We have a viessman combi gas boiler 100k 24kw and cant get the hot water to heat what have we done wrong?

bought a viessman secondhand but it has never been used. we needed to get the valves for it and a couple of other things like the flue, we then had it fitted by a boiler fitter, but he cant get the combi to work so as we get hot water,he says he may hav put the pipeing the wrong way.........we have heat in the rads but no hot water help...........