sigh.... really?
Combination boilers
no storage tank, water and heating are heated directly from boiler via cold mains.
Sealed system boilers (unvented)
A system boiler (also known as a closed or sealed system boiler) works on the principle of stored hot water. This means that you will need an airing cupboard or other space for the hot water storage tank.
Open vent and open vent sealed system boilers
An open vent boiler is the existing system in most people’s homes. This type of system uses two water tanks, both located in the loft, and a hot water storage cylinder in the airing cupboard so it is very space hungry. One tank draws water from the mains supply and feeds the storage cylinder which, when heated by the boiler, can release hot water to taps all over the house.
....or perhapes you could explain.
So how do you suppose you measure the pressure on a sealed system boiler then? How do you fill the CH loop on a sealed system boiler?
Oh, thats right, a filling loop and a pressure gauge.
Its not just combis that have a filling loop.