Got a question that may or may not be simple. When I built my house, 13 years ago, I had a local plumber help me with my heat piping. I don't know why he did what he did. When the PEX from my OWB comes into the house, it transitioned to copper to go through my flat plate HX. On the other side of the HX, he came out of it with copper and then transitioned to black iron for the main loop. Yet, every zone off that main loop (there are four heat zones and one hot water zone) are all transitioned to copper with a threaded fitting, then the rest of the zone is copper. The main loop is 1-1/4" black iron and the zones are all 1" copper.
I have never asked the guy why he used the black iron. I know all my circulator pumps are cast iron (all Taco 007) but why build some of the system with black iron and some with copper? All the black iron is rusting on the exterior, it's ugly and looks like crap.
I am planning on installing a new heat-pump electric water heater as a backup system in the spring, and as part of that work I would like to replumb the entire thing in 100% copper and get rid of all the black iron. Any reason this shouldn't be done? I have looked at dozens of photos of heating manifolds and piping systems online and most of them stick to copper pipe (although some are using more and more PEX).