CYA Question


LifeTime Supporter
Apr 30, 2007
Fort Worth, Texas
What level should I try to keep my CYA level at for a Vinyl lined pool? I was looking at Bens chart and it seens to me that the lower the CYA is kept at the lower the shock will have to be which will be better for the wear and tear on the liner?? How high can I shock for a short period of time with no ill effects on the liner?

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
0 ppm . . . . ... . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
10 - 20 ppm . .. . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
30 - 50 ppm . .. . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
60 - 90 ppm . . .. . 5 ppm . . . . . 10 ppm . . .. 20 ppm
100 - 200 ppm ... . 8 ppm . . . . . 15 ppm . . .. 25 ppm

The Mermaid Queen

TFP Expert
LifeTime Supporter
Mar 28, 2007
Northern KY
It is generally stated that a CYA of 30-50 is optimal. This keeps your FC from degrading so fast, while not requiring a huge amount of chlorine.

While CYA stabilizes the FC, it also decreases its effectiveness. So my understanding is that having 15 ppm for shock in a stabilized pool is not particularly more harsh than 10 ppm in an unstabilized pool.

If you work diligently at keeping your FC in the 3-5 range (assuming 30-50 CYA) you will not have to shock often... perhaps after a high bather load or a baby in the pool. :shock:


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida

The bleaching effect of high chlorine in vinyl pools, IMHO, is somewhat overated.

Last year, I took some small squares of vinyl liner and submerged them in solutions of 10ppm (cl), 25ppm, and 50ppm. At the end of 48 hours, I could see no difference in any of the liner samples. That doesn't mean it won't do it, but I do not find it to be as big an issue as some suggest. Prolonged high levels would have more influence, I believe. Have you ever figured the ppm in a typical load of clothes in the washing machine??

After all that, If your liner bleaches out, I'll swear I never heard of you!! :lol: :lol: :lol:

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Generally speaking for any pool, vinyl or otherwise, you want to use the smallest amount of CYA that corresponds to a level of FC that you can realistically maintain in your pool. In a pool with CYA in it, the sun will break down about half of the FC in the pool on a hot summer day. So using less CYA means a lower FC level so less total chlorine loss. The problem is that you need a certain minimum amount of FC in the pool to have enough in reserve to not get "used up" by oxidizing organics, consuming ammonia/urea, battling algae, or killing bacteria. In theory, you could have a pool with no CYA in it and have 0.1 ppm FC and be equivalent to 30 ppm CYA with 3.5 ppm FC, but how in the heck would you consistently maintain 0.1 ppm FC everywhere in your pool and replenish it as fast as it will get consumed? Also, you need to consider how often you plan on adding chlorine and what happens if you skip a day.

In practice, 30 ppm CYA works for most people. Some find that a somewhat higher CYA level works better, but remember that the half-life of chlorine (in direct noontime sun) without CYA is about 35 minutes while with 30 ppm CYA it is about 6 hours, but with 60 ppm CYA it is about 7 hours. So the biggest bang-for-the-buck comes from a small amount of CYA. In my own pool, I try an maintain 20 ppm CYA, but I have an automatic opaque pool cover so the sun is kept off the pool most of the time. That keeps my total chlorine consumption quite low at around 0.5-0.7 ppm FC per day.


Well-known member
Mar 28, 2007
To prove that high CYA levels do lock up the chlorine, in a previous pool I covered it in the winter
for about 5 months. That pool was a salt water pool and did have a CYA level of 90+ ppm. About a week after I covered the
pool for the winter I turned off the SWG. The pool maintained a chlorine level of 3 ppm for almost 5 months.
The combination of no UV light from the sun and the high CYA level froze the chlorine in its tracks.

Cliff s

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
I wouldn't characterize it as being locked up, but rather that the disinfecting chlorine concentration is far lower. At a CYA of 90, there is about a third of the amount of disinfecting chlorine as with a CYA of 30 so reaction rates are also about a third. However, cooler temperatures really slow down reaction rates significantly. So the combination is what effectively stopped chlorine from reacting. In my own pool with an opaque cover and pool temps in the winter of around 50F, my chlorine usage over the winter was somewhere around 0.25 ppm per week or 1 ppm per month at a level of around 4 ppm, but that was at 20 ppm CYA. At 90 ppm CYA, my usage would be around 0.2 ppm per month so would appear to be locked even over 5 months (1.0 ppm out of 4 ppm). It the temps were cooler, the loss would be even less.
I have a SWG and keep my CYA around 70 ppm and live in Florida so I do not cover my pool but when the temperature drops I have to turn the output on my SWG and shorten the pump run time to maintian the same FC level. The only factor that is changing is the temperature of the water. I have also seen this in my customers pools with and without SWGs in the colder months. The chlorine is not being "locked up" but the colder temperatures DO slow down processes and create much less of a chlorine demand (algae growth slows considerably in colder temps). Also, when a pool is covered there is less loss of chlorine. When I use a solar cover on my pool at night (I didn't bother this past year) I had even less chlorine loss but the pool was still exposed to the same UV during the day. (I uncover my pool in the morning and cover it at night to maximized heat retention when I use the solar cover.)