It Can Happen to Anyone - Zero Chlorine, CYA-->Ammonia

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Yes. CYA can turn into ammonia and then the ammonia can come out of the water and go into the air. This doesn't seem to be very common though. CYA appears to get turned into ammonia most effectively in pools with a solid cover, where there is little opportunity for the ammonia to go into the air.
 
Anything is possible. It's finding the appropriate explanation that's hard! A few people have reported CYA drops over the winter and not had huge chlorine demand upon opening. The only thing I can figure is that the ammonia outgassed or was otherwise taken up by something (algae?) that got filtered out.

If there is no measured ammonia or CC but there is a large chlorine demand, then that can mean there are intermediate by-products of the CYA oxidation on its way to ammonia and that makes sense and is probably what happened in my pool as well since it took more chlorine than expected based just on ammonia and CC measurements.
 
Alright, I'll give you more details, as they came available, so it won't look like a theoretical question.
Pool was closed last winter with CYA= 60. (Frankly, CYA was measured using the Taylor kit only at the beginning of the season. In theory at least it could be lower than 60 at season's end. It won't happen again...)
About two feet of water was drained upon closing, and then, at the end of winter, due to snow fill up, close to another foot was drained.
Pretty soon rain water filled up, so all in all it is fair to say that the pool had a 25% dilution.
The pool is still covered with the winter mesh cover, but submerssion pumps have been working 12 hours a day at about 3000 gph flow rate.
Numbers after a good circulation:
FC=CC = 0. pH = 7.3 - 7.4. CYA was measured at two different pool stores. One measured 0 and one measured 8 ppm CYA. Dilution only should have brought the CYA down to 45.
Bleach was added to reach 2-3 ppm FC. Winter cover is still on.
FC dropped to 0.8 after a few days. CC is still 0. Amonnia = 0.25 (difinitely not 0 and definitely lower than 0.5.)
Based on all these numbers and the pool's history, is it reasonable to say that my CYA was eaten up by bacteria?
Should I expect a large amount of FC to bring the ammonia down to 0?
What is the theorteical amount of FC needed to "burn" the ammonia?
Should it be done before or after adding CYA?
Thank you.
 
I'd just do a bucket test to see how much chlorine it will take before the FC will hold. The ammonia level is pretty low so you are probably OK. In the worst case where there are intermediates that aren't registering as CC (with no chlorine in the water, they wouldn't anyway) or ammonia, then the rule you can use is that it takes up to 2.5x the CYA drop as FC to clear it. If it were a 40 ppm drop, then that would be 100 ppm FC. Let's hope that's not the case in your particular situation. Again, do a bucket test if you are concerned or just start adding some chlorine to the pool and see what happens.
 
It happened again this winter. I opened and had <10 ppm CYA. I ended around 50 ppm last October. No ammonia, no CC, no chlorine demand when I opened. In fact, I had 5 ppm FC when I opened after 6 months being closed (October 20-April 6th). Lots of snow over the winter, so perhaps dilution explains it, but i open to almost zero cya every spring no matter what.
 
If you had algae in the pool when you opened, then the ammonia could be used by algae as their nitrogen source so you end up removing most of that via filtration rather than with chlorine alone. However, you said there was little chlorine demand when you opened so I presume the water was fairly clear. Other than dilution or some sort of evaporation of the ammonia, we don't have a good explanation for this.

You say you had 5 ppm FC after 6 months being closed. What FC level did you close with? The cold water temp probably kept the chlorine from getting used up, but would also eliminate the possibility of bacterial growth unless there was an isolated pocket without chlorine due to no circulation.
 
Well, even if the chlorine itself oxidized the CYA, the difference of 20 ppm FC would only be able to oxidize around 9 ppm CYA at best so would not account for the drop. So we've still got a mystery as I doubt that water dilution accounted for more than half or so of the drop.
 
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