So from what I've been reading, my initial understanding is that getting the CYA up is important because it protects FC from sunlight. I interpreted that to mean that the higher the CYA (without going too high of course) the less I will have to run my SWG because the FC won't be getting 'burned off' by the sunlight. But then I read in pool school : "The higher your CYA level, the more FC you need to use to get the same effect." So THAT makes it sound like I will have to run my SWG more once I get my CYA level up to their appropriate level. What am I not understanding here? WHy would I want to increase my CYA if it means that I am going to have to run my SWG more?
Thought I understood CYA, but maybe not....
- Thread starter Jim H
- Start date
Ironically your confusion tells me you almost get it. But in all seriousness, it comes down to this. What do you need to do to your pool in order to maintain your FC level above the safe minimum level throughout the day? And you want to do this, because as soon as you drop below the safe minimum, you are inviting trouble into your pool. If you don't have enough CYA, a hard day of sunshine will eat up all of the FC in your pool, so you have to add CYA to help save the FC. If you look at the reference chart, its about finding the happy medium. Without any CYA, you are asking your SWG to generate chlorine faster than the sun breaks it down
At a higher CYA you need to maintain a higher FC level to keep the pool balanced. But, a higher CYA protects a higher % of the FC from the sun, so you should actually lose less ppm of FC to the sun everyday even though your absolute FC level is higher. Clear as a green pool? 
I thought of a new way to explain it, with some hypothetical numbers.
If you start each day with a clean pool, with FC of 3, without CYA, you will probably end the day with 0.5ppm FC, losing 2.5ppm of FC to the sun. So your SWG has to make at least 2.5ppm FC each day. Not to mention that you spend most of the day below the safe minimum FC level. So without CYA, you lose almost all of the FC every day
Now, lets say you get some CYA to get your CYA up to 50. Now you have to start the day at 6 ppm FC (obviously much higher than 3). But the real benefit is that now you only lose 2ppm FC each day. So over time, you end up using less FC than if you had no CYA at all. And the benefit is that you never drop below the safe minimum FC level.
If you start each day with a clean pool, with FC of 3, without CYA, you will probably end the day with 0.5ppm FC, losing 2.5ppm of FC to the sun. So your SWG has to make at least 2.5ppm FC each day. Not to mention that you spend most of the day below the safe minimum FC level. So without CYA, you lose almost all of the FC every day
Now, lets say you get some CYA to get your CYA up to 50. Now you have to start the day at 6 ppm FC (obviously much higher than 3). But the real benefit is that now you only lose 2ppm FC each day. So over time, you end up using less FC than if you had no CYA at all. And the benefit is that you never drop below the safe minimum FC level.
Yeah, that is putting some numbers to what I said ... although, actually without CYA at all, all the FC will be gone in a few hours of direct sunlight.
