System boilers
A system (or pressurised sealed system) boiler uses a sealed hot water storage cylinder which is usually installed in an airing cupboard. There is no need for an expansion tank in the roof as the combination of boiler and cylinder allows you to run mains pressure hot water from more than one outlet at the same time.
Is this the boiler for me?  Yes if....
     You live in a larger home with more than one bathroom.
     Your household has hot water needs that can’t be met by a combi.
Heat only boilers
A heat only (or open vent) boiler is usually connected to two tanks in the loft and a hot water cylinder in the airing cupboard. A small expansion tank supplies water to the boiler and radiators, and a larger cold water storage tank provides the cold water feed to a hot water cylinder.
Is this the boiler for me?  Yes if...
     You already have a heat only boiler and want to replace it as inexpensively as possible.
     You want to run hot water from different sources at the same time.
Combination boilers
'Combi' boilers deliver hot water on demand and are very economical to run because they don’t use an external hot water tank. The fact they don’t take up much space and you never have to wait for hot water makes them a popular choice, especially in households where it’s not possible to have a storage tank.
Is this the boiler for me?  Yes if...
     You live in a small house or flat without many bathrooms, or want more space.
     You want to convert your loft but currently have a water storage tank in it.
     You want instant hot water in your home and drinkable water from every cold tap.