Sorry I posted this in plumbing but had to move it here. I have a question about a residential two pipe boiler heating system with cast radiators. If I wanted to replace a section, or all of it, with copper is there a need to reduce the pipe diameter or should I leave it the same? Also is it preferable to use type L or M for this purpose. Thanks

I would keep all the pipe the same type. Electrolosis can be a problem with disimilar metals. Is this hot water or steam?

Thanks for the response. It's hot water. I was going to use a brass fitting to hopefully minimize the reaction between metals. Given that would the diameters be the same? The copper would be so much easier to work with. Thanks again.

They make dielectric unions to solve the electrolysis problem and you would use the same size copper tubing as the pipe you are replacing (plumbing is sized by inside diameter so they would move the same amount of water) Type "M" is thinner wall and cheaper and used mostly for water. Mike