Water chemistry

Calcium Hardness and Scale in Pools, With Doses

Raise calcium hardness with calcium chloride, lower it by dilution, and spot scale early, with a dose chart and a worked 20,000 gallon example.

Calcium hardness (CH) protects plaster from etching when it runs too low, and it feeds scale when it runs too high. The target is 200 to 400 ppm for plaster pools and 150 to 250 ppm for vinyl and fiberglass. Raising it takes calcium chloride. Lowering it takes dilution.

Raise CH: dose chart

About 1.2 lb of 77% calcium chloride raises CH 10 ppm in 10,000 gallons.

Use the pool volume tool if the volume is a guess. Check the label of the product too. A percentage other than 77% changes the dose.

Worked example: raise CH on a 20,000 gallon plaster pool

Readings: CH 120, pH 7.4, TA 90. Target 250 ppm.

1. Raise needed: 250 - 120 = 130 ppm.
2. Multiplier: 20,000 / 10,000 = 2.
3. Calcium chloride: 1.2 lb x 13 x 2 = about 31 lb.

Steps:

1. Split the dose into two or three parts on separate visits or hours.
2. Pre-dissolve each part in a bucket of water. Calcium chloride gives off heat as it dissolves, so add it slowly to the water.
3. Pour the solution around the deep end with the pump running.
4. Let the pool circulate, then retest CH. Pool water can turn cloudy for a short while, especially when pH and TA are high.
5. Retest pH and TA when you finish, and bring them back to range if they moved.

Keep pH and TA at the moderate end of range while you add calcium. High pH and TA with a fast calcium raise is the recipe for cloudy water and scale.

Lower CH: dilution

Calcium does not leave the pool by itself. Only dilution lowers it, and the fill water carries its own hardness.

Replace fraction = (current - target) / (current - fill water CH)

Say the fill water reads 150 ppm CH, the pool reads 600 ppm and you want 300 ppm in a 20,000 gallon plaster pool.

1. Replace fraction: (600 - 300) / (600 - 150) = 300 / 450 = about 67%.
2. Gallons to replace: 20,000 x 0.67 = about 13,300 gallons.

If you used the shortcut 1 - (300 / 600) = 50%, you would drain 10,000 gallons and land near 375 ppm. The fill water adds calcium back.

This table shows the fraction to replace to reach 300 ppm at two example fill water readings:

If the fill water is hard, you cannot go below its CH by dilution. A source at 400 ppm cannot bring a pool to 300 ppm. In that case you need a softer fill source, and local rules on water sources and discharge apply.

Where the calcium comes from

• Hard fill water, and evaporation. Water leaves, calcium stays, so top-offs raise CH over time.
• Cal hypo, which adds calcium each time you use it. See chlorine types compared.
• Pools that were filled at high CH to begin with.

Spot scale early

• White or gray crust at the waterline tile.
• Rough or sandpaper plaster.
• Scale on salt cell plates, heater exchangers and return fittings.
• Cloudy water after you raise pH or TA.

When you see it, test pH, TA and CH and run the saturation index. The saturation index tool runs the math. A positive reading above +0.3 matches scale.

What to do about scale

1. Bring the water balance back to the -0.3 to +0.3 band. Usually that means lowering pH a little or lowering TA.
2. Brush light scale on the tile line with a soft brush.
3. Use a stain and scale product as the label directs. Some pools need an acid wash. Hand that to a licensed professional.
4. Retest after a week.

Before you drain

Check local rules for where pool water may go, and for who may discharge it. Rules vary by city, county and state. Know the groundwater risk on the site. Ask a licensed professional if you are unsure whether a plaster shell or vinyl liner can handle a drain.

Mistakes

• Using the 1 - (target / current) shortcut when the fill water is hard.
• Adding the whole dose of calcium in one pass.
• Adding dry calcium chloride straight to the pool in one spot.
• Raising CH in a pool where pH and TA are high.
• Ignoring salt cell scale until output fails.
• Using plaster targets on a vinyl pool.

Safety

Follow the product label and SDS. Never mix chemicals. Add chemical to water, never water to chemical. Wear gloves and eye protection, and use a bucket that can take heat when you dissolve calcium chloride.