Recharge in expansion vessel

Discussion in 'Plumbers' Talk' started by Hunters123, Jul 16, 2016.

  1. Hunters123

    Hunters123 New Member

    Question about pressure in expansion vessel(white)on unvented system. Was recharged to 3 bar
    5 months ago when new release valve was fitted, system serviced today and pressure in expansion vessel was under 1bar . This would mean it would have lost all its charge in approx 6 months. Is this normal?? Having to charge up at least twice a year?? Or new vessel needed??
    Thoughts appreciated
     
  2. Hi Hunt.

    Where is this expansion vessel - is it in a combi boiler or elsewhere in a system - eg a pressurised hot cylinder?

    That'll have some bearing on whether 3 bar is correct. Did you get that pressure from the instruction book?

    And, what side of the vessel is losing pressure - the 'air' side or the 'water'.
     
  3. Hunters123

    Hunters123 New Member

    No not in combi boiler, as previously mentioned it is the white expansion vessel which is for the hot water cylinder. The seperate expanion vessel for heating side of system is the red one which is pre charged at 1 bar. The vessel for hot water cylinder as per instructions on the vessel is charged to 3 bar. It loses pressure on air side. I realise like a car or bike tyre pressure will invariably drop over time but to lose its full 3 bar over 6 month period seemed excessive?? It is not the valve that has been checked.
     
  4. Cool.

    As you say, a 3 bar loss in 6 months is totally excessive - it shouldn't need anything other than the most perfunctory top-up every couple of years or so.

    How can the air be lost? Either through the Shraeder valve (what do you mean by "It is not the valve that has been checked."?) Has the valve been checked or not?

    The other most likely way is through a ruptured or perforated diaphragm, so the air gets out past the water. The simplest way of checking for this is to fully discharge the air side - press and hold that Shraeder valve and see if any water spurts oot. (That's how you'd test the exp vessel in a combi anyway...)

    And I guess a third possibility is via a poor seal in the vessel, say where the metal sides are joined. But I don't think I've ever heard of that happening.

    Hopefully plumbing pros will be along to advise further.

    (When you top-up a combi's exp vess, you are supposed to first fully depressurise the water side so that you get a true reading from pumping up the air side to ~1 bar. If you think about it, if the air side is empty and the water side is already at 1 bar, then the air side pressure will shoot up to 1 bar with chust a few pumps - well before it has actually been charged up the right amount. Do you know if the same applies to water system exp vessels? If so, it could be that the 3 bar reading you are getting is misleading; ie it is 3 bar at the time of testing, but that is within a tiny volume of compressed air in there.)
     
  5. Hunters123

    Hunters123 New Member

    The water was turned off and both hot and cold taps turned on to drain/de pressurise system before vessel was recharged. So would be correct reading. Not sure why losing charge over 6 month period? but guess as replacement vessel is only approx £30 might as well just replace it.
     
  6. diymostthings

    diymostthings Well-Known Member

    In my (limited) experience, expansion vessels have the cold charge pressure, working pressure and test pressure printed on the side...

    diymostthings
     
    Deleted member 33931 likes this.
  7. Sounds as tho' it was done properly, then.

    So clearly you do have an air leak. It'll almost certainly be via one of the three options I gave above.

    Faulty valve should be obvious - soapy water on the Shraeder outlet will form bubbles.

    Perforated diaphragm should be fairly obvious - water coming oot t'valve when fully discharged (but the water side should still have some pressure so's to push out any water on the 'sir' side).

    Leak via vessel body just seems unlikely.

    As you say, at £30 - and it almost certainly being the issue at fault - it makes sense to go for it.

    (Report back with the outcome, please :) )
     

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