noise.
    The valve should have been set by your plumber to operate only when most of circuits shuts off (TRVs on radiators or zone valves, etc) and on overrun.
    Not sure why your bypass valve is not now turning fully. Unless the boiler pump is operating at same time and the pressure has altered with some circuits off.
    Or valve has a fault.
     
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  4. tobers

    tobers

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    Thanks for the response. It’s a bit of a mystery to me as well. I am guessing that the boiler on overrun needs flow through the bypass valve to avoid overheating, and if the bypass doesn’t open enough there is insufficient flow and the boiler will overheat. Therefore adjusting the bypass valve slightly should allow more flow and prevent overheating on overrun.

    In any event, can you help with the questions re the way the bypass valve operates? Thanks!
     
  5. ReJect

    ReJect

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    A bypass valve is basically just a valve that uses a rubber washer under tension via a spring that is tensioned by number of turns of the valves head.
    The valve should normally be in a closed position allowing no flow through it until there is little or no flow in rest of system.
    If it bypassed continually, then your system would be out of balance and the pump would be bypassing the rest of system (short circuiting).
    So that is why bypass valves are automatic using a spring to ensure the valve pushes open when internal flow pressure reaches a certain point when system is closing down to few rads etc.
    The bypass valve should be a installed far from gas boiler to allow heat dissipation in pipework as well as circulation.
    It needs set to open when perhaps one radiator only working - as a very rough starter guide.
     
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  6. tobers

    tobers

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    Thanks ReJect.

    Is it normal for a bypass valve to turn plenty of full turns clockwise and go “twang” each full turn, but never turn more than 1 turn anticlockwise?
     
  7. Mottie

    Mottie

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    I’m not a plumber but the only valves I have on my system that 'twang' when turning them are pressure and temperature relief valves. My bypass valve doesn’t 'twang'. Looking at the picture you link to, I’m not sure you need to loosen the screw - you just turn the red knurled knob. Perhaps the twanging is the knob slipping on the shaft because you have loosened the screw? If I were you, I’d get the plumber who fitted it back in to sort it out. It shouldn’t take long to set it up.
     
    Last edited: 24 Oct 2019
  8. tobers

    tobers

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    Thanks Motman. The “twang” is definitely inside the body of the valve and not due to the knob slipping on the shaft. The screw needs loosening to enable the knob to be turned - it is a lock screw.

    The plumber will definitely be called.
     
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  10. Mottie

    Mottie

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  11. tobers

    tobers

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    Thanks for that old post.

    I have bought a new Honeywell ABV, and it definitely turns both ways, and does not twang when turning clockwise, so I think mine is up the swanny. What a drag.

    I imagine this could be causing my boiler to overheat on the overrun.

    Anyone know if it is possible to replace just the head of the valve without removing the entire valve and breaking into the water flow (ie no drain down needed)?
     
  12. ReJect

    ReJect

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    It might be possible to replace the works with identical works, but I would try to replace entire valve.
    If system is a sealed type, then the pressure can be reduced to zero by using a draincock or other drain off point and then it would be possible to do a quick swap of valve, using plenty towels.
    Get your plumber to do it as he is responsible for any new parts he supplied and installed and the precise setting of the auto bypass.
    The old valve should be replaced for free if faulty
     
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  13. raykf

    raykf

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    That "twang sounds very much like the behaviour of a pressure relief valve and NOT a bypass valve? Has it got 6 Bar or similar printed on the plastic part anywhere?
     
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  14. tobers

    tobers

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    It’s definitely an auto bypass valve - a Honeywell DU145.

    Further info...I managed to turn the valve anti-clockwise with some effort. It then loosened up and I was able to turn it all the way out, and miraculously my boiler overheat problems stopped. Turning it clockwise no longer “twangs” either. I reckon the internals of the valve went a bit wrong and whatever that is inside that tenses up the resistance spring had malfunctioned.

    So it seems the valve was set at too high a resistance which caused the boiler to overheat. My attempts to adjust the valve resulted in it only turning clockwise, so increasing the resistance further and preventing the boiler pump overrun from operating, causing it to overheat.

    Plumber coming on Monday to replace the faulty valve (hopefully free of charge!!)
     
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  15. Mottie

    Mottie

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    Sounds like it was turned fully off in the first place.
     
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