Hand wash units. Storage and instaneous water heaters. Large capacity water cylinders. Water boliers. Vented and unvented by Redring, Heatrae,Dimplex, Zip, Hyco.
Unvented water heaters store water at the incoming mains pressure, which is good in terms of offering high flow rates and permitting use of normal taps, but pressure relief valves are always needed and sometimes other accessories must be fitted as well.
Vented water heaters are low pressure, gravity fed devices, with the storage tank open to the atmosphere. This generally makes for simpler installation since excess pressure build up is not a concern, but there is a catch in that water pressure will be lower, there are more restrictions on siting and - depending on the product - it may not be possible to use normal taps.