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Discuss Proper functionality of a pressure relief valve in the Central Heating Forum area at

  1. jrtrrror

    jrtrrror Member

    Hi. Just wondering how a properly functioning pressure relief valve operates.
    Does it have it's its own mechanism that detects when the pressure is above 3bar? Or does in rely on the water pressure sensor? The two don't appear to be linked in any physical way.
    Also, when a correctly operating valve does 'open', does it release pressure quickly, in a large dump of water. Or does it release the water slowly?
  2. DuncanM

    DuncanM Plumber GSR

    The prv is a spring loaded device that the water pressure forces open st about 3 bar.
  3. jrtrrror

    jrtrrror Member

    Thanks for your reply DuncanM.
    Does it release the water in a large 'dump' of water and then close. Or does it release the water slowly. Obviously, if operating correctly.
    Thanks again
  4. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    It will release as quickly as it can until correct pressure tolerance is achieved. May I ask why???
  5. jrtrrror

    jrtrrror Member

    You may
    Haha. Our boiler has been losing pressure, quite slowly over a week or so. And then, two days ago, very quickly and the boiler switched off with a E119 low pressure code. I noticed that the discharge pipe connected to the PRV was leaking at a connection. Strange, because ideally there should be no water in that pipe unless the PRV was leaking. I also, noticed that the automatic air valve was, leaking from its plastic headed screw. (It was slightly open). I repressurised the boiler, after which I closed the automatic air valve. I then sat and watched the gauge as the boiler fired up and began heating the rads. I noticed that the needle rose through the gauge quite quickly. In probably less than ten minutes, it was in the red, passing 3 bar, hovering around 3.5 bar. I went outside and sure enough the PRV discharge pipe was leaking water. Quite fast. I think I was expecting, to hear the valve open and dump a lot of water, reducing pressure to a point that the boiler would recognise a too low pressure and subsequently switch off displaying a E119 code.
    Thanks for showing interest.
    I like a good mystery and like to understand what's happening.
  6. townfanjon

    townfanjon Trusted Plumber GSR Top Contributor!!

    Not my favourite part of a boiler .
  7. shyguy2005

    shyguy2005 GSR

    Need a gas engineer in to check something else that’s causing the rapid rise in pressure when on heating.
  8. jrtrrror

    jrtrrror Member

    What are any common causes for a rapid rise in pressure?
  9. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    Yes there are common faults however we can’t discuss them because they are a safety feature of the boiler and we have a no DIY Gas Appliance Stance
  10. jrtrrror

    jrtrrror Member

    Hi shyguy.
    May I ask what makes you suggest there is a rapid rise in pressure. How long would it ordinarily take to rise from cold (actually the boiler was displaying water temp at 40c) at 1.5 to 2.5 bar on a properly functioning boiler?

    Sorry. Just trying to understand.
    My brother was ripped off by a GSE recently. Spent loads of money, without success. When another plumber came, he correctly diagnosed the problem and sorted it in no time.
    I don't want to be victim to my own ignorance.
  11. jrtrrror

    jrtrrror Member

    Also, it has been suggested that the expansion vessel may need charging or replacing. Could that be a cause of high pressure? What are the symptoms of a discharged expansion vessel?
    Ta again everyones
  12. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    There should be a steady pressure rise and then fall once heating demand is satisfied. And please know that we aren’t all rip off merchants. I think shyguy is picking up on the fact that the pressure is not being regulated and is able to rise to 3 and above which is a big no no
  13. jrtrrror

    jrtrrror Member

    Thanks Riley.

    Two questions if I may.

    Could a faulty or discharged expansion vessel be a cause.

    What constitutes a steady rise in pressure. 10-15 minutes to reach 2.5 bar?

    What actually regulates the pressure? As in, what keeps the pressure at the normal operating pressures. That's a third question
  14. Knappers

    Knappers Plumber GSR

    The pressure relief can discharge at any rate but should only do so at 3bar and over.
    Issues with the expansion vessel often lead to irreparable issues with the relief valve.
    As always faults are best dealt with professionally and quickly to avoid danger and further damage
  15. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    It could be one of a number of causes yes.

    Your second question is how long is a piece of string as everybody has central heating on at different times And there are many factors affecting it such as water temperature, hot water usage to name just a couple. Just know that if your pressure is going above three then you have a problem

    Correct system design, incorporating correctly sized expansion vessel
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