Do Expansion Vessels Go gradually?

Discussion in 'Plumbers' Talk' started by richhand, Nov 25, 2020.

  1. richhand

    richhand Member

    I'm thinking that there would be signs of an expansion vessel diaphragm degrading very gradually over time with loss of pressure becoming more apparent gradually too. But can they simply fail suddenly?
  2. quasar9

    quasar9 Screwfix Select

    Loss of pressure by itself is not indicative of a issue with EV. It all depends on how the loss occurs. Classic is the PRV opening because a failed EV cannot absorb the expansion of water which increases the pressure over 3 bars. That said sudden failure is not common as usually the rubber diaphragm perishes over time but anything can fail suddenly, especially if there was a manufacturing defect.
    Astramax likes this.
  3. rogerk101

    rogerk101 Screwfix Select

    There are no signs of a gradual decline. It's like a puncture. One day the EV works and the next it got a puncture. The puncture might be small and take a day or even more to 'go flat', but you wouldn't be able to see that without daily tests for water on the air side (not recommended). The first real sign is, as @quasar9 said, when your pressure relief valve releases water.
    kiaora likes this.
  4. kiaora

    kiaora Screwfix Select

    Good explanation!
    Well done ..
    rogerk101 likes this.
  5. There are two ways an expansion vessel can loose its internal pressure. One is that the diaphragm can split or pinhole, the other is that the Schrader valve lets the air out
    To determine which is which you can do the following
    Take off the dust cap on the valve and put a pressure gauge on the valve (bicycle pump with gauge, car tyre gauge etc) to check what the pressure in the vessel is "live"
    (It should be the pressure of the incoming mains water). Take off the gauge and press the valve. If water comes out of the valve, the diaphragm is damaged and the vessel needs replacing.
    Next, turn off the mains water (either near the cylinder or the mains house stop cock) and draw off an amount of hot water until it stops running. Check the pressure of the vessel again, it should be between 1 and 3 bar depending on your system pressures.
    If the pressure is below this, you can top up the pressure to the correct level with a car or bike pump. To check for Schrader valve leakage, drop a small amount of non corrosive fluid into the valve, if it starts to show a small bubble, then the core of the valve needs replacing. replace and re-pressurise the vessel.
    Close all hot taps, make sure nothing that you have been working on has been left open, and open up the stop cock.

    Diaphragms even if they split suddenly, will not cause any issues as such, other than they fill up with water, and then the PRV will start to drip water through the tundish as the expanding water cant go anywhere and the pressure climbs and opens the PRV. However, the pressure relief valve on the cylinder is set to open at between 6 and 10 bar, so each time the PRV (on the cylinder T/PRV discharges, the entire water system is subject to that pressure, this can happen each time the system comes up to temperature if there is no draw off from a hot tap) The loss of air from the vessel should be repaired as soon as possible. Keep checking your tundishes!

    There is an in-tundish water alarm unit that has recently come onto the market that will give you an audible warning that there is something wrong, so takes the hassle out of checking the tundish.
  6. David Goodwin

    David Goodwin New Member

    I discovered the other day that you can get replacement diaphragms for expansion vessels. About £10.
  7. dcox

    dcox Screwfix Select

    How do you put them in? I thought the vessels were sealed.
  8. David Goodwin

    David Goodwin New Member

    I haven't done it but it looks like you unbolt the ring around the inlet and swap the balloon looking thing. Although the vessels are not that expensive I can imagine there would be faffing about with pipes and wall brackets if you had to replace the whole thing. So if mine is playing up (I am currently monitoring it because I got a bit of Tundish action the other day) I will try the replacement diaphragm route.
  9. cleggie

    cleggie Active Member

    you can also fit an external expansion vessel instead of the built in one.Plenty of info on the internet and youtube

Share This Page