IntelliChlor 40 with decreasing free chlorine demand?

[alpha1]

My indoor pool will be one year old at the end of this month (specs in signature). For the past 8 months, the free chlorine demand has been pretty stable, with overnight FC loss of about 0.2 ppm, and a few bather-hours per week. Keeping the IC40 at 4% (with the pump running 24/7 in conjunction with the dehumidifier) had been keeping the FC level stable at 1.5-2.0 ppm ... until a couple of weeks ago when the FC level increased from 1.5 to 2.5 ppm over one week with the IC40 setting unchanged at 4%. The only change I made around that time was to decrease the pool temperature from 88 degrees to 86 degrees. Everything else remained unchanged, with the same few bather-hours per week, with environmental factors also unchanged in the indoor environment.

Could the functioning of my SWG be increasing/malfunctioning (producing more FC at the same 4% setting), or could the decrease in the pool temperature by only 2 degrees have such an effect on decreasing FC demand?

Pool water clear
FC 2.0
CC 0.5
pH 7.5
TA 80
CH 290
Salt 3600
CYA 15 approx. based on prior Tri-Chlor use

 

[JasonLion]

Chlorine gets used up more quickly at higher temperatures, so I'm fairly sure it was just the change in the water temperature. An indoor pool doesn't usually maintain FC levels at constant levels, it tends to have FC either go up or go down, often in a fairly dramatic seesaw. What I find surprising here is that you were able to keep things balanced for as long as you did.

 

[alpha1]

Jason -- Thanks for the reply. What makes the FC levels in a typical indoor pool so relatively unstable? I figure that my levels were stable because the environmental factors are very constant: pool covered except for a coupe-few hours per week, no dirt, leaves, etc blowing into the pool, light bather load. It surprised me when things changed a couple of weeks ago, and the only variable that changed was the water temperature.

 

[duraleigh]

When you are dealing with the minute percentages (parts per million) in pool water chemistry that's not really so unstable. In fact, I agree with Jason that you were able to keep them so close to your target is pretty remarkable.

 

[JasonLion]

Outdoors if you raise the FC level too high, you lose more to sunlight. If you let FC get too low, you lose less chlorine to sunlight. Those factors tend to push the FC level towards the middle. It is hardly perfect, and even with everything just right you will still wander around a bit, but FC will never get out of control high and only algae drives chlorine to zero.

Indoors you have two processes, chlorine addition and chlorine consumption which have no feedback with each other besides what you, the person caring for the pool, provide. If you have extra people swimming and use up more chlorine than normal, nothing pushes FC back towards where it needs to be unless you do it. If the SWG is turned up a little too high, FC can just keep going up practically forever.

 

[alpha1]

Thanks for the explanations (Jason and Dave). I'm thinking then that it's probably the fairly constant low bather load and otherwise fairly controlled conditions (always covered) that contribute to the relative stability, plus perhaps diligently keeping the chemistry balanced?

 

[JasonLion]