someone told me it's acceptable to use black pipe for hydronic radiators. i always thought you need galvanized steel pipe. is this true?
Copper is more commonly used but just as steel is used in boilers it can be used in piping and galvanized pipe is not necessary--the key is no oxygen supply for the water. So long as the hydronic system is closed and it is heated to 180 F initially to drive out dissolved gasses you will not have a problem corrosion problem. Personally I prefer to also use a boiler additive that adjusts pH and includes an oxygen getter but that is considered by many to be like belt and suspenders--not really necessary.
commercial building use black on hot water and steam to heat fan coils and perimeter,but copper is less labor and easy to lay down...plus the leak situation can be more riskier... nice clean copper joint soft soldered and you'll bang the job out..cutting pipe fittings turns to line up ...who needs it