[TreeFiter]

I take care of swimming pools for a living, and for years, I've been told that it is important to maintain Calcium Hardness levels non-concrete pools because low Calcium Hardness levels will cause corrosion to the copper heater elements.  I've contacted a few different people in the industry that have made the claim that this is the case, and I've spoken to others that are both knowledgeable with regard to swimming pools and chemistry that have told me that Calcium Hardness has virtually no effect on metal components in a pool. 

So I'm interested in understanding how Calcium Hardness would change how corrosive pool water would be.  One of the reasons I started questioning this concept is because we treat hard water in residential water supplies because it is corrosive to plumbing and fixtures.  So why would it be the exact opposite in swimming pools. 

To be clear, I'm not asking about the interaction with concrete pools.  I'm only concerned with how it affects metal components like copper heater elements or steel ladders (usually stainless). 

[Furanone]

Calcium being divalent will form cross-linked insoluble scale in the form of calcium carbonate on the copper pipes and therefore prevent a barrier for the heat to be conducted by the water making the heaters much les efficient. By treating with softeners such as sodium, it helps dissolve the calcium carbonate changing to sodium carbonate and calcium chloride thereby giving more direct contact of the water with the heating elements.

On the flipside, having too little calcium is bad too since the sodium and potassium can help erode any  concrete surrounding the pool over time by the same mechanism.

[unsu]

sodium, it helps dissolve the calcium carbonate changing to sodium carbonate and calcium chloride thereby giving more direct contact of the water with the heating elements.

Few questions:

1) do you know the formula of the sodium compound used?

2) how exactly does it help to dissolve calcium carbonate? I do not understand. When Ca(2+) is present with CO3(2-), the formation of the precipitate is favoured. Na+ ions do nothing.