In an earlier contribution to this discussion,
Eithera wrote:

I have a Saunier Duval ISOFAST 28 E whose pressure varies from 2.4bars
(when on) to 0.4 bars (when off for some time). I have to frequently
fill in. Sounds like the vessel is packing up. Any idea what is the
normal life of a vessel and what causes its failure? Many thanks in
advance

It's probably lost some of its air - and thus reduced its expansion
capacity - causing the pressure to rise too much, and water to be expelled
through the pressure relief valve.

With the water system unpressurised, you should have something like 0.7 bar
(10 psi) on the air side of the pressure vessel. There should be a Schrader
valve (like a car tyre valve) where you can measure the pressure and pump
more air in. If this is low, top it up to 0.7 bar and then pressurise the
water system to 1 bar when cold. Hopefully it won't then rise above 2 bar
when hot.

Note: If water comes out of the Schrader valve, the vessel has a split
diaphragm, and is shot.
--
Cheers,
Set Square
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