Q:

I have some questions about hydronic heating: What is the difference between black steel pipe and galvanized steel pipe? Can copper and black pipe be mixed in the same system? Can copper pipe be connected to cast iron radiators without risk of corrosion? Is copper a good choice of tubing?

Peter Sheridan, Westmount, None

A:

Rich Groff, a heating and cooling contractor in Neffsville, Pennsylvania, replies: Black steel pipe offers no resistance to corrosion. It can be used for hydronic heat systems using recirculated water. Galvanized steel pipe is coated to resist corrosion and was generally used for domestic hot and cold water piping.

Copper and black pipe can and do co-exist in hydronic heating systems. If dissimilar metals are used in a corrosive environment, electrolysis will take place at the point where the two metals are joined. Dialectric unions provide a barrier and prevent electrolysis.

Hydronic systems normally work under low pressures. Residential applications rarely exceed 30 psi. This pressure is very forgiving when joining metals that expand at different rates when heated. However, well-seasoned cast iron will not tolerate an overtightened fitting as the tapered thread will try to stretch the casting, and a crack will result.

Copper tubing is my choice for piping. Copper pipe will not corrode in a recirculated-water hydronic heating system.