can I drain and unvented hot water cylinder?

Hi

Due to issues with pressure I'm thinking of changing from a traditional hotwater system and having an unvented hotwater cylinder installed - I am aware that this is a specialist job.

Currently I am able to turn off, drain, refill etc etc the system myself to do the odd plumbing job (change a tap or replace the bathroom) if I change to an unvented system do I need a professional each time I want to drain and refill the system? - If so can anyone recommend a good pump!

Thanks