Colder Weather Chemistry

RexO

Well-known member
May 24, 2020
47
Melbourne, FL
Hi All, this is my first Winter for owning a pool. I test the basics ever day or every other day so I noticed right away the chlorine demand going down as the days got shorter and water temps decreased and I adjusted accordingly. At water temps at about 65 degrees and the short days (even while sunny) the chlorine demand was very low (about .3 ppm). I lowered the output to 20% and ran the SWCG to 3 hours. I was still running the pump for 10 hours per day (5 hours normal speed/5 hours low speed) It was keeping up nicely and sometimes getting above FC 6, so I would shut it off for a day or two. With family visitors coming for the holidays I bumped the Heat up to 85 degrees for a little more than a week so we could enjoy some pool days and ran the pump 24/7. I noticed the chlorine demand go up slightly, but not dramatically. I was keeping a FC level of 4-5. Then, in a 24 hour period the FC dropped from 4 to 1.5 ppm. This was at the same time I turned off the heat pump and was letting the water temp go back down. With one gallon of bleach and the SWCG running high I quickly bumped the FC up to 10 ppm and did OCLT for a couple of days. The OCLT was about 1 ppm, not really calling for a SLAM, but I didn’t think all was OK because that 1 ppm was greater than what the demand had been in a 24 hour period previously. I didn’t really want to SLAM so I just kept up the FC level for a couple more days while conducting OCLT each day. Now, I have zero loss on the OCLT. The water temp is back down to 65 degrees after several days of the heat pump not coming on.

So, my questing is kinda broad. What do you think caused the rapid chlorine drop? The pool was being used very lightly. The FC was according to the CYA/FC chart. Except, my CYA is on the low end (60). I did not want to bump back up to 70-80 during the winter because the FC demand had been so low anyway. Should I be running my SWCG over a longer period of time (more than 3 hours). As a new pool owner, what should I learn from this and should I have done something different?

Current Chemistry with Taylor 2006: pH 7.7, TA 80, FC 10, CYA 60, Calcium 350, Salt 3400
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
23,756
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Sounds like some algae got into your pool. When the water was cold the algae was dormant. When you heated the water the algae began to bloom and multiply. That then caused your chlorine demand.

When you raise the temperature in your pool you should bump up your FC by a few ppm inc ase algae gets active. Running your Fc a few ppm above target is better then having it fall low.
 
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RexO

Well-known member
May 24, 2020
47
Melbourne, FL
Sounds like some algae got into your pool. When the water was cold the algae was dormant. When you heated the water the algae began to bloom and multiply. That then caused your chlorine demand.

When you raise the temperature in your pool you should bump up your FC by a few ppm inc ase algae gets active. Running your Fc a few ppm above target is better then having it fall low.
Thanks for the analysis. I might do a precautionary SLAM before bring the water temp back up next time.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
23,756
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Thanks for the analysis. I might do a precautionary SLAM before bring the water temp back up next time.

You probably want to get the water temp Between 70-80F for the SLAM Process. Raise the FC as you begin heating the water.
 
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