All your existing control system will be scrapped as well as your hot water tank. Everything needed including the pump will be provided inside the combination boiler. This answer assumes you mean a combination boiler(sealed heating system and instantaneous hot water)
David, October 2010
can't have pump
p bonney, September 2010
your combi's comes with a self contained pump ,and a diverter valve to change from hot water to central heating
wirecutterman, September 2010
edit
I can answer this question...
Changing to combi from gravity feed do I need a new pump?
