Radiators with 10mm or 15mm

[mm]

I want to replace 2 of my radiators because I believe they have been sized wrong for the living room (plus they are in poor visual condition).
I reckon they are about a 3rd of the recommend BTU.
The existing ones have standard 15mm copper pipes fed from upstairs (its an old house). I have 2 new Stelrad double rads. 2 questions:

1) I want to remove the copper pipe and replace with speedfit/hepworth piping. Is it feasible to replace the existing 15mm copper with 10mm plastic, or should I play safe and stick with 15mm plastic? (the theory behind exact calculations was way out of my depth!)

2) I've heard a few scare stories about the pushfit plastic connectors being at slight risk of seperating. Not having used these before is this likely to be garbage or would you recommend compression fittings?

thanks in advance, Mark.

 

[oilman]

Do the radiators keep the room warm for most of the year? If they do, don't change just because some theoretical calculation indicates you should. Why do you want to get rid of the 15mm copper pipes? Plastic has the usual problems of plastics, and copper will last for decades. Definetly don't change to 10mm, and think again about plastic.