Hard water in the pool: calcium, scale and how to keep them away
Hardness shows how much calcium the water contains. In a liner pool we usually want it at about 150–250 mg/l. Much higher than that, and the calcium settles as scale at the waterline, in the filter and on the equipment.
What calcium hardness is
Calcium hardness measures the dissolved calcium in the water, in mg/l as calcium carbonate. As long as the water can “hold” the calcium, you see nothing. But when high hardness, high pH, high alkalinity and warm water come together, the calcium comes out of solution: first as cloudiness, then as a white or greyish crust.
In a liner pool, very low hardness does not threaten the lining, but it can make the water more corrosive to metal parts, such as ladders and heat exchangers. The more common problem in Greece is the opposite: very hard water.
Where hard water shows
- Waterline: a greyish-white band on the liner and at the skimmer mouth, which hardens and holds dirt if it is not cleaned.
- Cloudy water: a milky tint, often after hot weather or after a shock with calcium hypochlorite.
- Filter: the sand clumps and the pressure rises faster.
- Equipment: deposits in the heat pump exchanger and on the salt chlorinator electrodes, which reduce their efficiency.
Islands, boreholes and water tanker trucks
Hardness depends mainly on the water source, which is why it is always tested at the first fill.
| Water source | What to expect, usually | What to watch |
|---|---|---|
| Mains water | From soft to fairly hard, depending on the area | One test at the start shows your baseline |
| Borehole | Often very hard, sometimes with iron or manganese | Water analysis before filling, risk of stains |
| Desalinated (common on the islands) | Very soft, with low alkalinity | Unstable pH; hardness may need raising |
| Water tanker truck | Depends on where it is sourced | Ask where the water comes from |
Hardness also rises during the season: water evaporates, the calcium stays behind, and every top-up brings in more. In hot, windy places where evaporation is high, the pool gets harder year after year.
How to manage it
- Keep the pH at the lower end of the range, around 7.2–7.4, and the alkalinity within limits. That way the calcium stays dissolved even in hard water.
- Use a scale inhibitor (calcium sequestrant), which keeps calcium in solution. Add it from time to time, at the dose on the packaging, usually after topping up with water.
- With hard water, choose a chlorine that does not contain calcium.
- Clean the waterline regularly, before deposits harden, with a mild product suitable for PVC; more on this under stains on the liner.
- If hardness has risen a lot, the answer is a partial water change with softer water. A liner pool is never emptied completely; see how a partial water change is done.
If, on the other hand, the water is very soft, e.g. from desalination, hardness is raised with calcium chloride, in small doses with a test in between.
Water balance
Whether scale forms does not depend on hardness alone. It is decided by a combination of four factors: hardness, pH, alkalinity and temperature. Water that is balanced in May can leave a crust in August, simply because it has warmed up. So if you have hard water and heat your pool, pay closer attention to pH and alkalinity in the hot months. When you cannot easily change the hardness, adjust the other two so the water stays in balance.
Questions
Does scale at the waterline damage the liner?
It does not puncture it, but if it is left for a long time it hardens, holds dirt and is hard to remove. Clean it early with a mild product for PVC, and never with wire wool or abrasives.
How often should I test hardness?
At the first fill, at the start of the season and about once a month in summer. In areas with high evaporation or with borehole water, it is worth testing more often.
See also
Water alkalinity
Total alkalinity in pool water: how it keeps pH stable, what range to aim for, how to raise or lower it and…
Cloudy water
Cloudy pool water: how to tell whether filtration, pH, hardness or dust is to blame, and when a clarifier or…
Liner stains
Pool liner stains: how to tell metal, organic and waterline stains apart, how to clean them without damage,…