Quote Originally Posted by njpool
My CYA is between 100 and 150, so I wonder if the FC readings I am getting is really a reading of CC? Does CYA also stabilize chloramines?
Yes and no....When stabilizer combines with FC it forms a groups of compounds known as 'chloramides' such as chloroisicyanuric acid. These are a "combined chlorine" becuase the chlorine has combined with the sabilizer. However, these will test as FC, not CC, and have sanitizing and oxidizing ability...just not as much as FC. In the old days they used to stabilize chlorine by adding anhydrous ammonia to the pool to form monochloramine! (Some pool wholesale suppliers still sell anhydrous ammonia!) Monochoramine does have sanitizing ability and is a very good algae killer but it is short lived. It does possess more staying power in sunlight than FC, however. They also used to use sulfamic acid which formed chorosulfamates. Neither of these was as effective as cyanuric acid.
In short, stabilizers combines with the chorine and create what really is a form of combined chlorine. If you cya is beween 100 and 150 then you need a lot of FC in the pool to make sure there is some residual that has not combined or your pool will not be properly sanitized. My suggestin would be to lower the level of CYA by dilution (drain and refill) to get it into the range of 30-50 ppm.

Hope this didn't confuse you too much.