Calcium Hardness - Further Reading

What is Calcium Hardness?

Measuring calcium hardness (CH) is a straightforward process that directly quantifies the amount of calcium ions (Ca2+) in pool water. It's important to note that CH is distinct from total hardness (TH) or general hardness (GH), as these encompass magnesium hardness as well.

Magnesium ion concentration is irrelevant for pool water and does not need to be included in hardness measurements.

Calcium Hardness, along with pH and TA, is one of the parameters needed to calculate the calcite saturation index or CSI. The CSI tells you the degree to which your water is saturated with calcium carbonate. More details about the CSI can be found in Calcium Saturation Index in Pool School and CSI and LSIalcium Hardness

Is Calcium Level Important in Plaster Pools?

Over time, water with low calcium levels will tend to dissolve calcium from plaster, pebble, tile, stone, concrete, and, to some extent, fiberglass surfaces. You can prevent this by keeping the water properly saturated with calcium.

Is Calcium Level Important in Vinyl Pools?

There is no need for calcium in a vinyl-liner pool, though high levels can still cause problems, mostly from the scaling of calcium carbonate.

The short answer is this: Most vinyl liner manufacturers get their starting vinyl materials from different manufacturers. Those vinyl sources all use proprietary recipes for their finished products, and none will disclose what specific compositions they use. Therefore, short of some exhaustive, reverse engineering study of dozens of different liner materials, there is no way to know precisely how much CaCO3 is used or how susceptible it would be to leaching. There is also a further complication, given that liner makers will use different material batches in the same liner fabrication. So there is no way to know what a particular liner will do.

That said, the recommended levels for vinyl are set to create comfortable water that is free of foaming. For a vinyl pool, controlling saturation balance is more important for the equipment pad (not scaling calcium in an SWG, etc.) than it is for the pool surface.

Is Calcium Level Important for Fiberglass Pools?

Calcium helps fiberglass pools resist staining and cobalt spotting. Over time, water with low calcium levels will tend to dissolve calcium out of fiberglass surfaces to some extent.

Modern fiberglass pools do not need calcium. However, if you have waterline tile with grout or a gas heater, you will need calcium in your fiberglass pool.

Is Calcium Level Important to Spas?

If you have a spa, you might want to keep CH at at least 100ppm to 150ppm to reduce foaming.

Is Calcium Level Important to Heaters?

Direct-fired gas heaters tend to build up a thin layer of calcium carbonate on their internal surfaces over time. This thin layer acts as a barrier to physical metal erosion (from high water flow rates) and chemical attack (from chlorine). However, this film is not indestructible, and it can come and go over time depending on CH and pH levels.

This does not apply to Heat Pumps, electric heaters, or solar heaters.

Low CH inhibits this film formation, leaving the internal metal surfaces more exposed to pool water. There is no way to assess the quality of this scale layer precisely, but having some CH will ensure that it is present.

Very high CH can lead to thick and flaky scale formation inside the heat exchanger, which can be highly detrimental to it. Scale can plug up the coils, leading to over-boiling of the water, banging or knocking sounds, and/or a plugged-up header/bypass. All of these can shorten the life of a heater and lead to less efficient heating. See this thread for what scale can do to a heater.

Keeping the CH around the minimum recommended value, ~ 200ppm, seems to be a reasonable compromise between forming and maintaining a thin scale layer and low risk of boiler scale.

Chronically low CH leaves the heater more susceptible to chemical attack should the other chemical levels get out of range.

What Calcium Level Should be Maintained in a Pool?

A plaster pool without a SWG should have CH levels between 250ppm and 650ppm, with levels between 350-550 being ideal for proper maintenance of your pool surface and equipment.

See Recommended Levels by Surface Type in Pool School for more about recommended levels.

How to Increase Calcium in Pool Water

Calcium increases in the pool water can occur for at least five different reasons:

• aggressive water (low CSI)
• bad plaster mixing and application - Calcium will naturally bleed out of poorly mixed plaster that also has a high amount of calcium chloride added to the plaster mix.
• adding tap water that replaces evaporated water
• adding calcium hypochlorite (powdered chlorine shock)
• adding calcium chloride to the water to intentionally raise the calcium level.

You can increase CH with calcium chloride, which is sold as a deicer, and by pool stores, or calcium chloride dihydrate, which pool stores sell to increase calcium.

In some parts of the country (the southwest, for example), high water hardness causes CH to naturally increase in pools when there is more evaporation than precipitation.

Beware of buying ice melters to increase CH. Most of the cheaper ones are salt-based. Some will increase your salt as well as your Calcium. Check the MSDS of non-pool products to find the ingredients.

Calcium products should be spread across the surface of the pool's deep end. Do not mix calcium in a small bucket of water. Calcium is exothermic and generates heat when mixed with water, which can cause burns or fires.

YouTube: Dissolving Calcium Chloride: An Exothermic Process

https://www.troublefreepool.com/threads/calcium-chloride-not-getting-hot.308676/

https://www.oxycalciumchloride.com/where-to-use/other-uses/pool-spa/

Adding Calcium Chloride May Reduce FC to 0

Over the years, there have been reports of members finding their FC at 0 after adding calcium chloride. There is no chemical explanation for this, but it is happening.

A guess is that there are probably brands of calcium increaser that use ferrocyanide salts (calcium-, sodium-, or potassium ferrocyanide) as a flow and anticaking agent. Ferrocyanides undergo reversible redox (reduction/oxidation reaction) -

[Fe(CN)6]4− ⇌ [Fe(CN)6]3− + e− ferrocyanide ⇌ ferricyanide plus an electron

That electron can be taken up by the chlorine ion (Cl+) in hypochlorous acid and converted to chloride (Cl-). Because of the reversible nature of the redox reaction, even trace amounts of ferrocyanide could deplete higher chlorine levels. The ferrocyanide will break down quickly, so it’s not a permanent issue, and the amount of iron exposure is insignificant.

Check your FC often after adding calcium chloride; add liquid chlorine if you see it drop.

How to Lower Calcium in Pool Water

You lower calcium by replacing water or using a Reverse Osmosis water treatment.

Managing High CH Levels

Some areas have high CH in the pool-fill water, such as areas in the Southwest US that get water from the Colorado River.

Water of 250 ppm CH or more out of the tap and high evaporation will lead to rising CH levels in pool water. CH levels up to 600 - 800 can be managed with careful control of water balance and the Calcium Saturation Index(CSI).

There are two practical methods to combat this continuous rise in CH:

• Periodically replace the high CH pool water with new tap (city) water by using the partial drain and replace method
• Use the soft water from the house to compensate for pool water loss due to evaporation.

One member plumbed his pool auto refill valve to allow him to select either soft or hard water as described in Water Meter Solution.

A few things to consider if using fill water from a water softener system:

• A water softener is required, and it must be of a make/model to handle the extra load of approximately 100 gallons per day. Preferably, it is a non-electric type that allows for regeneration at the same time that water usage is allowed (dual tank type design).
• Salt usage for the softener will increase. One member found his salt usage doubled from 6 to 12 bags a year. He felt the cost increase of about $36.00 per year was a small price to keep his pool water CH constant at about 200ppm to 250ppm.

Does Adding Calcium Chloride Increase Salt Level?

Calcium chloride raises the salt level equivalent to adding the same amount of salt. However, the amount of salt added from the calcium chloride is a rounding error compared to the 3000-4000 ppm of salt an SWG usually requires.

So, we can say that calcium chloride does not add a significant amount of salt and does not change the salt level of the SWG.

The chemistry for this is:

CaCl2 Molar mass: 110.98 g/mol

NaCl Molar mass: 58.44 g/mol

110.98 grams of CaCl2 provides 1 mole of calcium ions and 2 moles of chloride ions.

CaCl2 --> Ca2+ + 2Cl-

116.88 grams of sodium chloride provides 2 moles of sodium ions and 2 moles of chloride ions.

2NaCl --> 2Na+ + 2Cl-

110.98 grams of CaCl2 is equivalent to 116.88 grams of sodium chloride.

One gram of CaCl2 is equivalent to 1.053 grams of sodium chloride.

Thirty-eight pounds of calcium chloride anhydrous is equal to adding 40 pounds of sodium chloride salt.

Calcium Reducers

Calcium reducers in a bottle do not REMOVE the calcium from your water; the reducer binds to calcium and prevents it from plating out on surfaces. You must keep up with the maintenance doses for it to work until it no longer does because the calcium in the water gets high enough to overwhelm it.

It is a band-aid in place of managing your CSI and water chemistry.

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