Can I remove the header tank on an open central heating system (oil boiler) and replace it with an expansion tank. I would connect the original header tank supply to the fill side of the expansion tank. On the original expansion pipe I would fit a pressure release valve and automatic air bleeder.

By "header tank" do you mean an open tank on or above the highest point in the heating system?

By "header tank" do you mean an open tank on or above the highest point in the heating system?
Yes that is correct, an open system with open expansion tank. I want to replace the open tank with an expansion vessel, so changing it to a closed system.

The pressure relief should be screwed directly into the boiler & be verticle (both if possible). You will also need to add a pressure reducing valve (feed valve) or a low water cut-off (or both).

Ok thanks. So my idea will work. It means that my expansion vessel will be up in the attic whereas with most systems it is placed near the boiler.

Silly question but why not put it near the boiler?