How do I reduce the pressure on my Valiant Turbomax boiler?

My valiant tubomax 823 boiler is showing a pressure of 3 bar (in the red). I found a knob to turn which releases water out of the overflow and the pressure reduces, however as soon as I switch heating bavk on pressure increases to 3 bar again??? Pressure should be about 1 to 1.5 bar I think.