Need Help with Explaining FC Loss

[Kit]

I previously posted a similar concern regarding FC loss. However, I did another SLAM raising the FC ltarget level from 15 ppm to 25 ppm. With the SWCG removed for the upcoming winter, I began chlorinating the spa manually with liquid chlorine. Based on Pool School guidance, I did a SLAM with a FC target level of 25 ppm. However, the FC numbers below seem very perplexing to me. I would think that (for a 2,600-gallon spa) after 4 days the FC levels would have fallen considerably since any remaining organic matter would be dead. As mentioned previously, the spa is always covered with an auto cover. In addition, the spa has not been used for a couple of months. During the SLAM, the outdoor temperature has been between 60F-70F. Why do I continue to lose FC? Are there other explanations for FC loss? Or are the results below "normal" for adding liquid chlorine? Would the fact that the spa water has been maintained at 85F influence the FC numbers? I'm trying to make sense of all this.

Thank you so much for your help. I truly appreciate it.

SLAM RESULTS

Starting Levels
FC = 4 ppm
CYA = 50 ppm
FC SLAM Level = 25 ppm

Hours Lapsed FC Loss
Day 1 2 hrs. 15 ppm
3 hrs. 8 ppm
3 hrs. 7 ppm
3.5 hrs. 7 ppm
Day 2 10 hrs. (overnight) 8 ppm
3 hrs. 7 ppm
3 hrs. 4 ppm
5 hrs. 2 ppm
3.5 hrs. 6 ppm
Day 3 10 hrs. (overnight) 8 ppm
5 hrs. 4 ppm
4.5 hrs. 2 ppm
4 hrs. 3 ppm
Day 4 11.5 hrs. 8 ppm
4.5 hrs. 4 ppm
4 hrs. 6 ppm
4.5 hrs. 5 ppm
Day 5 10 hrs. (overnight) 7 ppm

 

[PoolStored]

Can you post some picture of the spa so we can get a sense of what we are dealing with?
Have you scrubbed everything?

 

[Kit]

Please see photo below. Yes, I initially scrubbed the spa. Also, I have attached more complete SLAM information. Thank you so much.

SLAM RESULTS

Starting Levels
FC = 4 ppm
CYA = 50 ppm
FC SLAM Level = 25 ppm

Hours Lapsed FC Result FC Loss FC Added
Day 1 2 hrs. 10 ppm 15 ppm 64 oz.
3 hrs. 17 ppm 8 ppm 34 oz.
3 hrs. 18 ppm 7 ppm 30 oz.
3.5 hrs. 18 ppm 7 ppm 30 oz.
Day 2 10 hrs. (overnight) 17 ppm 8 ppm 34 oz.
3 hrs. 18 ppm 7 ppm 30 oz.
3 hrs. 21 ppm 4 ppm 17 oz.
5 hrs. 23 ppm 2 ppm 9 oz.
3.5 hrs. 19 ppm 6 ppm 26 oz.
Day 3 10 hrs. (overnight) 17 ppm 8 ppm 34 oz.
5 hrs. 21 ppm 4 ppm 17 oz.
4.5 hrs. 23 ppm 2 ppm 9 oz.
4 hrs. 22 ppm 3 ppm 13 oz.
Day 4 11.5 hrs. 17 ppm 8 ppm 34 oz.
4.5 hrs. 21 ppm 4 ppm 17 oz.
4 hrs. 19 ppm 6 ppm 26 oz.
4.5 hrs. 20 ppm 5 ppm 21 oz.
Day 5 10 hrs. (overnight) 18 ppm 7 ppm 38 oz.
6 hrs. 24 ppm 1 ppm 4 oz.

 

[Kit]

Oops. Sorry for the jumbled format of the columns above. Don't know why that happened. Let me try attaching it as a PDF file.

 

[PoolStored]

You should brush at least once a day. Remove any drain covers and scrub them and the drain (turn the pump off when you do).

 

[Kit]

Your point on scrubbing is well-taken. I have brushed the spa a couple of times during the SLAM; but have not removed the drain covers. But after 6 days of maintaining FC at 25 ppm, I am still perplexed as to why the water is still consuming so much chlorine without any significance decrease.

 

[PoolStored]

You need to scrub everything at least once a day. Algae has a biofilm. That protects it. You need to break the biofilm to kill it.

This pic is all it took to extend my slam 2 days and consume lots of chlorine.

3.jpg

• PoolStored
• Jul 16, 2024

 

[Kit]

Thank you for the helpful info.