Why Are Systems With Unvented Cylinders Better For Some Homes Than Combination Boilers?

Boilers which use unvented cylinders as part of their heating and hot water system are able to provide both heat and hot water to a household at a relatively low cost. They are one of the most popular types of boiler system that are available, along with combination boilers. But what makes boiler systems with unvented cylinders so popular, and why are they better than combination boilers for some people's houses? ..»