I'm hoping to put a new radiator in my central heating system. Some of the radiators have microbore piping leading to the valves. When I went into the cellar to check where I could run the pipes to the new rad I noticed that the microbore coming from the rads was joined to 15mm pipe. Is there any reason to do this? why not just use 15mm pipe throughout the system? any help with this would be appreciated, so that I can buy right pipe etc before I start grovelling around in the cellar putting pipework