Last year my CYA was between 30 - 40.
In Northern California I switch off the solar and run my pump at low speed during the winter months and use tabs to maintain some chlorine in the coolish (`40-50F) water. We normally get a lot of rain in the winter months (1700' in the Santa Cruz Mountains) so I find draining the rain water periodically and using the tabs normally leaves a good CYA for the next season.
However this year we have had almost no rain so I was expecting my CYA level to have risen.
Yesterday I cleaned the filter - started the solar and ran my normal tests with TF-100 and was suprised to find my CYA was reading at about 20 (certainly below 30 and close to 20 as best I can tell). I tried using some 50 ppm CYA solution to double check that I was reading the test correctly and frankly I would probably have read 60 ppm using this sample - so if anything I am reading a little high - again suggesting that 20 is probably the correct result using my pool water.
Yesterday evening I raised the FC to 6 ppm - this evening when I tested the pool it was 4.5 ppm which surprised me as I would have thought the FC would have dropped more if my CYA was really 20 ppm.
Today was a sunny day and the pool gets direct sun until 5PM. Water temp is up to about 74 today after one day of solar (from about 68 yesterday).
If my FC levels seem to be holding OK I probably won't do anything about the CYA - but I am a little puzzled.
EDIT I should add that I only use liquid chlorine in the summer months. For the last two years all I have had to add is chlorine and acid (and tabs in the winter months) with no need for shocking and beautiful clear water that everyone remarks on.