True "chlorine lock" doesn't happen too often in my experience. It is a real thing that can happen, but often when people think they are dealing with it, they actually aren't and they can get their pool back to normal pretty easily. Over-stabilized water is often the main cause of true chlorine lock. To break chlorine lock, it can help to partially drain the pool and refill it. This helps to adjust pH and dilute CYA levels in your pool pretty effectively, and from there it's easier to effectively shock the pool and get levels back to normal. Using unstabilized chlorine tends to work best. Make sure to do this at night. The best preventative measure is simply testing your pool water regularly. Instead of testing it weekly, try to test it daily or at least every other day.
In my time working with private pools, I've seen chlorine lock crop up more often than you might think. Main triggers include improper balancing of chemicals and irregular maintenance routines, which can cause combined chlorine levels to rise or CYA (cyanuric acid) to reach saturation. Particularly during hot seasons or after heavy pool use, these factors kick in heavily, as increased swimmer load and sun exposure chew through free chlorine, leaving the pool vulnerable. When it comes to fixing a chlorine lock, the first step is always to test the water to confirm that we're truly dealing with a lock, and not just a simple imbalance. If tests show that combined chlorine levels exceed free chlorine, or CYA levels are off the charts, I start with partially draining the pool—about 20-50% depending on how severe the imbalance is. Then, I add a non-stabilized chlorine shock to help break the bond between the combined chlorines. This is followed by thorough aeration; installing air stones in the pool works great to circulate and oxygenate the water, accelerating the process. After 24-hours, re-testing ensures we're back on track. To prevent future incidents, I set up a maintenance routine for every client based on their specific pool usage and environment. Regular shock treatments—once a week during active use—and careful monitoring of CYA levels are crucial. Keeping free chlorine levels steady, usually between 2-3 ppm, depending on the pool’s exposure to sunlight, also helps ward off potential issues. Weekly or bi-weekly testing, more frequent during peak season, helps nip problems in the bud before they can develop into a full-blown lock. Just remember, consistent care is your best defense against the hassle of dealing with a chlorine lock.