Expansion vessel?

Discussion in 'Plumbers' Talk' started by Gazowl1975, Oct 28, 2019.

  1. Gazowl1975

    Gazowl1975 Member

    Do you have to drain boiler down when recharging a expansion vessel on a combi? Thanks
     
  2. kiaora

    kiaora Guest

    Hi
    If you get someone to hold the safety valve open, you can pump up the vessel, and ensure the water is expelled from the vessel.
    If you don’t do this, the expansion vessel, may have the correct pressure, but not the correct volume of air !


    Good luck
    Peter
     
  3. Gazowl1975

    Gazowl1975 Member

    I drained it first now pal but thanks
     
  4. kiaora

    kiaora Guest

    Hi
    The trick is, even if you have drained it down, is always better to have an open end somewhere, sometimes just charging up the vessel is not enough.
    If the pressure is down, it’s (the vessel) has been replaced with water, this needs to be let out as you pump it up, water cannot compress, so the pressure will read correctly, but you won’t have the expansion available.

    I do hope I’m making sense !
    Good luck

    Peter
     
    rogerk101 and Heat like this.
  5. Gazowl1975

    Gazowl1975 Member

    Perfect sense thanks mate I recharged vessel Monday to 0.75 bar and its now holding 1.5 bar system pressure since then where as before it fell by around 0.2 bar every couple of days cheers
     
  6. Heat

    Heat Screwfix Select

    Normally you would need to recharge air to close to the water pressure (system cold).
    So if system is filled to 1 bar cold pressure, then air charge should be nearly 1 bar.
     
    rogerk101 likes this.
  7. Gazowl1975

    Gazowl1975 Member

    Well I have my boiler at 1.5 bar pressure and the vaillant manual says vessel pressure should be 0.75 bar thanks for replying mate
     
  8. Heat

    Heat Screwfix Select

    I am not sure why instructions would say the vessel should have 0.75 bar air charge to a water system of 1.5 bar!
    Less air, less expansion available.
    Interesting to note how much the system pressure will rise when system fully hot then.
    The function of an expansion vessel is to take up most of the extra pressure that the expanding water causes.

    The air charge and water charge must be set in relation to each other, lowered or raised.
    Note that the expansion vessel inside the boiler tends to be inadequate for many installs due to the actual water content of entire system. It is said to cause premature failure of the vessel diaphragm.

    I will stick my neck out on this one and disagree with the 0.75 bar.
    At 1.5 bar cold water pressure, the air charge would need to be well over 1 bar.
    Common sense would say that when system is cold there is no need for any expansion whatsoever, therefore why would you have the air side of diaphragm inside expansion vessel under much higher water pressure, crushed by the cold system water side?
    That reduces available air volume before the system is on. :)
     
    Last edited: Nov 5, 2019
    rogerk101 likes this.
  9. Gazowl1975

    Gazowl1975 Member

    I agree the vaillant ecotech manual says 0.75 bar for vessel but says nothing about the cold pressure which I just set at 1.2 - 1.5 bar because anywhere below 1bar the f22 fault code appears (low water content) and to answer your question the pressure when heating at its hottest never goes over 2.2 bar so it's probably due to me having ten rads in close proximity in a smallish house ie not excessive water content really thanks mate
     

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