Randy G. wrote:There seems to be a lot of confusion here. There are different pressures at different times in different places in different machines. In a HX machine the boiler will normally run at about 1.2BAR +/- . The heat exchanger will run at about 9BAR during the extraction.

Now, in a single boiler (like Silvia) the boiler will idle at various pressures but it should be around 1.2BAR or so when it is just sitting around MOST of the time. But, when the boiler is quite full and the heating element is on, the pressure will build. That makes sense because it is a sealed chamber. There has to be some sort of safety and that is the job of the over pressure valve, sometimes called the pressure relief valve. BUT, in that same single boiler machine that same valve also controls the brew pressure (the maximum pressure allowed to hit the coffee). So at no time can the pressure in the boiler exceed the setting of the pressure relief valve.

Some of that is explained in my article HERE