What is causing my Ariston boiler microsystem 21RFF1 to gain pressure causing the safety valve to open and shuts the boiler down - not due to overheating.

John Shailer, November 2008

I have changed the expansion vessel and it is now working perfectly

John, November 2008

There are several things that can cause this problem, does the pressure increase when the boiler is not in use? if so maybe the filling loop is letting by or your water to water heat exchanger has failed, try disconnecting the filling loop and check if the pressure still increases. If the problem is when the boiler is running then it could be your expansion vessel has failed or just needs to be re-pressurised (with air).

Martin, November 2008