# Thread: Why must FC be below 5 if CYA is zero?

1. ## Why must FC be below 5 if CYA is zero?

In the Basic Pool Care, I read the following:

"When CYA is really zero you should not bring the FC level above 5."

My question is why? Thank you.

2. ## Re: Why must FC be below 5 if CYA is zero?

High chlorine levels are corrosive and will damage hair, metals, vinyl, and some other materials. Remember that CYA dramatically reduces the effective chlorine level. With CYA at 50 it would take an FC level of about 46, more than double shock level, to have the same effective chlorine level you have with CYA at zero and FC at 5.

Depending on the materials your pool is made of, it may be possible to use extremely high FC levels for brief periods, but you wouldn't want to swim in them.

3. ## Re: Why must FC be below 5 if CYA is zero?

Originally Posted by JasonLion
High chlorine levels are corrosive and will damage hair, metals, vinyl, and some other materials. Remember that CYA dramatically reduces the effective chlorine level. With CYA at 50 it would take an FC level of about 46, more than double shock level, to have the same effective chlorine level you have with CYA at zero and FC at 5.

Depending on the materials your pool is made of, it may be possible to use extremely high FC levels for brief periods, but you wouldn't want to swim in them.
I had a cloudy, green tinged pool. I added bleach a couple of days ago and vacuumed the pool. It is clear now. After testing, the results are:

FC=10
CYA~0
TA>300
pH=8.2

Hence, the question that started this post. I do not plan to swim until the end of May. Am I going to have any issues?

Thank you for the quick response

4. ## Re: Why must FC be below 5 if CYA is zero?

In an outdoor pool the CYA level is almost never zero, even when it tests as zero. If there was ever CYA in the pool, there is going to still be some in the water. It only takes a very little, 1 or 2 ppm of CYA which will test as zero, to dramatically reduce the effective chlorine level to something much more reasonable.

How are you testing?

5. ## Re: Why must FC be below 5 if CYA is zero?

I am testing using the pool kit purchased here. I am following the directions and essentially filled the tube with the proper mixture of pool water and #13 chemical. Hence, my almost zero CYA. I just am not sure what happens with an almost zero CYA. My thinking is that the chlorine will "evaporate" very quickly.

I do have left over bags of chlorine (dichloro) from Leslies. I was thinking of throwing in a bag or two to bring the CYA to 30-50 into my outdoor pool, which did have a CYA level of 50 last year...or should I wait until my FC drops to 5?

6. ## Re: Why must FC be below 5 if CYA is zero?

The first thing to do is to get the PH down. Lower PH to around 7.2 with acid.

If you want, you can use the dichlor to add both chlorine and CYA at the same time. The best way to approach that is to figure out the total amount of dichlor you want to use based on the CYA increase you are looking for, and then actually add portions of that total amount based on how much chlorine you need to add. That means not adding any right now since the chlorine level is fine.

7. ## Re: Why must FC be below 5 if CYA is zero?

Originally Posted by JasonLion
The first thing to do is to get the PH down. Lower PH to around 7.2 with acid.

If you want, you can use the dichlor to add both chlorine and CYA at the same time. The best way to approach that is to figure out the total amount of dichlor you want to use based on the CYA increase you are looking for, and then actually add portions of that total amount based on how much chlorine you need to add. That means not adding any right now since the chlorine level is fine.
Thank you very much...will do the recommendations.

Have a great day!!!

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