Chlorine lock/demand

  • Last Post 02 June 2021
Scott1205 posted this 28 May 2021

Hello I have a 10k gallon pool and I can't seem to hold any chlorine in it. No matter how much I put in it will all be gone within 5 days tops. Last water test showed ph 7.2 alkalinity 120 stabilizer 100.  I just replaced the filter a couple months ago. I have a pentair fns plus 36 sq ft. I backwashed last weekend and added 3.5 pounds of granular shock in an attempt to break the lock. Well here we are 5 days later with no chlorine again. This also happened to me last summer but once I added the same 3.5 pounds it was good the rest of the season. I have now done this two consecutive weekends. 7 pounds of chlorine in 2 weeks and I'm showing zero when I test 5 days later. I live in central florida so I know the sun will eat it but no one has been in the pool yet this year. I have a screen over it so not much organic material gets in. I did have some green algae a few weeks ago which seems to be gone now and I did backwash after I got rid of it. I don't know what else to do or what I'm doing wrong. 

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JCMC70 posted this 29 May 2021

Hi Scott

Based on the information you have provided, the chlorine should be gone within 5 days. 
Your stabilizer is 3 times what it should be (about 35-40ppm). As CYA levels rise, chlorine becomes much less efficient so you need more of it to sanitize the water. At a normal CYA level (35-40ppm) the free chlorine should be 4ppm daily. At a CYA level of 100ppm in a 10000 gal pool, you need a bare minimum of 8 ppm daily (the actual range is 8 minimum and 12 maximum). 
If you put 3.5 lbs. of shock in a 10000 gal pool, it raises FC 27ppm.(I'm basing this on 65% cal-hypo since you didn't list the shock you used). If you don't add anything for 5 days and the pool uses only 5ppm of FC per day, you can see that all the chlorine will be gone in that 5 day span. 
l won't get into why you are chlorinating the pool every 5 days. Chlorine levels should be checked daily. 
The bottom line is you need to reduce the CYA level in the water and the only way to do that is to drain water from the pool and replace it. I would start with about 1/3 of the water and then check the CYA levels. 

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  • Service
Service posted this 02 June 2021

JCM70 is spot on!

Thank You