A long time ago Building Regulation G3 was only about unvented hot water systems and demanded that anyone who fitted such a system had to hold a qualification (G3 certificate). That's long since gone though.
Nowadays G3 covers all water heating systems (vented as well as unvented) and generally expects unvented systems to be a pre-manufactured 'package' rather than being assembled on-site from random components (so much less for the installer to get wrong). The demand for specifically qualified personnel for unvented went, but there's still the 'notification' arrangements like we're used to for part P. G3 "qualifications" are now only useful for joining a competent persons scheme to allow self-certification - and the pre-notification route is open to anyone else. As far a I can tell, notification isn't required for small unvented systems (≤15 litre) - although as with part P you should still follow the demands of building regs as far as the technical details are concerned, and of course follow manufacturer's instructions.
Does anyone notify works on a traditional vented cylinder with header tanks then ? I'm sure I never have, but luckily I've only done my own and my parents, and no one has complained.
