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Replacing iron pipes with copper
I'm going to be replacing/moving a few old decorative cast iron radiators as part of a complete hot water heating system replacement. The existing piping is a mix of 1.25" and 1.75" iron, as well as 1.5" copper. Is there any reason not to replace all of it with 3/4" copper? I don't know if the increased water velocity through the pipes would be a good or a bad thing, or whether the greater water mass in the old piping is good or bad, or even whether it makes any difference at all.