So there is an expansion valve in addition to the OVP set to 12 bars? I though that the OVP handled the expanding water. Since this machine has a rotary pump, the pressure should never get above ~9 bars (except during heat up). Why are both valves needed?

A rotary pump has a built in bypass valve to regulate the brew pressure, there is no brew pressure OPV line on a rotary. It is all internal in the rotary pump which is why they can be out-boarded under a cabinet and need nothing but a water input and output to the machine. The pressure release valve is designed to release excess pressure in the sealed pipe of the machine. As the machine heats, the cold water in the tubing/boiler/heat exchanger expands. That pressure can get very high. That valve opens at around 12 bar to allow that excess pressure to drip out. You usually only see them drip when the machine is turned on from a cold start. It also acts as an emergency release, simular to the boiler over pressure valve, just in case your pump bypass does fail for some reason and you get extreme pressure from the pump. The valve will open instead of a fitting or solenoid rupturing.

Dave, are you saying that the Duetto II does not have an OPV discharge to the drip tray, or that it does not have an OPV at all?

Its not an OPV like you would see on a Vibe pump setup.

For example.. If you had a machine, with a Vibe pump/tank, and NO OPV then you would adjust your grind to get the proper pressure/extraction time. I believe it is something like for a traditional double you would see 10BAR, and for a ristretto/single you would see about 11-12BAR. The Over-pressure, or Expansion valve regulates the flow from the Vibe pump. A Vibe pump, in it of itself, cannot be adjusted for pressure or flow rate. It puts out however much water it puts out. There is a graph somewhere buried in the forums showing different flow rates of the ubiquitous Ulka 52W pump(which is on most Vibe machines) and the Fluid-o-tec pump. The FOT has a slightly higher flow rate, IIRC. This is why we use an Over-pressure valve. It is placed AFTER the vibe pump, and depending on how you set it, "controls" the pump. As I said above, no OPV, and you rely on your grind/dose to get the rite pressure/flowrate for your shot.

What dave is talking about is something different. Its a safety of sorts, to relieve any pressure that may build up in the system.

A Rotary pump doesnt need an OPV, because, unlike a Vibe pump, a Rotary pump CAN be set to a certain pressure.

JKolmo wrote:Dave, are you saying that the Duetto II does not have an OPV discharge to the drip tray, or that it does not have an OPV at all?

If it is a rotary, it will have no discharge line at all because there is no OPV. All the magic happens inside the rotary pump. The little pipe that goes to the drip tray is an internal safety valve output, unless they did something very unusual in the Duetto.