Chlorine Lock &
CYA Drain Calculator
Is your pool still green after shocking? You might have Chlorine Lock. Find out exactly how many inches of water you need to drain to fix it.
How to Use This Calculator
If your Cyanuric Acid (CYA) level is too high, it "locks" your chlorine, making it impossible to clear up a green pool. The only reliable fix is a partial water drain. Here is how to find your target drain depth:
- Enter Your CYA Levels: Input your current CYA (stabilizer) level and your target goal (usually 30 to 50 ppm).
- Provide Pool Dimensions: Enter your total volume and average depth. (Tip: To find average depth, add the shallow end depth to the deep end depth and divide by two).
- Get Your Drain Target: The engine will calculate the exact percentage, gallons, and inches of water you need to pump out of the pool.
What is "Chlorine Lock"?
Cyanuric Acid (CYA), also known as "stabilizer" or "conditioner," is essentially sunscreen for your pool. It protects your chlorine from being destroyed by the sun's UV rays. You need a little bit of it (around 30-50 ppm) to keep your pool balanced.
However, if you constantly use 3-inch chlorine tablets (which contain CYA), the CYA levels will continuously rise because it never evaporates. Once CYA crosses 100 ppm, it completely over-stabilizes the chlorine, preventing it from killing algae. No matter how much liquid shock you pour in, the pool will stay green. This is called Chlorine Lock.
Why "CYA Removers" Fail
Many pool stores sell "bio-active" CYA reducers. Unfortunately, these enzyme-based chemicals are highly temperamental. If your water temperature or chlorine level is slightly off, the enzymes die immediately, wasting your $50. Draining is the only guaranteed fix.
The Dangers of Draining
Never drain a vinyl liner or fiberglass pool completely empty. The groundwater pressure beneath the pool can cause the entire pool shell to "pop" out of the ground, or cause the liner to collapse and shrink. Always leave at least 12 inches of water in the shallow end.
Frequently Asked Questions
Can I use my pool pump to drain the water?
No, you should not use your main pool pump for a significant drain. Your skimmer sits at the top of the water line. Once the water level drops below the skimmer, your pump will suck in air, lose its prime, and the motor will quickly overheat and melt. You must use a submersible utility pump placed at the bottom of the deep end.
Do I have to re-balance my chemicals after refilling?
Yes. When you drain the water, you are also draining out your salt, calcium, and alkalinity. After you refill the pool with fresh hose water, you must run the pump for 24 hours, take a fresh water sample, and re-balance everything from scratch using our Master Water Analyzer.
How do I prevent Chlorine Lock in the future?
Stop using 3-inch Trichlor tablets as your only source of chlorine. Tablets are great for vacations, but for daily maintenance, you should switch to Liquid Chlorine (Sodium Hypochlorite) or install a Salt Water Generator. Neither of these add CYA to the water.
Ready to Shock It?
Once you have successfully drained, refilled, and lowered your CYA below 50 ppm, the Chlorine Lock is broken. Now it is time to SLAM the pool and kill the algae.