Joining 15mm pipe to older pipe

[Rolyata]

My central heating system was installed many years ago, presumably when copper was in short supply. The pipe-work is in what appears to be steel tubing with an integral plastic coating. The metal is about 15mm diam.

It seems to be in good condition, so I'm not planning on ripping it out, but I would like to modify it a little so that I can replace a radiator.

What would be the best way to join 15mm Cu to this piping ?

The original joints were made with quite bulky looking end-feed solder fittings. Some other work has been done using compression fittings, probably at a later date.

[John Stumbles]

fairly shiny? I've come across stainless steel, but not with plastic coating. How thick, and what colour and consistency, is the coating?

Some copper pipe is plastic sheathed - it couldn't be that, could it?

Compression. If the pipe's stainless I'd use brass olives.

[Rolyata]

It's certainly not stainless steel. I assume that the plastic coating is to avoid corrosion problems.

The plastic coating is less than 1mm thick, cream coloured and is some sort of thermo-plastic. If a knife is used to slit it, it peels off dead easy.

I thought that compression fittings would be the most likely to seal properly, but just how much tolerance is there regarding the pipe size ?

I've only measured the diameter with a vernier caliper, but it seems to be about 15mm.

Can you easily get olives that are a touch bigger or smaller than normal ones for 15mm Cu if needed ?