Cya From 50ppm To 160ppm In 1.5 Months

[bobnc]

I have a 25,000 gallon inground pool that was installed in late March of this year. I use Trichlor 3" tabs to chlorinate in an automatic feeder and shock weekly with non-stabilized Calcium Hypochlorite. I normally keep the chlorine between 3 and 6 ppm. The water has been crystal clear all summer without any trace of algae.

I took some water to the pool store that installed the pool to have it tested. I got these results:

FC: 6.0 ppm

TC: 6.0 ppm

Alkalinity: 130 ppm

PH: 7.8 ppm

CYA: 160 ppm

I was shocked when I saw the CYA number since it was only 50 ppm when I had the water tested at the same place about 6 weeks ago. I know CYA builds up in the water with the use of stablized Trichlor, but is it possible that it has gone up that much in such a short time?

[chem geek]

For every 10 ppm FC added by Trichlor, it increases CYA by 6 ppm. At 2 ppm FC per day chlorine usage, after 6 weeks that would be an increase of 50 ppm. If you've been using chlorine at the rate of 4 ppm FC per day, then that could account for the rise, but I doubt your chlorine usage has been that heavy. I suspect one or both of the two tests is wrong. You should really get your own Taylor K-2006 or TF100 (from tftestkits.com) test kit.

If the CYA did rise to or is at 160 ppm, then it's a good thing you've kept your FC higher, but I really think this number isn't correct.

[bobnc]

For every 10 ppm FC added by Trichlor, it increases CYA by 6 ppm. At 2 ppm FC per day chlorine usage, after 6 weeks that would be an increase of 50 ppm. If you've been using chlorine at the rate of 4 ppm FC per day, then that could account for the rise, but I doubt your chlorine usage has been that heavy. I suspect one or both of the two tests is wrong. You should really get your own Taylor K-2006 or TF100 (from tftestkits.com) test kit.

If the CYA did rise to or is at 160 ppm, then it's a good thing you've kept your FC higher, but I really think this number isn't correct.

Actually, I have my feeder set to the lowest setting (1 on a 1 to 9 scale) and whenever I test with test strips the chlorine is always above 3 and usually around 5. To be quite honest, the pool doesn't get much use. I swim laps in it for about 30 minutes 3 or 4 times a week and that's about it. The pool is situated in area without a lot of trees, so nothing much gets in the water. I have an Aquabot that I use to to vacuum it about once a week, and there really isn't much for it to pickup.

I wonder if the chlorine feeder isn't working properly and is putting too much chlorine in the water? I run my pump/filter 24 hours a day ... does that contribute to more chlorine in the water?

I have ordered a Taylor K-2006 test kit and hopefully that will give some accurate readings.

[chem geek]

Running the pump longer will erode more Trichlor pucks in the inline chlorinator, but usually you would see a rise in FC unless you had simultaneous increasing chlorine demand. With the feeder at the lowest setting, I don't think you're putting that much FC nor CYA into the water so I still suspect that the latest CYA test result is wrong.

[Mark357]

Get a sample of your pool water in a clean 20 oz container. Pour out half of the water and fill it back with tap water. Have the water retested and then double the result. Don't tell the testers that you diluted the water by half. If it really is 160 the result should come back as 80. Also, is there any way that the shock you are using might be Dichlor? And, how many tabs do you normally keep in the feeder?

[Mark357]

Here is the formula for Trichloroisocyanuric acid………………..(C3Cl3N3O3)

Active Chlorine 45.76% by mass

C3Cl3N3O3 + 3H2O <-> 3HOCl + C3H3N3O3

Every pound of Trichloroisocyanuric acid (C3Cl3N3O3) contains 0.4576 lb of chlorine and the rest is 0.5424 pounds of cyanuric acid. A 35 lb. bucket of trichlor contains 18.98 lbs of cyanuric acid. That will raise the cyanuric by 110 ppm in 20,000 gallons.

How much trichlor have you used in the 6 weeks?

[bobnc]

I have a 25,000 gallon inground pool that was installed in late March of this year. I use Trichlor 3" tabs to chlorinate in an automatic feeder and shock weekly with non-stabilized Calcium Hypochlorite. I normally keep the chlorine between 3 and 6 ppm. The water has been crystal clear all summer without any trace of algae.

I took some water to the pool store that installed the pool to have it tested. I got these results:

FC: 6.0 ppm

TC: 6.0 ppm

Alkalinity: 130 ppm

PH: 7.8 ppm

CYA: 160 ppm

I was shocked when I saw the CYA number since it was only 50 ppm when I had the water tested at the same place about 6 weeks ago. I know CYA builds up in the water with the use of stablized Trichlor, but is it possible that it has gone up that much in such a short time?

Update:

I got my Taylor test kit about a week ago and ran the CYA test, it was above 100ppm. At that point, I decided I would partially replace the water (drained about 30%) at retest. After replacing the water the CYA was between 80 and 90ppm. While it may not have been 160, it probably was well over 100ppm before the drain and refill.

I would still like to get the CYA down even further, and may do another partial drain and refill. Right now I am keeping the FC at about 5ppm and the water is staying perfectly clear. Since the pool doesn't get a lot of use and I run my filter pump 24 hours a day, I have the auto chlorinator on the lowest setting of 1 and have not had any problem maintaining the 5ppm FC reading. For the rest of this season I am continuing to use the trichlor pucks, but will probably look into other options for next spring.

Question ... I normally just put 5 or 6 trichlor 3" tabs in the feeder and let them dissolve over time. Is it better to just put in one tablet and let it completely dissolve before adding another, or is it ok to keep the feeder topped off with 5 or 6 tablets at a time.

thanks for the help.

[chem geek]

The more tablets you have in the feeder, then the more chlorine and CYA you are adding to the water per time period. It's similar to having fewer tablets but increasing the dial setting to increase the flow rate. So for convenience, having more tablets you replace less often makes sense, but if dialing down to 1 isn't low enough (i.e. if it causes the FC to get too high) then cut back on the number of tablets. It sounds like you're on the edge right now of being where you want.

How often do you replace with new 5-6 tablets? That would give me a rough idea of your FC and CYA output.

As I mentioned in a previous post, your CYA rise would be explained if you were using Trichlor tablets at a rate of 4 ppm FC per day. This translates into roughly 1.8 3" 8-ounce tablets per day so you'd have to add 5-6 new tablets every 3 days. Does that sound about right to you? If so, then what is strange is why your FC usage is so high.

Richard

[bobnc]

The more tablets you have in the feeder, then the more chlorine and CYA you are adding to the water per time period. It's similar to having fewer tablets but increasing the dial setting to increase the flow rate. So for convenience, having more tablets you replace less often makes sense, but if dialing down to 1 isn't low enough (i.e. if it causes the FC to get too high) then cut back on the number of tablets. It sounds like you're on the edge right now of being where you want.

How often do you replace with new 5-6 tablets? That would give me a rough idea of your FC and CYA output.

As I mentioned in a previous post, your CYA rise would be explained if you were using Trichlor tablets at a rate of 4 ppm FC per day. This translates into roughly 1.8 3" 8-ounce tablets per day so you'd have to add 5-6 new tablets every 3 days. Does that sound about right to you? If so, then what is strange is why your FC usage is so high.

Richard

Back when the pool was first completed, the builder set the chlorinator at 5, and yes, I was going thru about 6-8 tablets a week. In July, I turned it down to 1 and now 5 or 6 tablets in the chlorinator lasts for about 14 days or so. As I mentioned in my earlier post, the FC is now stable at about 4 ppm with a CYA of 80, so I feel like that is a good place for right now.

The combined chlorine test in the Taylor kit never shows any combined chlorine, so I have discontinued shocking the pool weekly. Is there any need to shock the pool when you have no combined chlorine and a stable FC reading with perfectly clear water?

[chem geek]

No need to shock the pool when you have no CC, clear water, AND no large drop (> 1 ppm) in FC overnight. You can have algae growth started without seeing it so the FC drop overnight is one way to tell, a day or two before the water starts to turn dull, cloudy or green.

Your 4 ppm FC with 80 ppm CYA is taking a risk of getting green algae unless your pool is low in algal nutrients (phosphates, nitrates). A safer target is 6 ppm (an FC that is 7.5% of the CYA level). Also, if you continue to use Trichlor tabs, you may find your CYA continue to increase which only makes the situation worse. If you want to continue to do this and to prevent algae you either need to raise the FC level proportionately or need to use a supplemental algaecide (PolyQuat 60, phosphate remover, or 50 ppm Borates).

If you keep your FC low on the edge, then shocking the pool would help kill any algae that slowly started to take hold, but this can also be avoided by a somewhat higher FC level as noted above.

Richard