Water chemistry

Green Pool Cleanup Steps With Chlorine Doses

A step-by-step green pool cleanup plan with a shock-level table by CYA, liquid chlorine and cal hypo doses and a worked 20,000 gallon example.

A green pool clears fastest with a plan: remove the load, set the chemistry, hold a chlorine level high enough for the CYA, and keep it there until the water proves itself. Here is the sequence, with doses you can pour from.

Step by step

1. Test before you touch anything. Take FC, CYA, pH and TA. If you can, test combined chlorine too. Photograph the pool for your service record.
2. Remove the load. Skim, net, bag leaves. Dead algae and debris use up chlorine.
3. Clean the filter. Backwash or clean the cartridge, and clean the baskets. Expect to do it again during the cleanup.
4. Run the pump around the clock until the water is clear. Circulation brings algae to the filter.
5. Bring pH toward 7.2 to 7.4. Chlorine works faster at the low end of the range. Use the muriatic acid dose chart if you need a number.
6. Set the shock level at about 40% of CYA. Use the table below.
7. Add chlorine, preferably at dusk, with the pump running. Dosing at dusk limits loss to sunlight.
8. Brush walls and floor, and vacuum to waste if you can. On plaster, a stiff brush helps on black algae. Never use a stainless brush on vinyl or fiberglass.
9. Hold the level. Test FC every few hours on day one and top up to the shock level.
10. Check the overnight loss. Test FC at dusk and again the next morning. A loss of about 1 ppm or less is the pass mark many techs use.
11. Finish. Water is clear, FC holds, and combined chlorine is near the goal of under 0.2 ppm. Clean the filter once more and return to normal levels.

Shock level table

Shock FC is about 40% of CYA. This table shows 12.5% liquid chlorine to raise 10,000 gallons from 0 ppm. Subtract the current FC from your target when you do the real math.

128 fl oz is 1 gallon. The figures come from 10.2 fl oz of 12.5% liquid chlorine per ppm per 10,000 gallons.

Worked example: 20,000 gallons, CYA 50

Readings: FC 0.5, CYA 50, pH 7.8. The multiplier is 20,000 / 10,000 = 2.

1. Shock level: 40% of 50 = 20 ppm FC.
2. Raise needed: 20 - 0.5 = 19.5 ppm.
3. Liquid chlorine, 12.5%: 10.2 fl oz x 19.5 x 2 = about 398 fl oz, roughly 3.1 gallons.
4. Cal hypo, 65%: 2 oz x 19.5 x 2 = 78 oz by weight, about 4.9 lb. A cross-check agrees. 1 lb raises FC about 7.8 ppm per 10,000 gallons, which is 3.9 ppm in 20,000, so 19.5 ppm takes about 5 lb.
5. pH: going from 7.8 to 7.4 is two steps of 0.2. Each step takes 12 to 16 fl oz x 2 (the multiplier) = about 24 to 32 fl oz of 31.45% acid. Dose the first step, retest, then decide on the second.

Add the acid and the chlorine separately, with time between them and the pump running. Dissolve cal hypo in a bucket of water first, adding the product to the water. Do not put it in the skimmer.

When CYA is high

At CYA 100, the shock level is 40 ppm. That is a lot of chlorine, and a pool that high may also need dilution before the shock works well. See high CYA and what to do for the percentage to replace, and decide whether to dilute first.

Why FC drops fast at the start

While algae are alive, they consume chlorine. FC may read near zero a few hours after dosing. Keep adding until FC holds. Stopping early means the algae regrow behind you.

After the pool clears

• Return FC to the normal minimum for the CYA, about 7.5% of CYA or more.
• Clean the filter again, since dead algae go there.
• If algae return even though FC meets that minimum, look at circulation, dead spots and brushing. A phosphate test is a reasonable later step in that case.
• If combined chlorine is still high after the cleanup, raise FC to about 10 times the combined chlorine reading and hold it until it drops.

Talking to the customer

Tell the customer what you found, what you added, how long it will probably take and when swimming can resume. Safe swim levels come from the chemical label and local code. Whether the extra chemicals are billed is a pricing decision. See chemicals included or billed if you have not set a rule.

Mistakes

• Shocking before removing the debris.
• Skipping the filter cleaning, then wondering why the pool will not clear.
• Dosing the shock level from a CYA guess instead of a test.
• Using unstabilized chlorine at noon in full sun.
• Quitting when the pool looks clear. Run the overnight check first.
• Using a stainless wire brush on vinyl or fiberglass.

Safety

Follow the product label and SDS. Never mix chemicals, especially acid with chlorine. Add chemical to water, never water to chemical. Keep swimmers out until the label and local code say the water is safe.

The chlorine dose tool works out any volume and any target.