What hot water system do I have

Typically if you have a cold water storage tank in the loft space and a hot water cylinder in an airing cupboard, you will have a conventional open vented Gravity fed system. The water is heated either directly by an immersion or indirectly by a boiler. Mains fed systems can include a combination boiler, typically a wall mounted unit where cold water is heated on demand. An unvented system is a pressurised cylinder (unvented) fed and pressurised by the incoming mains, the stored contents are heated (various methods) and hot water pushed out by the force of the incoming mains on demand.