I am adding radiators to my 2-pipe pumped, vented central heating (UK). The whole piping is copper 22mm and microbore, and I'm extending it with 15mm pex barrier pipe graded for central heating. I know that the first bit of pipes connected to the boiler must be copper, which is the case. However, I couldn't find whether there is a minimum distance between the boiler and plastic pipes merely passing near to it, without being directly connected to it. Is there such a thing? Any other comment on what I'm trying to do is welcome!