How much water to drain


Well-known member
Mar 17, 2010
Spring, TX
How much water do you have to drain, natural splashout, or evaporation before you see a change in levels such as CYA or CH or any other levesl that may be affected. I usually do not drain any as it is so hot here it will evaporate soon enough. My levels are fine, but about a month ago I tested and got a CYA around 40 and tested last night and got 30 doing it 2-3 times in a row. Also my CH dropped from 350-325. Nothing to worry about, but just wondering how much water loss you have before those change.



LifeTime Supporter
Nov 18, 2009
Sacramento, CA
CH and CYA are not lowered by evaporation.

As far as draining, you can assume even distribution throughout the pool so it is straight percentage. If you remove 20% of your water and refill with tap water you will have lowered CYA by 20%.

With CH it is a little more complicated because the fill water has CH in it. A good estimate would be:

  • new ppm = ( % drained * ppm fill water ) + ( % remaining * ppm pool water)

So if you have CH = 1000ppm and your fill water CH = 300 ppm and you drain 20%:

  • new ppm = ( 20% * 300 ) + ( 80% * 1000) = 860

Obviously splash out is a very small percentage of the total pool volume, so it will have a small effect over the short term.

Darkside of the Pool

LifeTime Supporter
Jul 20, 2010
Technically it is a proportion... like a percentage. Were your pool a CLOSED system (no sunlight, algaes, outgassing) if you'd remove 10 percent of your water, AND replace it with the same amount of distilled water and it should have 10 percent less of everything: TDS, Salt, Calcium, Alcalinity (correct me if wrong on that one), CYA... you name it.

But it does leave a question behind: Why did the CYA drop 25% when calcium dropped only 8%? If you're using Cal-Hypo, that would answer it...


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