Does CYA get used up, or does it remain?

[67mike]

Just curious if CYA levels drop with time?

Also, it sure seems to take a long time for CYA to dissolve.....I am doing the sock hanging infront of return jet method suggested by a member here.

 

[chiefwej]

They drop only through splash out and back washing. CYA remains in the water. It is not used up, nor is it lost with evaporation. Only in rare cases is it converted to ammonia by a bacteria between seasons. Other than that it stays in the water.

 

[67mike]

Thank you, good to know.

 

[jblizzle]

Some of the CYA is in fact broken down by the chlorine. But it is usually only around 2 to 3 ppm per month.

 

[chem geek]

There are also some inconsistent reports of people losing 10 ppm CYA per month so it is possible that in full sun the losses might be somewhat higher, but not everyone sees this. You just can't count on the CYA dropping enough to be able to use only Trichlor, unless you have other sources of significant water dilution such as a small pool with backwashing a sand filter and summer rain overflow.

 

[Jo Jesty]

Hope this might be useful from an amateur... CYA might drop very slowly. After my last year's 2/3 draining and getting it down to 50, it's now about 40, probably mostly from backwashing and winterizing. However, using just hypochlorite all the time (12.5%, aka liquid shock), the CYA (measured by Taylor kit) continued to drop very slowly. I now sorta alternate liquid and occasional dichlor powder, the latter to maintain the CYA in the 40 ppm range. I simply switch the filter valve to Recirc to add them thru the skimmer and then switch back to Filter; then stir it up for 2-3 hours/day. Works well. (But make sure--as I learnt--to add stuff and stir either in early morning or evening to reduce loss from sunlight breakdown.) In my 16x32 IG pool, I use about 1 qt 12.5% liquid or 1/3 lb of dichlor per day. And it' sparkling clean, free chlorine (at midday) 2-3 ppm. Note too that I keep my pH fairly low, around 7.2 (chlorine effectiveness goes down drastically with higher pH, > 7.4). Note also that although liquid hypochlorite is extremely alkaline out of the bottle, it ends up neutral when it breaks down and releases its chlorine, and doesn't raise the pool pH.

 

[jblizzle]

@ Jo Jesty, I have added a few comments below in red.

Hope this might be useful from an amateur... CYA might drop very slowly. After my last year's 2/3 draining and getting it down to 50, it's now about 40, probably mostly from backwashing and winterizing. However, using just hypochlorite all the time (12.5%, aka liquid shock), the CYA (measured by Taylor kit) continued to drop very slowly. I now sorta alternate liquid and occasional dichlor powder, the latter to maintain the CYA in the 40 ppm range. I simply switch the filter valve to Recirc to add them thru the skimmer and then switch back to Filter; then stir it up for 2-3 hours/day. Works well.
TFP does not recommend added any chemical directly through the skimmer to avoid potential damage to the equipment.

(But make sure--as I learnt--to add stuff and stir either in early morning or evening to reduce loss from sunlight breakdown.) In my 16x32 IG pool, I use about 1 qt 12.5% liquid or 1/3 lb of dichlor per day. And it' sparkling clean, free chlorine (at midday) 2-3 ppm.
If your CYA is maintained at 40ppm, to be sure of keep algae away, you need to keep the FC ABOVE 3ppm at all times and not let it drop below that by the next time you add chlorine.

Note too that I keep my pH fairly low, around 7.2 (chlorine effectiveness goes down drastically with higher pH, > 7.4).
This is not true when there is CYA in the water. With CYA present, the chlorine effectiveness degradation is greatly reduced to the point where it is nothing to worry about when the pH in the in normal pool ranges.

Note also that although liquid hypochlorite is extremely alkaline out of the bottle, it ends up neutral when it breaks down and releases its chlorine, and doesn't raise the pool pH.