The External expansion vessel stainless steel unvented cylinder has the pressure expansion vessel fitted on the pipe work by the cylinder which takes up a bit more space. It still gives you the same Megaflow performance of high pressure hot water when used in conjunction with a heat source and attached to the mains pressure cold water. They are available in different versions Thermsaver vertical, horizontal, solar single/twin coil, ACV Smartline and Smartline multi Energy.