Chlorine Question

[Kit]

After having my in-ground 2,600-gallon spa re-plastered, I decided not to add salt since I will disconnect that SWCG next month (being the end of the season). In the meantime, I am simply adding liquid chlorine bleach a couple of times a week to maintain the chlorine level within 2-4ppm. However, I have noticed that after adding 2 quarts of bleach, the chlorine level drops to 0ppm in 2days. I might add that (1) the spa has not been used, (2) the spa is covered & (3) during the day the water temperature is maintained at 83F. The air temperature has been high for this time of year (83F-95F).

My question: Is it normal in a spa this size to add a couple of quarts of bleach every other day under the conditions I listed above? It seems odd that the chlorine level would drop from 4ppm to 0ppm that quickly. Thank you so much for your thoughts.

 

[ajw22]

It sounds like your spa has some algae in it and would not pass the Overnight Chlorine Loss Test

Have you put any stabilizer in the water since the refill?

 

[Kit]

Thank you for the response. I would assume that I had added CYA after the water refill. But maybe I forgot since I just tested the CYA level & it was zero. So, I will bring the CYA level up to spec & then do the OCLT. I will report back.
Again, thank you for your suggestion.

Kit

 

[Kit]

Well, the OCLT showed a drop of only 0.5ppm in FC overnight. So that tells me that the issue of previously adding more & more chlorine was likely due to no CYA in the water. However, I will keep an eye on things & SLAM if necessary. Thank you again for your help.

Kit

 

[ajw22]

(2) the spa is covered

How is the spa covered?

If the spa loses little chlorine at night then the cover is letting light and UV in during the day.

 

[Kit]

The cover is an auto cover with side tracks. The water is completely covered. As I indicated above, yesterday afternoon when I checked the CYA level there was no indicated CYA at all. Apparently I might have forgotten to add stabilizer after the spa was refilled. Now that I adjusted the CYA to 40ppm, the FC seems to have stabilized. Question: Although the spa is covered & assumingly no UVs directly hit the water, can there be chlorine loss due to the sun still heating the water through the cover?

 

[ajw22]

All the signs are the cover is letting sunlight and UV hit the water and consume chlorine.

 

[Mdragger88]

It’s also possible that you were simply adding more fc with the swcg than you’re adding manually.
What % and how long were you running the swcg previously?
What model/size is your cell?
2quarts of 10% lc should add 19ppm in 2600 gallons -
Is it possible your liquid chlorine is old & degraded?

 

[Kit]

It’s also possible that you were simply adding more fc with the swcg than you’re adding manually.
What % and how long were you running the swcg previously?
What model/size is your cell?
2quarts of 10% lc should add 19ppm in 2600 gallons -
Is it possible your liquid chlorine is old & degraded?

Mdragger88: Soory for the delay in responding, but I have been out of town for a couple of days.To answer your questions,
1. 10% for 8 hours/days.
2. Pentair IntelliChlor IC20
3. The liquid chlorine is new. I have been using a 7.5% generic store-branded bleach.
Your comment that the swcg might have been generating for chlorine than what I have been adding manually, seems plausible. However, ajw22's comment that all signs point to sunlight & UV directly hitting the water just doesn't seem logical since the auto cover is a very tight fit.

Nevertheless, I might simply rule out the possibility of any organic matter in the water by doing a SLAM.

 

[Mdragger88]

Sometimes that household stuff can be old (so can the dedicated pool stuff) so always check the date codes.

 

[Kit]

Will do. Thank you.

 

[Kit]

Although my spa passed the Overnight Chlorine Loss Test (a decrease in 0.5ppm) & the auto cover completely shuts out any sunlight, I opted to do a SLAM to rule out organic matter in the water. I attached the results below. To me, these results are confusing. For example, FC seems to fall by about 3.5-5.0
ppm regardless of the time lapse since the chlorine level was was checked & increased back up to 15 ppm. But I hoping to get some insight from those of you who have far more experience in these matters than me. If relevant, I might add that the water temperature is maintained at 83F (although recent warm daytime temperatures have raised the water temperature a few degrees). Is it possible that the decrease in FC (as noted in the attachment) is "normal" considering the spa temperature, as well as the air temperature at this time of year?

Thank you for any help. I truly appreciate it.