How to balance CSI before closing


Aug 2, 2019
Fredericksburg, Virginia
We have been limping along with high CYA levels this season (I discovered this site halfway through our first summer with a pool after we had already developed this problem) and are hoping to get the CYA down after we drain/winterize/refill in the spring. The water is clear and we aren't losing free chlorine overnight- everything seemed ok during late summer but when I adjust the temperature in pool math down to 40, it looks like our CSi will get too low while we're winterized and risks etching the plaster.

Here are today's readings:
TC: 12
CC: 0
pH: 7.6
TA: 80
CH: 370
CYA: 160
temp: 63
CSi: -0.52

We were doing ok with the warmer temps but the temp is dropping fast and if I winterize now with these values and the temp drops to 40 over the winter, the CSi drops to -0.74.

I was playing around with the numbers, and if I shoot for the scenario below then I would have a better CSi through the winter.

pH: 7.8
TA: 100 (as an aside, the Taylor book lists the ideal range for sodium hypochlorite as 80-100...does anyone know why TFP lists it as 70-90?)
CH: 450
CYA: 160
temp: 40
CSi: -.025

Should I add calcium and baking soda to get to these levels before we winterize? How much should I be worried about CSi over the winter?

Also, am I correct in assuming that we are supposed to shock the pool to 1/2 slam level prior to closing, even though that means we're looking at a FC of 31? Is it ok to add polyquat, drain, and cover (loop loc) with the FC at 31 or do we need to let it come down? Will shocking that high knock anything else out of balance?

We want to drain as much as is safely possible to get rid CYA over the winter; how far below the returns is safe in a plaster pool?

Thank you!


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Don't add chemicals to adjust your CSI for the winter. Your pH will rise above 8 over the winter and the CSI will rise. Leave your chemicals within TFP range.

Don’t worry about your CYA level. You will see what you have at opening in the Spring and can take action then as necessary. CYA can degrade and drop over the winter.

Raising your FC to 31 will not knock anything out of balance. It is not a problem if you go to 31. Going ot a FC of 20 plus adding Polyquat 60 should work just as well.
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