Calcium Hardness - Further Reading

What is Calcium Hardness?

Calcium hardness (CH) is the direct measure of the amount of calcium ions (Ca2+) in your pool water. Calcium hardness is different from total hardness (TH) or general hardness (GH) as those two parameters include magnesium hardness as well.

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

CH, 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 LSI.

Is Calcium Level Important in Plaster Pools?

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

Is Calcium Level Important in Vinyl Pools?

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

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.

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 flow rates of water) and chemical attack (from accidental pH drop). 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 and leaves the internal metal surfaces more exposed to pool water. There is no way to precisely assess the quality of this scale layer but having some CH will assure that it is present.[1]

Very high CH can lead to thick and flaky scale formation inside the heat exchanger that can be highly detrimental to it. The coils can get plugged up with scale which can lead 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 the formation and maintenance of 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 maintaining of your pools surface and equipment.

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

How to Increase Calcium in Pool Water

You increase CH with calcium chloride, sold as a deicer and by pool stores, or calcium chloride dihydrate, sold by pools stores for increasing calcium.

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

Beware of buying ice melters to increase CH.[2] 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.

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 fill water used for pool 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:

  • Of course a water softener is required and it must be of a make/model that will handle the extra load of approximately 100 gallons per day. Preferably a non-electric type and which 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 bags a year to 12 bags a year. He felt the cost increase of about $36.00 per year was a small price to pay to keep his pool water CH constant at about 200ppm to 250ppm.[3]

Calcium Reducers

Calcium reducers in a bottle do not REMOVE the calcium from your water, it binds it to prevent it from plating out on surfaces. You must keep up with the maintenance doses for it to work, until it no longer does because your calcium in the water gets high enough to overwhelm it.[4]

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