Took out a conventional boiler and tanks and installed a new Worcester 37CDi combi. System kept draining out till I realised that the patio was steaming. A 1 1/4 barrel heating pipe was buried under under slabs in MUD. Pipe was massively corroded then you see what happens when I tapped it with a hammer to see how brittle it was. Nightmare job now. Oh well, got to laugh about it.