I had a Bosch tankless natural gas water heater that stopped working with C7 and EA error codes and the plumber indicated that the unit had failed due to water getting into the internals like the fan an circuit board. The plumber said this was due to the tankless never having been descaled and that the unit should be descaled annually.

Under what situations should a tankless water heater be descaled? Does it depend upon how hard your water is? Can you add some kind of filter before the intake to reduce the need for descaling? If descaling should be done periodically, how can I determine the right frequency for my situation?


The manual will say something like

Flushing the heat exchanger with a descaling solution if mineral build up is evident. Scale build up will shorten the life of the water heater, descale heat exchanger thoroughly and repeat annually depending on mineral content of ground water.

(From Bosch 2400E NG user manual)

or maybe something like

Periodic descaling may be necessary in areas with high mineral content in the water. Scale buildup in the heat exchanger may result in lower flow rates, error codes of A7 and E9 and boiling sounds (knocking and banging) in the heat exchanger.

(From Bosch GWH 2400 ES NG User Manual)


Under what situations should a tankless hot water heater be descaled?

When mineral build up is evident on annual inspection.

Does it depend upon how hard your water is?


If descaling should be done periodically, how can I determine the right frequency for my situation?

By inspection.


Scale will always occur except in extremely unusual circumstances that only cover 10% of the human population who obtains extremely low-calcium water in extremely wet regions like the Pacific Northwest of North America.

For everyone else, scale is a fact of life. Certain minerals dissolved in the water, for instance calcium carbonate, have solubilities that decrease with temperature (odd when most things get more soluble with temperature).

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This means cold water that is flowing through any device that heats it will precipitate these bizarrely-behaving minerals onto the device's heating elements.

This is simply a fact of water physics. Harder water precipitates more material, of course, but even seemingly soft water will become overloaded with calcium after it reaches 200F in contact with the heater.

Removing the calcium before it enters the heater is challenging because there is a lot of water going through. Like thousands of gallons a month. This is why water softeners are enormous. But they work.

You need some way to visually monitor the scale. Certainly scale is problematic once it significantly reduces thermal conductivity and/or when it constricts flow rate. As for when exactly "too much scale is too much" is hard to define. Perhaps 1/4in thickness of scale should prompt doing something.

Scale can cause acoustic noises like popping, crackling, gurgling, due to poor water circulation and excessive temperatures due to poor thermal conductivity. Certainly if you hear unusual noises time to descale.

When descaling is required, use an acid-based descaler. However minimize contact between the acidic descaler and the heater because acid is corrosive to metal. Any acid solution put in your water heater should have nontoxic acid corrosion inhibitors in it. This is something I am currently researching: https://chemistry.stackexchange.com/questions/111546/simple-acid-and-corrosion-inhibitor-combination-for-descaling-steel-boilers

  • Please include an answer to the question of when to/frequency of descaling. – WilliamKF Mar 27 '19 at 16:34
  • That is a fantastic answer, what household solution, or cheap and readily available chemical would you recommend? I read your chemistry stack exchange question. I see the inhibitors are the issue in regards to your research. I can tell you that several flush kits and solutions are available. I don't know what's in them but you can try searching tankless flush kits. – Joe Fala Mar 28 '19 at 3:37
  • @JoeFala right now the retail situation for descaling solutions is insane. There is no commodity descaler, and all of the formulations are trade secrets and expensive. The Safety Data Sheets (SDS) for the products disclose usually what kind of acid they use (hydrochloric, phosphoric, sulfamic, etc) but never what else they use. The literature is absolutely horrendous, seems "PhD Factories" in India and China produce 1000s of crap papers on it every year. Any product you find that has corrosion inhibitors and acid will work, but I'll let you know once I figure out specific solutions. – OrangeSherbet Mar 28 '19 at 23:20

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