Just re-read what i wrote, some of it might sound confusing so i'll get to the bit I suppose most of you want to know.

Older boilers were not subject to such high heat so: the primary heating element, if it has coiled pipes they are usually silvery coated copper. I have no idea what they coat it in or why they coat it. If it's a block its usually a large copper finned block at the bottom on the boiler.



New boilers: the primary heat exchanger which is either a block at the bottom or a cylindrical container by the side and quite large is either stainless steel or aluminium. The secondary heat exchanger is a stainless steel or aluminum block that looks like a brick, weighs around 2kg and is finned internally.

I will post some pictures in the future when I have figured out how.