Is it possible for minerals in drinking water to solidify when high temperatures are applied? The minerals in mind are: iron, copper, aluminum, magnesium.

Background: I have small, brittle, darkish flakes appearing in my hot water system. The manufacturer of my tankless hot water heater said it's possible for these elements to solidify in this manner over time.

Looking at their website, I found this brief explanation: http://support.noritz.com/article/faq-what-is-water-hardness-46.html

"Water hardness is the measure of dissolved calcium, magnesium, and other minerals in your drinking water. It is typically measured in grains of hardness which equates to 17.1 mg/L of dissolved calcium carbonate. If not treated hard water can cause mineral build-up in your heater. Mineral build-up reduces flow and produces flakes at your fixtures."

We got out water hardness measured, and it came back as 68 mg/L.

The temperature of the water heater is set to 130F. I believe that the heater has a higher internal temperature.

I'm not sure why they are dark/black in color, all the pictures I'm seeing for calcium carbonate are white.

Thank you

  • $\begingroup$ How often do you drain your hot water heater? If you have ever done it, you will see all kinds of gunk coming out. Exactly what depends on the details of the water coming in to your house and the condition of the tank and any sacrificial electrodes in it. $\endgroup$
    – Jon Custer
    Mar 30, 2016 at 18:36
  • $\begingroup$ Hi, it's a tankless hot water heater, not sure if there is a place for them to collect in the same manner. $\endgroup$ Mar 30, 2016 at 18:39
  • $\begingroup$ Well, my tankless states to flush it every year (or sooner if you have hard water). You get gunk out... $\endgroup$
    – Jon Custer
    Mar 30, 2016 at 22:27
  • $\begingroup$ en.wikipedia.org/wiki/Hard_water $\endgroup$
    – Mithoron
    Mar 31, 2016 at 10:00
  • $\begingroup$ @JonCuster thanks I updated my question with a test result of the water's hardness. It is 68 mg/L. $\endgroup$ Apr 1, 2016 at 11:55

2 Answers 2


Limescale (calcium and magnesium carbonate) precipitates in the heater, the brown colour is most likely some iron compound that co-precipitates. A fresh surface of limescale likes to adsorb other ions, which then get trapped in it as the scale grows. If you had a water kettle, you'd see the same stuff in there after using it a few times. (Not exactly, because carbonate precipitation depends on the pressure, but iron (hydroxide?) not. I wouldn't want to predict.)

The limescale first appears on the surface of the heater cell, and then breaks off during temperature changes, i.e. when you open the faucet. As you have no tank, the flow rate of the water is high in the heater and carries the scale out of there.

Very convenient, that. In a tank, the scale builds up and can fill the whole thing after a few years, depending on your water source.

  • $\begingroup$ Thanks, do you know what could be causing the dark color? They flakes are generally black / dark grey in color (a few have brown or green smatterings). All the pictures I'm finding for calcium carbonate appear to be white. $\endgroup$ Apr 1, 2016 at 11:54
  • $\begingroup$ also I updated the question with some test results we got from city water tests, the water hardness was measured as 68 mg/L fwiw. $\endgroup$ Apr 1, 2016 at 11:56
  • 1
    $\begingroup$ It's most likely iron (or manganese). The stuff in the water heater in my kitchen has a light brown colour. The colour might vary with the pressure at which the stuff precipitates. Larger pressure will let less carbonate precipitate, so the amount of adsorbed ions (like Fe,Mn) could be higher. Thats speculation from my side now. $\endgroup$
    – Karl
    Apr 1, 2016 at 16:15
  • $\begingroup$ Ok thanks - sorry last question - if the flakes are calcium carbonate, will they dissolve in vinegar overnight? Just want a household way to test if that's what it is. $\endgroup$ Apr 1, 2016 at 17:39
  • $\begingroup$ The part that is calcium carbonate in them, definitely. $\endgroup$
    – Karl
    Apr 1, 2016 at 18:02

Your heater is set way to hot. Set it at 108° then turn your shower as hot as it goes. You can't handle any hotter. Your adding cold water to cool it down to 108. Calcium starts dropping out of solution around 125° Two birds with one stone!


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.