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Often called a three-way valve. When you press the button for coffee is there an audible "click" from inside the machine just before the pump runs? If so you have a solenoid valve.This is a smiple electricaly controlled valve that directs water to the group head/steam wand when the coffee/steam button is switched on. Any water/steam (under pressure) left between the valve and the group head/steam valve is then then exhausted into the tank as it closes. The clicking sound is the valve opening and closing. In simple terms this is how the valve works; The valve comes in two main parts, the Body & the Coil. The body is essentially a metal tube containing a small slug of metal with a small rubber seal on each end and a spring (the nucleous). When the valve is in the natural closed position, it is held closed by the spring. It will usually hold back pressures of upto 10 bar. It is controlled by the coil, a black square that sorrounds the body. As the name suggests contains a load of coiled wire. When electricity is applied to the coil (usually 240v AC or 24V DC) it acts as a strong electro magnet. The magnetic force then pulls the nucleous along the tube compressing the spring thus opening the valve. When the electricity is removed the spring snaps the nucleous to the closed position once more. The valve ensures that water is started and cut of in a controlled way. What if I don't hear a click? Then you don't have a valve. Instead, inside the group head is a small insert containing a rubber bung on a spring. The spring has enough power to hold back the 1 bar pressure of the boiler, but not the 9bar pressure of the pump. When the pump runs the water pressure pushes on the bung compressed the spring. The water flows. When the pump is switched off unlike the solenoid valve the residual pressure is not relieved to the water will continue to flow until the pressure dissapates. Helpful links. Wikipedia