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hot water tank operation - expansion "vase"


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Would appreciate some clarification of how French systems are required to work. 

We have an old vilage house with 4 hot electric water heaters (complicated story but basically due to it being 4 original houses that were combined decades ago). A single supply feeds all of them. The most recent is a large replacement tank installed about 6 years ago with an expansion "vase", which has just had to be replaced together with its safety valve. I was given to believe by the plumber (who I did not fully understand, as will become obvious!) that the near spherical "vase" has a sort of diaphragm so heated water from the tank which is completely full can expand into it (pressurising the air space above) so the tank itself won't split. Water had leaked through through the diaphragm almost filling the air space, and resulting in a slow drip from the safety valve between the tank and the "vase". 

What I am puzzled by is this:  if this is correct why there is no such "expansion vase" with any of the other heaters? And why isn't it an integral part of the heater itself? 

  

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In general, French hot water central heating systems are sealed and use the spherical expansion vessel for the same reason.

In hot water tanks, the expansion vessel is often used where it is difficult to run a drain pipe from the tank the a drainage system to get rid of the dripping water as the tank heats up.

IMO the great advantage of this system over the UK "tank in the loft" is the shower works at mains pressure, rather than the "head" (tank to shower head) pressure.

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