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Green rust on water heater discharge pipe, what causes it? The tank is pretty old, almost 20 years.

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Copper oxides = Green "rust"

Water causes that, can be aggravated by failure to clean acid flux from soldered fittings, but since yours starts right up by the threads I'd guess you have some "weeping" out the valve and the threads are none too watertight (sloppy workmanship - "just a drain, no need for it to hold water, never going to be used anyway...")

Weeping can be caused by a failing valve, or by a lack of expansion tank combined with a check valve or thing that acts like a check valve (such as a pressure reducing valve or PRV) causing the pressure to rise when the tank is full of cold water, no water is used, and the cold water in the tank is heated and expands.

At 20 years old, have you changed the anode and flushed the bottom regularly, or just let it sit? You might be reaching end of life, if you have not maintained it. The oxides on the drain line don't indicate one way or the other on that, really, but it's worth thinking about anyway.

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  • I'd probably say condensation. Metal tends to have condensation first so periods of high humidity coupled with the flux serving to allow the water to stay in that area of longer periods.
    – Fresh Codemonger
    Commented Oct 18, 2020 at 20:16
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    It's attached to a tank full of hot water, and in my experience tends to be warm since it's a hunk of copper thermally and mechanically attached to a tank full of hot water, so condensation is very unlikely on this particular pipe, IME.
    – Ecnerwal
    Commented Oct 18, 2020 at 20:19
  • Even if you've maintained it, 20 years is a long time +1
    – JACK
    Commented Oct 18, 2020 at 20:41
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    Normally they just start leaking. The whole point of pressure/temperature relief valves is NOT turning into "a bomb." Beware falling for stupid internet/TV videos where all the safety mechanisms have been bypassed to make dramatic footage...
    – Ecnerwal
    Commented Oct 19, 2020 at 0:34
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    Over many years ,I have had 3 hot water tanks leak, all were pinhole corrosion drips in weld seams.
    – blacksmith37
    Commented Oct 19, 2020 at 2:15

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