Combination boilers combine most of the components of a conventional system i.e. boiler, cylinder, pump, control valves and thermostats into one unit so are much better for smaller houses.
The advantages of this system are:
No stored water in loft or airing cupboard, therefore saving space and no risk of freezing pipes
Instantaneous hot water twenty four hours a day, seven days a week
High pressure showers without the need for mechanical power showers
The most economical type of boiler, only pay for exactly the hot water you use
Combis can be sited almost anywhere in the house because of the enormous flueing options i.e. loft, airing cupboard, garage etc.
The disadvantages of this system are:
The main disadvantage of a combination boiler is that it primarily only serves one tap at a time, this means if someone is in the shower upstairs, and someone turns on the hot tap at the kitchen sink, whoever is in the shower will recieve reduced hot water, regardless as to how big the combination boiler is
This type of boiler is generally more suited to smaller homes with up to three independent hot water users (users over 10 years old).
Although very energy efficient, the problem with running more than one tap at a time is exaspirated if more than three people live in the home or if there is more than one bathroom