Condenser Boiler Pipe Diameter?

It is seen as good practice to have a combination on a 22mm gas pipe. This is not always necessary but I would recommend it as it can effect boiler performance. British gas used to refuse to take people on to a contract that had 15mm gas supply. Obviously the smaller the input of the boiler the more chance that 15mm will be fine, check the manufacturers instructions for the boile, (avail on line and see what they say)