Ariston Micro Combi - water constantly coming out of overflow pipe?

Constantly leakage from the overflow usually means that the pressure release valve isn't sealing properly - once activated they often do this for some reason.
If the pressure fluctuates when the heating is on, this often means that the pressure vessel inside the boiler is low on air pressure. An average air pressure inside the pressure vessel is 10 psi.
If the pressure constantly increase in your boiler, then the filling loop tap is slightly on. The average pressure for a combi boiler is between 1 and 1.5 bar.

John
April 2008