Can i pick your brains please . I am a week into replacing 12 combi boilers and the new are condensing boilers .The only thing id like to ask is the condensing pipe can i just use overflow pipe and can i link them up as there flats and the top floor has 4 flats ,so can just run 1 pipe and tee in then drop it down into the grid. I was going to run the 1st and 2nd floor boilers (8) into a rain water pipe with a strap on boss.Any ideas please