I HAVE A STANDARD BOILER AT PRESENT. I NEED TO REPLACE IT BUT CANNOT RUN A 22 MM GAS PIPE THROUGH THE HOUSE AS I WILL HAVE TO RIP UP NEW WOODEN FLOORING AND NEW BUILT IN WARDROBES. IS THERE ANY WAY AROUND IT BY STILL USING THE 15MM PIPE ALREADY IN PLACE?
Not down to any regs other than those regarding pressure loss. The further the boiler from the gas meter, the larger the interconnecting pipework needs to be in order to cope with the pressure loss. The majority of boiler manufacturers do require a 22mm supply, but not all. Had you considered running the gas supply up the outside wall, into the loft space and then down to the boiler? Would eliminate the need to destroy new floors!