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I'm replacing pretty much everything in my boiler/DHW/radiator systems. Keeping a couple of the decorative cast iron radiators, but virtually everything else needs to be replaced. Existing piping to radiators is iron; some left over from gravity-feed days, some more modern 1.25" OD iron. I'm inclined to just start from scratch and re-pipe the radiator system with copper. Is type M OK, or should I stick with L? I can find GPM flow data and head loss data for pex and copper, but nothing for iron. Would 1" or 1.25" copper on the radiator runs cause excessive head loss compared to the existing iron? I'd like to go with 1" obviously, but since this is a one-time expenditure (knock on wood) the additional cost of 1.25" can be dealt with.