I have a 18 X 36 28,000 inground pool with a sand filter. The first test this season with the pool store gave us a reading of 90ppm in cyanuric acid. The pH was low and chlorine was low.
After balancing the pH, and shocked with liquid chlorine, the pool was still green. Pool store checked again now the cyanuric acid level is 160ppm.
Said drain 24 inches and refill. We did that. My home test is showing about 100 on stabilizer, but I am afraid to shock or add chlorine.
We have granular and slow release tabs, both dissolve into CYA, right? How do I chlorinate without raising the CYA?
Thanks for the question
Some granular chlorine and most chlorine tabs do have CYA and will raise the level.
The most potent form of stabilized chlorine is Dichlor. For every 10ppm of chlorine using Dichlor, your CYA will increase by 9ppm.
For every 10ppm chlorine using Trichlor tabs your CYA will increase by 6ppm.
Your Chlorine should be at least 7.5% of your CYA level and the normal CYA range is 30 - 50ppm. As the CYA increases through using Trichlor tabs the chlorine needs to increase as well.
If you allow the CYA to get to 100ppm, you need to run the chlorine at 7 - 8ppm for the chlorine to be effective. When the CYA gets this high the only way to reduce it is a partial drain and refill. There's no chemical to reduce the CYA. If your CYA is at 100ppm, and the range is 30 - 50ppm, then by draining 1/2 of the water and refilling it you'll drop the CYA to 50ppm.
The first thing to do is remove any chlorine tabs. You don't want more CYA in your pool.
Normal liquid chlorine and much of the granular chlorine is unstabilized, so it won't raise the CYA. You can use those to chlorinate your pool.
If you have hard fill water, use liquid chlorine because you don't want to add more hardness to your pool water.
Hard fill water is anything over 100ppm.
Unfortunately there's no chemical to lower the CYA. The only way to reduce it is to drain 1/3 - 1/2 of the water, refill, and balance the chemicals out.
Hope this helps and have a fun and safe swimming season.