reduce combine chol.

[robinixie]

My 20,000 gal. pool has a combined chlorine reading of 3.4 ppm (FC is 1.4 ppm). I believe I need to raise my FC by 34 ppm. If this is correct, what is the easiest way to do this?

[chem geek]

You do need to add more chlorine to shock the pool, but you should also expose the pool to sunlight as that helps break down the Combined Chlorine (CC) faster. The best way to add chlorine to the pool is with chlorinating liquid (usually 12.5% though 10% is also available) or unscented bleach (though it takes a lot more at 6%). You actually don't have to have 34 ppm FC -- the 10x rule makes the process go faster, but with sunlight exposure 20 ppm FC should be fine. Just make sure to keep the FC up high if it drops.

Richard

[robinixie]

How much bleach do you suggest?

[chem geek]

It depends on your Cyanuric Acid (CYA) level. The normal shock level for green algae or clearing CCs is 40% of the CYA level, but if your CYA is very high then this may be impractical and a partial drain/refill to get the CYA level lower may be better. If your CYA is much higher than 50 ppm, then dilution of water to get it lower might be a better option. Otherwise, you'll have to maintain a higher FC level with the higher CYA level (normal target FC is 11.5% of the CYA level; absolute minimum at all times is 7.5% of the CYA level).

One gallon of 6% bleach will raise the FC in 10,000 gallons by about 6 ppm. That's a general rule you can use. So in a 20,000 gallon pool, it would take about 6.5 gallons of 6% bleach to raise the FC by 20 ppm. Just be sure to keep the pool uncovered and exposed to sunlight during the shock process.