I've read in a water heater manual, and was told by a friend, that water filtered with a water softener is bad for tank water heaters, without being given any other explanation.

On the website of a major water conditioner company however, it is said that it is only naturally scale-free water that presents a risk to water heaters, and that filtered water will not damage but instead preserve equipment.

Does a salt-based water softener damage or prevent an electric tank water heater from fonctionning optimally? If so, how does it happen, and what are the risks?


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    In my experience of one gas water heater and softener setup, NO. My softener feeds into my water heater and our last water heater lasted more than 25 years. What finally got it was rust on the outside, not any failure on the inside. We noticed that the tank was leaning because one of the feet had rusted away and thought we should replace it before it collapsed, flooding the basement and possibly ripping open a gas line.
    – FreeMan
    Sep 27, 2023 at 10:55
  • Water softeners don't "filter" water -- they soften it through ion exchange instead. Sep 28, 2023 at 4:15
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    Also: that water conditioner company is selling total balderdash -- naturally soft water has a low conductivity (low ionic content) and thus poses less risk to a tank water heater than softened water Sep 28, 2023 at 4:17
  • @ThreePhaseEel Thanks, I wasn’t sure which word to use as I don’t normally talk about this in English. I even thought "balderdash" was an actual thing upon reading your comment.
    – Mat
    Sep 29, 2023 at 19:41


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