Steve wrote:
Our house, built in 1950, has a one-pipe diverter tee system with
radiators (not cast iron, aluminum fin on copper, but they're not
baseboard) for our first floor heat (they used copper for the second
floor, but did it more like a two-pipe system). For some reason, it was
plumbed with a combination of cast iron and steel pipe rather than
copper. The steel pipes are now starting to rust and leak, and I need
to redo the system.


If I understand you correctly, the plumbing is a combination of copper
and steel components.

This is a classic way of having corrosion, because of the dissimilar
metals being in contact. There are special connectors that prevent the
two metals from coming in contact, but this would be practical only for
a few connections.

You may want to consider picking a single metal and being consistent.