A "megaflow" is just a brand name of one make of pressurised hot water cylinder, but like "hoover", "megaflow" now seems to be used for any of the several different makes of pressurised hot water cylinder.
They are just like any other hot water cylinder, except instead of being supplied from a header tank, they are fed directly (via a pressure reducing valve) from the water mains. The benefit being you get hot water at the same pressure and flow of the cold mains water, rather than the dribble of hot water from a header tank fed ordinary cylinder.
From a sparky's point of view they are exactly the same as any other hot water cylinder. They usually have 2 immersion heaters, and can be direct or indirect so can be heated by a central heating system, in which case they need the same controls and valves as any other hot water system. They do have a cylinder thermostat built in to make life a little easier.
