I have extended my house from 4 bed detached house to 6 beds with 4 bathrooms. I have also replaced all the windows and upgraded the CH to all underfloor, both upstairs and downstairs. 3 manifolds, each serves 6 zones. both DHW and CH are currently supplied by 28cdi Worcester combi boiler that was installed 1999 which I am thinking of replacing with system boiler and tank as I led to believe that is the most efficient heating system; is that true? if so what boiler type I need and what size.