The radiators on my pressurised system have been bled, the pressure vessel set @ 1 bar system cold, with zero pressure in the heating system, pump vented, heating system re-pressurised to .75 bar.
PROBLEM; the pressure rises to 3.5 to 4 bar when operational, so the 3 bar relief valve opens. Once the system returns to cold the system pressure is below zero.
Any ideas

Den