Replacing iron pipes with copper

TomTA

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.

Jamie Hall

You will have an additional head loss. Roughly 4 times the loss between the two copper sizes, and -- depending on the condition of the iron pipes -- any where from say twice the loss to 6 times the loss for the bigger iron pipe. This may throw balancing off, but otherwise -- assuming the pump(s) can handle it, should be OK.