Hello, copper would have been better, but if you add suitable water treatment,and fit electrolytic unions between the black iron any of the copper components, and Califfe can recommend the water treatment, then black iron pipe will be just fine.
Calgon makes a wide range of water treatment chemicals for boilers, they are broadly distributed, and dealer can help you specify what you need, or you can call calgon directly,
Lacking that the black iron pipe will develop pits near the connections to any copper attached to it.
Regarding back flow preventers, you need one in the city water make up line used to charge the system with water. double check valve type with vents... the check valves in the Califfe manifold are to insure water flow stays in the same direction when operating they do not protect the domestic water supply from contaminated back flow.
The double check valve back flow preventer is to prevent contaminated water from the closed heating loop from back flowing into your domestic water,
Some municipalities are stricter than others on this issue, most are strict.
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