High CYA… calculating how much water to replace?

[AveMaria472]

So this question is for my son’s pool. He bought a house at the end of last summer. Came with a 24’ above ground with sand filter. I tested his CYA and it was sky high. Since the season was almost over, I told him we’d fix it this spring. A test today registered the CYA is still over 100. Is there a way to calculate how much he needs to drain and replace to bring it to a manageable range? For instance, if we want to reduce that reading in half, do we drain and replace 50% of the water? Something tells me it can’t be that easy to calculate. Thanks for any help!
@chem geek @Newdude

 

[kul]

Since your CYA is over 100 you need to do a diluted CYA test to get at the actual number before calculating how much water to remove.

If it was 100 and you’re trying to get to 50, you’d have to remove 50% of your water.

 

[Newdude]

+1. Reduction is an even % because the fill water has 0 CYA.

If draining a big chunk anyway, just drain most of it. You need to leave 12 to 18 inches of water or the liner may shift. Start watching it around 18 inches and stop if you notice it moving at all on the bottom. Go a little bit more and start mostly fresh.

Get the FC up upon refill and Overnight Chlorine Loss Test