There are two ways to end up with high persistent combined chlorine (CC) in your pool:
  1. Add nitrogen into the pool in a form that can react with chlorine, OR
  2. Put something in the pool that screws with the test.

In the first category are things like:
  • foamy algicides
  • granular algaecides based on ammonia
  • degraded stabilizer (ammonia, again)
  • urine or lots of sweat
  • some fertilizers
  • other stuff I haven't though about, or heard of

In the second category, there are several possibilities but "oxy-shocks" or "non-chlorine shocks" based on potassium monopersulfate stands out. MPS will test as CC, even though there's no chlorine in it, and it's completely ineffective as a sanitizer ( unlike combined chlorine).

Of course, I'm sure there are possibilities I don't know about.

But, if you can't think of any one of those that might be the explanation, it's probably best to simply hang on, and see if the problem resolves itself.

Oh, just to rule out one more possibility: exactly what test kit, and what test method are you using to test for chlorine?

Ben / PoolDoc