Major CYA Decrease in Indoor Pool

[fairway221]

I have a curious question about the CYA in my indoor pool. By the time I discovered this site and made the conversion from Trichlor tablets to liquid chlorine in May 2022 my CYA had creeped up to about 50ppm. This was after a full water replacement when a new liner was installed in November 2020.

Based on the CYA-FC charts from this forum I then targeted and maintained my FC between 6 and 8 for the next 18 months - surrendering to the fact that my CYA would remain around 50ppm for the long term. During this time, my water remained clear, stable and with no signs of black algae or mold. I also performed a small 15% no-drain water exchange in May 2022 and it had a similar 15% reduction in the CYA. But I have not done any further exchanges. But then - throughout 2023 I was noticing that my CYA was slowly decreasing and is now at a level I can no longer detect on my K-2006C test kit. When I had the water tested at my local pool store in October 2023 it was 7ppm on their test system.

In October, I began to slowly lower my target FC and I have been managing it at around 2.0 to 2.5 for several weeks - with still no issues in water clarity or algae. The water only needs about 8oz to 12oz (depending on age of chlorine) of the 12.5% sodium hypochlorite per day.

I am quite happy about all of this but it was my understanding that CYA will not normally decrease on its own in an indoor pool that is covered for probably 95% of the time. There is no splash-out with this minimally used pool and I only need to add a 1/2" of water every few months or after a backwash - so further dilution is not a likely candidate for causing this.

Any ideas what might have happened? Thanks.

Latest Test Result Summary:

FC: 2.2
CC: 0.0
pH: 7.7
TA: 150
CYA: undetectable
Water Temp: 89°

 

[mknauss]

There is a degradation of CYA over time. Albeit will be slow in an indoor pool, as you have seen.

Add a touch of stabilizer to get the CYA near 30.

 

[Mdragger88]

The hotter the water the faster the cya degrades.
Get cya back to 30ppm as @mknauss suggested & follow the FC/CYA Levels.
Check cya at least monthly & replenish as necessary.

 

[fairway221]

Thank you both. I still have some trichlor pucks leftover and will use them in my feeder to get that CYA up a bit. Seems ironic that after years of trying to get my high CYA down (it was higher than that before the new liner and water) I now need to march it back up some. I still love this overall process though - it is sooooooo easy.

 

[JamesW]

The chlorine binds to the nitrogen atoms in the cyanuric acid and it tries to take the electrons from the nitrogen atoms.

As the chlorine takes the electrons, the nitrogen becomes nitrogen gas and the CYA molecule falls apart.

The carbon is already oxidized into a +4 state, so it becomes carbon dioxide.

 

[JamesW]

There is a bacteria that can break down the CYA into ammonia (NH3), but that will usually result in CCs that are elevated for a period of time.

If the FC ever goes to 0, then the bacteria is a possibility.

If the FC is always good and Never too low, then the bacteria is unlikely.