How long has it been running outside? The pressure is constant and is not regulated by any device. Pressure is created by filling the system via a tap or taps on the filling loop from the mains. Once the system is pressurized (normally 1 - 1.5 bar when cold) the filling taps should be turned off. The pressure only rises when the heating is on and normally rises .5 to 1 bar depending on the system. If you have constant running water, somebody has left the filling tap on. If the pressure only rises above 3 when the heating is on there is a very serious problem with the expansion vessel and this can be dangerous. However if the pressure rises above 3bar because of a defective expansion vessel it will only overflow once unless somebody re-pressurises the system. Check your filling taps you can normally here water passes through if they are not properly closed.
ac, October 2010
