That valve is only to bring the boiler up to pressure and is not connected to you rads. The meter that is showing red should be showing between 1 and 2 bar. There should be a little hose( usually a flexi metal weave clad hose). If it is fitted, turn the valve clockwise with the boiler off, until the gauge rises to about 1.5 bar. If it is not fitted find it(it will be nearby) and fit it to the water main connected to the valve and the boiler inlet. It will not fit to the wrong connections.