I experimented with no CYA when I first got my pool this year. There was a discussion a few pages long... Lots of good info in that.
Is CYA Absolutely Necessary???
Now I will say that I do use CYA. but I went a good month without any and truthfully my pool was fine.
Now what
@Richard320 said was right. You cant keep up with solar losses without CYA, but if you have an auto cover as I do, you can get by.
Now I had to run my SWG at around 60% with a pump speed 3 to keep up with solar loss on days we used the pool, and dialed it down to 20% at speed 3 on the days we didnt use the pool at all.
Keeping up with the swings was the hardest part.
The water wasnt harsh (even though a few times the FC got up over 10) and I felt no adverse effects of no CYA.
I added it because I was sick of the swings in FC and having to change the SWG settings based on usage.
Now with CYA in the pool I havent touched the pool in 2 months. Literally not once. 20% swg at speed 3. I only test PH and FC about once a week to make sure its in range.