Expansion vessel sizing query

Discussion in 'Plumbers' Talk' started by AndBlue, Nov 20, 2020.

  1. AndBlue

    AndBlue Member

    Hi, about replace yet another expansion vessel to unvented household water system, currently its an 18L vessel working with a 210L unvented tank. House has 2 showers each with a cascade overhead shower head as well as the usual one on a flexi-pipe, and at times the pressure does seem to fade off fairly quickly. Is there a benefit for fitting a slightly larger expansion tank - such as a 25L instead?
  2. andy48

    andy48 Screwfix Select

    1. Legally, only a G3 registered engineer is allowed to work on that system.
    2. Water expands by about 4% on heating from 0 to 100 degrees. On that basis 8.4 litres would be sufficient. However, I don't know how they quote the sizes of those vessels. I suspect it is the volume available for water expansion, but it could be the total size including the air side. 18 Litres is plenty.
    3. Is it installed properly with the pipe going in to the bottom of a vertical unit?
    AndBlue likes this.
  3. AndBlue

    AndBlue Member

    Hi Andy 48, thanks for the comments, yes all suitably installed / competent, I just provide the kit. Seems the diaphragm goes so frequently, with varying makes so wondered if it was the sizing? When you add the materials and the labour, replacing becomes quite an additional overall cost if they keep failing.

    Or perhaps it is just one of those consumables expected of an unvented system?
  4. Make sure the baloon pressure is checked annually.
    I expect that reduction in pressure accelerates failure.
    AndBlue likes this.

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