I  am No plumber , but know enough that a combi boiler does not normally have a Hot water tank,(have seen them added! though!)  it generates the hot water as and when needed, taking cold water in and heating it,
the pipes with a cross feed! Quote:

well as there is no longer a "header" tank, which keeps the system topped up (radiator system!) if bleeding/removing radiators for such like as decorating behind, then the valve would be opened to allow mains water to refill the system and apply the required pressure to it! ( the small water pressure guage on boiler! , baring in mind you should not apply more pressure than required and go above the marked green line! )

no doubt Scruff will be along soon and amend my posting!  

Its the way ya read it  

When the cylinder was removed, the pipe that fed the hot water would be dry, likewise the combi would require somewhere for the hot water to go.