JoyfulNoise is right. You can spend a lot of money and eliminate the calcium but it's only temporary. It's much easier and cheaper to just balance it out to avoid scaling. It's all about temporary hardness and permanent hardness. When the TA exceeds the CH all hardness is temporary, which means no scaling. When the TA is less than CH, both permanent and temporary hardness are present. Permanent hardness will produce scale. pH also has an effect on this, but generally speaking TA-CH=TH or CH-TA=PH
Temporary hardness is defined as the hardness that is removed (through precipitation of calcium and magnesium carbonate) after boiling the water while permanent hardness is that which remains soluble after boiling (as calcium chloride, calcium sulfate, calcium nitrate, etc. -- see
this link and
this link). In practice in pools, most of the initial hardness from fill water is "temporary" in that it is from calcium and carbonate and not from other salts such as chloride and sulfate (i.e. we usually find that high CH fill water also has high TA) while the hardness added from calcium chloride product is permanent. It is
not true that when TA exceeds the CH that there will be no scaling nor is it true that the "temporary" hardness does not produce scale. One can absolutely positively get scale by raising the TA too high even if the CH isn't particularly high. This can most easily happen in hot tubs because the water temperature is higher and we've seen scaling there if the TA is not lowered or the pH gets too high.
Again, "temporary" hardness absolutely positively can produce scale. What is "temporary" is that the hardness (calcium and magnesium) can be precipitated by boiling so is no longer measured as hardness in the water that remains. IF you do this in a tea kettle, the calcium carbonate can coat the metal of the kettle (i.e. scale).
One way to prevent scaling is to ensure that the Calcite Saturation Index (CSI) is not greater than 0 though in practice we don't normally see scale actually form until the CSI gets to around +0.7 or so in pools though in hot spas it can be seen at around +0.3 or so. The CSI does not determine the
rate of scale formation; only whether it is thermodynamically
possible. Likewise, a negative CSI says whether it is possible for solid calcium carbonate to dissolve into water (i.e. whether plaster can dissolve), not the
rate at which it dissolves. The rate appears to be more related to the pH where the rate is higher/faster at lower pH (at least for dissolving when the CSI is negative and speculatively for scaling as well when the CSI is positive).
The reason that CSI is the best scale predictor for pools is that calcium carbonate is generally the first solid to precipitate out of the water because it is the least soluble at normal CH, TA, pH and temperature in pools. Magnesium carbonate is much more soluble and we have never seen a pool that had enough magnesium to precipitate magnesium carbonate. Very rarely there are pools with very high calcium and very high sulfate that have precipitated calcium sulfate (see
The Mystery of the Pointed Crystals,
Large Crystals in My Pool, and
Sediments/Crystals on Pool Bottom). So these are examples of "permanent" hardness being able to produce a different kind of scale (calcium sulfate, in this case).
There is a lot of incorrect science on several websites such as
this one that pretends that there is this chemical called calcium hydrogencarbonate, Ca(HCO
3)
2, when in fact almost all of the calcium (Ca
2+) and bicarbonate (HCO
3-) in the water are separate ions. There is a small amount of various calcium ion pairs, but they normally represent less than 3% of total calcium. These sites show that boiling water creates calcium carbonate via creation of carbon dioxide, but what is really going on is that there is already carbon dioxide in the water because it is over-carbonated and boiling the water drives it out faster which raises the pH thereby increasing the carbonate concentration. The higher temperature also precipitates more calcium carbonate because it is less soluble at higher temperatures. The following shows the CH and TA combinations that saturate the water with calcium carbonate (CSI is 0) at 212ºF and at a pH where carbon dioxide outgassing has stopped (is in equilibrium with the air). You can see that such saturation has nothing to do with whether CH is larger or smaller than TA and obviously one could have both CH and TA be very high and would clearly scale even if TA > CH so long as the pH were not very low. To imply that 500 ppm TA with 200 ppm CH or that 400 ppm TA with 300 ppm CH won't scale at a pH of 8.0 is ludicrous.
CH ....
TA ....
pH
. 25 ... 49 ... 7.95
. 50 ... 35 ... 7.8
100 ... 26.5 . 7.65
200 ... 21 .... 7.52
I don't see the usefulness of distinguishing between temporary and permanent scale or comparing CH vs. TA as this has nothing to do with actually predicting the potential for scaling. Calcium carbonate scale can occur when the product of concentrations of calcium ions and carbonate ions exceeds the solubility product of calcium carbonate and it cannot occur when such product of concentrations is less than the solubility product.