Vaillant Ecotec Plus 630 - third expansion tank failure

[WilfW]

A Q on a boiler that was installed in 2016, ran fault-free for years, in Dec 23 started loosing pressure / refilled every week > blown expansion vessel replaced under warranty.
Same symptoms again Jan 24, same fix.
The same again this week, with a little occasional rusty water coming from the underneath of the case.
I've not looked inside as it's under warranty for another 14 months, and a Vaillant engineer is coming tomorrow.
Vaillants are normally good, the Q is if this is a known fault on Ecotecs, and if so is there a fix?

 

[Johntheo5]

If it was loosing pressure and refilling it and the boiler pressure wasn't exceeding say 2/2.25 when hot then a system leak which still shouldn't have had any effect on the EV as the rubber diaphragm is at the water end so no potential EV corrosion due to "fresh" water.
Try and get the engineer to tell you exactly what has failed in (if) the EV, the engineer probably won't agree to it but better off blanking off the internal EV water connection to the system and installing a external EV, you can change it yourself then after the warranty expires.