Reading all the threads on "what's the best boiler" I've learned that the most important thing is that it's fitted correctly to a flushed system. So if we take the flushed system for granted, what makes the difference between a well fitted boiler and a badly fitted one? Clearly you connect up the flow and return pipes to the corresponding pipes on the old system. Hot water connects to the old hot water pipe, and cold to the rising main; gas pipe to the gas supply. So long as it's screwed securely to the wall, what can a bodger possibly get wrong?