High combined chlorine level won't budge

[Carolina]

After dealing with an algae problem, I now have a free chlorine level of 7, and a combined chlorine level of 14. In an attempt to get rid of the combined chlorine, I shocked 3 times in one week - but I can't get that CC to move. Strangely enough, the combined chlorine/free chlorine has remained elevated for 2 weeks now, even though the algae is long gone and I have not added any more bleach. I had assumed these numbers would start drifting down by themselves.

My water temperature is 71, but I really don't think that is cool enough to affect the chlorine levels. Can anyone suggest a way to get the CC level down?

I should also add that these chlorine levels are from my local pool store. They use a spectophotometer, but not sure if they employ DPD or ortho -Tol.

[chem geek]

I'm not sure I trust their numbers. Has your pool been exposed to sunlight at all (this time of year the sun is lower in the sky and it may be cloudy where you are)? Also, what is your CYA level? If it's really high then yes, it is possible to not breakdown the combined chlorine very quickly because with high CYA your disinfecting chlorine level will be low so the rate of breaking down the combined chlorine will get slowed down proportionately.

See if you can get a good test kit, such as Ben's or the Taylor K-2006. You can then know more than your pool store.

Worst case, if you can't get rid of the CC through normal means (high FC chlorine shocking and sunlight) you can use a non-chlorine shock (potassium monopersulfate, KPMS), but that's an expensive last resort and I don't think it's needed in this case. Also, using KPMS will register as CC for at least a couple of days (though there's another expensive Taylor reagent that will remove the KPMS interference on the CC test).

Richard

[Carolina]

We've been having sunny weather, and the pool has been getting the normal amount of exposure for this time of year. Cyanuric acid has been running 25 -30, and that is also the norm for this pool - thus my concern about those chlorine levels. Maybe I should invest in a kit myself, as who knows how accurate that pool store data is. Thanks!