My mom's house has two hot water heaters (duplex). The first I replaced 7 years ago and everything is fine. The second I replaced 5 years ago and the unions are very corroded.

The two water heaters are the same make. They use the same dielectric unions. They are both bonded. The both have anode rods installed. They both use the same city water.

The only difference is the newer heater has a blinking light controller the old one has a dumb controller.

The old heaters used regular unions and were not bonded. They had lots and lots of corrosion. After the first heater didn't develop any corrosion I figured that I solved the problem. I also don't see any leaks.

What could be causing this corrosion and how can I fix it?

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  • 1
    Does the newer heater's controller require 120VAC power? Jul 29, 2017 at 16:28
  • @ThreePhaseEel No, it does not. Both controllers are only connected to natural gas. The newer controller has some diagnostic capabilities through the blinking light. (I think it's powered by the thermocouple)
    – vini_i
    Jul 30, 2017 at 11:33

1 Answer 1


The di-electric unions are supposed to have a rubber gasket to insure a water tight seal. The installer may not have gotten the gaskets correctly in place when the unions were installed, did not tighten the union nut correctly, or may have not take the rubber gaskets out before soldering thus ruining or distorting the rubber.

  • I was the installer. The gaskets were removed for soldering. Extra care was taken not to damage the plastic insulators as well. I'm not sure what constitutes tightening the nut correctly. They are not cross threaded and they are tight enough not to leak immediately. Do you think replacing the rubber gaskets is enough to stop the corrosion or does more need to be done?
    – vini_i
    Jul 31, 2017 at 14:52
  • Since the piping above the unions is clean and free of any noticeable corrosion, I have to assume that the rust build up is coming from water leaking at the unions.
    – d.george
    Aug 1, 2017 at 9:42

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