Can I pressurise system myself?

Hello, I have an Ariston Genus boiler downstairs, a hot water cylinder upstairs and in the roof a white Velum pressure vessel, teed off a 15mm with a release valve at the top of that.Next to that is another 15mm with another release valve.The guage on the boiler never gets above the second notch in the red '0' section and the radiators struggle to get warm most of the time. The boiler can be very noisy sometimes (like a squeeling Pig), usually on start-up. Any ideas please?