The pressure gauge on my Ariston Micro Combi boiler is usually quite high. There is no leak from any of the radiators but water constantly comes out of the overflow pipe. Only when the water supply to the house is switched off for any reason does the pressure fall to 0 and the boiler stops working. When the water supply is resumed, the boiler comes back on of its own accord without needing to be topped up.
Any ideas?

J Tempest, April 2008

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