How can I have the water running to increase pressure on my boiler?
This is how I found the solution: on vaillant eco pro filling loop pipe there is only one visible valve with a proper handle located on the left side. Follow this pipe and where it connects on the right side instead of a normal valve with a handle there is a kind of hidden screw on top of it. Turn it anti clockwise with a screwdriver to have the water running, then you can open the valve with handle on the left to top up your boiling pressure. Be careful only topup to 1.5 bars, about half of the grey area and please Don't forget to turn back all the open valves at the end to its initial position.

Hope this helps, because it took me hours to locate this hidden valve in the right side.