Why the pipe on top of a water heater can suddenly let go: galvanized steel meets copper

Ron George, who writes the code column in Plumbing Engineer magazine, describes a corrosion cell as a small battery with four parts: an electrolyte, an anode, a cathode and a return path. Water carries the current. The metal the current leaves is the one that corrodes. In the electromotive series he reprints, zinc and carbon steel sit well apart from copper, so when copper threads straight onto a galvanized nipple, the zinc coating and then the steel under it give themselves up to protect the copper. George adds that hotter water speeds chemical corrosion, which puts the hot side at higher risk.

The codes spell out the fix. Section 605.23.1 of the 2021 International Plumbing Code says copper-to-galvanized joints take a copper-alloy fitting, a dielectric fitting or a dielectric union meeting ASSE 1079, with the copper soldered to the fitting and the fitting threaded onto the steel. The 2021 Uniform Plumbing Code, section 605.16.1, also allows a copper-alloy nipple at least 6 inches long. George notes that the 2021 International Residential Code section on corrosion protection does not address dissimilar-metal joints, so the local plumbing code adopted where the house sits decides.

Manufacturers say the same. A. O. Smith's service handbook for its hybrid heat pump water heaters states that if copper piping is used, the unions at the inlet and outlet must be dielectric.

To check a heater, look at the first fitting above each tank nipple. A copper adapter threaded directly onto a galvanized nipple, with no brass or dielectric piece between them, is the joint the codes are written to prevent. George also says routine tank upkeep means inspecting the anode rod and swapping it when it is used up; A. O. Smith's handbook calls for pulling and inspecting it after six months of service and at least once a year after that.