Water chemistry

High CYA in a Pool and What to Do About It

How to confirm high cyanuric acid, work out the dilution percentage and plan the drain and refill, with a worked 15,000 gallon example.

Cyanuric acid (CYA) protects chlorine from sunlight, and it also slows it down. Once CYA climbs, you need more free chlorine to get the same result, and a shock gets expensive. The only dependable fix is replacing water.

What high CYA costs you

The target range for CYA is 30 to 50 ppm, or 60 to 80 ppm for salt chlorine generators. FC should stay at about 7.5% of CYA or more. A SLAM-style shock runs at about 40% of CYA. Here is what that looks like as CYA climbs:

A pool at CYA 100 and FC 3 reads fine on the kit, yet that chlorine works slowly. This is one reason a pool can turn green while the test shows a chlorine residual.

Confirm the reading first

The CYA test is a turbidity test. You add reagent and watch a dot at the bottom of a tube disappear.

1. Fill the tube to the sample line with pool water and add reagent as the kit says.
2. If the dot never disappears at the top mark, CYA is at or above the top of the scale.
3. Mix the sample half and half with tap water, test again and double the result.
4. Run the test twice. Turbidity tests are easy to misread, and a decision to drain should rest on a repeat.

How to calculate the dilution

Replace fraction = 1 - (target / current).

This assumes fill water with no CYA. Say a pool is at 100 ppm and you drain half and refill. You land close to 50 ppm.

Worked example: 15,000 gallon pool, CYA 100

Readings: CYA 100, FC 5, a plaster pool that has run on dichlor. Target CYA 50. Use the pool volume tool if the volume is a guess.

1. Replace fraction: 1 - (50 / 100) = 50%.
2. Gallons to replace: 15,000 x 0.50 = 7,500 gallons.
3. Drain and refill in one pass if the site allows, then let the water mix and circulate.
4. Retest CYA. Expect a reading close to 50 ppm.

Two rounds of 25% leave 100 x 0.75 x 0.75, about 56 ppm. To land at 50 in two rounds, replace about 29% each time (100 x 0.71 x 0.71 is about 50).

See the cost of waiting. At CYA 100, a shock level of 40 ppm from zero takes 10.2 fl oz x 40 x 1.5 = about 612 fl oz of 12.5% liquid chlorine, roughly 4.8 gallons. At CYA 50, shock level 20 ppm takes about 306 fl oz, roughly 2.4 gallons. The dilution pays for itself on the first shock.

Before you drain

• Check local rules. Where you may discharge pool water, and how, varies by city, county and state. Check your municipality or a licensed professional.
• Check the ground. In areas with high groundwater, draining a pool can lift or crack the shell. Vinyl liners can shift. Know the site before you pull water.
• Plan the refill. Fresh water dilutes everything: TA, CH, salt and chlorine. Retest after refill and rebalance. On a salt pool, recalculate salt (see salt pool service).
• Check the fill source. Some fill water has its own hardness or metals. Test it before it goes in.

Stop adding CYA

Dilution only helps if you stop the climb. Trichlor adds about 6 ppm CYA for every 10 ppm FC, and dichlor adds about 9 ppm. If the pool needs a feeder, move the sanitizer to liquid chlorine or cal hypo, which add no CYA. Chlorine types compared lists the side effects.

Mistakes

• Waiting for CYA to burn off. It does not evaporate or break down in the sun in any useful amount.
• Raising FC every week to chase algae without checking CYA.
• Trusting a single turbidity reading before a large drain.
• Draining without calculating the percentage, then draining twice as much as needed.
• Forgetting to retest TA, CH and salt after the refill.

When the pool is already green

A green pool at high CYA is a hard case. Raising FC to the shock level costs a lot of chlorine, and part of the work is better done by dilution first. See green pool cleanup for the sequence.

Safety

Follow the product label and SDS. Never mix chemicals. Add chemical to water, never water to chemical. Check with a licensed professional before draining a pool where structure or groundwater could be an issue.