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Help! My Pool has Chlorine Lock

Chlorine has stabiliser added to it, usually in the form of Cyanuric Acid. The stabilised element is the part that stops the Chlorine from immediately being burnt off by the Sun, thus allowing it to stay in the water longer. Chlorine Lock can happen when there is too much Cyanuric Acid (stabiliser) in the water, therefore ‘locking’ the Free Chlorine and preventing it from working. It can also happen when the pH is unbalanced.

Chlorine Lock is fairly common in Pools and is not something you need to be overly worried about, but it does need addressing.

Signs of Chlorine Lock

  • A strong smell of Chlorine. This is commonly mistaken for there being too much Chlorine present, but it is in fact a build-up of Chloramines that produce this smell. There can be different reasons for Chloramines being present but more often than not it’s when you have Chlorine Lock as there isn’t enough active Chlorine available.
  • Low Chlorine reading after adding Chlorine to increase it but with no improvement.
  • Total Chlorine (TC) level being much higher than the Free Chlorine (FC) The Total Chlorine and Free Chlorine readings should be the same.

What to do

To break the Chlorine Lock, you need to reach the breakpoint. This is where a very high level of Liquid Chlorine is added to break the chemical bond of the Chloramines. This is known as ‘shocking’ the water. By breaking the bond, the inactive Chlorine level drops rapidly. If you add too little Chlorine, then the problem can worsen so its important to add enough Chlorine to reach the breakpoint.  If you can get your head around the maths, here are the equations to work out the amount of Liquid Chlorine you need.

This example is based on a 15,000-gallon pool and using 14/15% of Liquid Chlorine (typically raises ppm by 1 in 10,000 gallons of water);

Step 1. (TC-FC) x 10 = ppm e.g. 6 – 3 = 3 x 10 = 30 ppm

Step 2. 400ml x (gallons in pool/10,000) x (ppm needed/ppm dosage) e.g. 400ml x (15,000/10,000) x (30 ppm/1 ppm) this then becomes 400ml x 1.5 x 30 = 18,000ml (18,000 litres).

If you would prefer, give us a call (or your local Pool Supplier) and we can let you know the amount of Liquid Chlorine needed based on the size and depth of your Pool.

Before adding the ‘Shock’ to the Pool, the pH needs to be balanced. After ‘shocking’ the Pool, wait a few hours and test the Chlorine levels again. If there is no improvement, repeat and retest.

An alternative to ‘shocking’ the Pool is partially draining and refilling it. This may need doing several times before it comes back into balance and you must consider the type of Pool you have. A Liner Pool can’t be drained down too far as it can cause the liner to lose its fit. A concrete/tiled Pool could collapse without the pressure of water in it, depending on its age and construction. If you choose to do this method, make sure you drain it to a safe level. Its better to repeat the process several times than risk destroying your Pool altogether!

It can take a bit of work to break the lock, but it is important to do to maintain a safe and healthy Pool.