Unvented Hot Water Systems are more commonly used in Europe and the US. It has only become legal to install in the UK since 1989.
The main advantage of the use of Unvented Hot Water is you can easily have the use of several outlets working at the one time ie Kitchen Tap, Bath and Shower all running at once.
The water flow rates available from an unvented system are in most cases double that available from even the largest of Combi Boilers.
One of the main advantages of Unvented Hot Water is that the Cylinder doesnt have to be fed in the conventional way of having a cold water tank in the loft feeding your hot water cylinder; this leaves all of your loft space available should you wish to carry out a loft conversion.