My own pool's CYA crept up to 90 and I never added a single tab

Started by tampapoolguy · · 2 replies
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#1
Kind of embarrassing since I test this stuff for a living on other people's water and forgot my own. Been topping off with liquid chlorine all summer thinking I was doing it right, but the well water I fill with around here has some cyanuric acid in it apparently, plus whatever residual was left from years ago when I did run tabs. Crept up slow enough I didn't catch it till my chlorine wasn't holding overnight and I finally ran a real test instead of eyeballing it.

Drained about a third of it out and refilled, coming back down slow. Just goes to show it's not always the tabs, sometimes it's the fill water or old habits catching up years later. Worth checking yours even if you think you know your own program cold.

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#2
CYA in fill water is a new one for me, we're on municipal here so mine's basically zero unless I put it there myself. Only reason mine stays around 30-40 is old tab residue from previous owner, been chasing that down with dilution for two years now.

Funny thing is my chlorine burns off so fast in this sun that a higher CYA almost helps some days, 100k gallons a year evaporating off will humble you either way.
#3
Ray, I'd push back a little on "higher CYA almost helps some days" because of faster burn off, that framing gets the mechanism backwards even if the outcome feels similar. CYA doesn't make chlorine last longer against sun by being generically protective at a higher level, it works by binding a percentage of the FC into a reserve that UV can't touch, so yes some chlorine survives the day, but the tradeoff is your effective free chlorine at any given FC reading is lower too, since more of it is bound up. That's the whole reason the FC/CYA chart has minimums that climb with CYA, at 30-40 you need more FC just to have the same killing power you'd get from a smaller number at CYA 20. So it's not that higher CYA helps, it's that it's masking a faster depletion problem by also weakening how much of what's left is doing any work. Worth running your numbers through the calculator on here (/pool-calculator/) just to see where your actual FC minimum should sit at your CYA, because "chlorine lasts longer" and "chlorine is working harder" are two different things and pool math tends to punish people who mix them up.