I bought a house with a 20,000 gallon fiberglass pool. I used tabs for the first few years until the stabilizer levels got too high (200 ppm).
I drained the pool 2 years ago and started over with fresh water. Now, 2 years later I have the same problem. I use about 4 tabs a week during the summer and about 2 per month during the off season. Why has the level gotten so high so quick.
I was told that it would take 10 years to get levels above 100 ppm.
Thanks for the question Jim
Many people who use Trichlor tabs never have problems but some pool owners do. This problem manifests itself by an increase in the CYA.
When you have a filter that requires regular backwashing, a shorter swimming season, lots of swimmers with lots of splash-out, and water replacement for winter closing, Trichlor use and an increase in the CYA can be managed pretty well.
Many pool owners use Trichlor tabs but sometimes that promotes a false sense of security. Trichlor tablets doesn't mean a trouble free pool. Many people like yourself do run into theese situations.
The constant use of tabs does keep the pool chlorinated throughout the week, but as you know, it does add CYA. Many pools can experience a more rapid increase due to heavy use of the pool, less backwashing (this is replaced with about 250 gallons of fresh water to dilute the CYA), less splash-out, longer filtering cycles, sun and heat, using too many tabs at one time or too many in relation to the size of the pool. A 25,000 gallon pool with a Summer temperature of around 90 - 95 degrees can go through 4 - 5 tabs per week. When I had my route in Arizona, some pools would go through 6 - 8 tabs with outside temperatures reaching 115 degrees. All of these scenarios will chew up chlorine tablets at a faster rate.
I would also ask if you've used Dichlor chlorine at any time. This is a stabilized form of chlorine and should NEVER be used for weekly chlorination or pool maintenance. This is only reserved when the CYA is low, below 20ppm, and you need to get the CYA in the pool quickly to get the correct range of 30 - 50ppm.
You can find some chlorine tabs made out of cal-hypo which dissolve much more quickly and do not contain CYA. Some cal-hypo tabs are wrapped in plastic so to slow the rate they dissolve. The calcium hardness will increase due to the calcium found in the tabs.
If you put the tabs in the skimmer (you should never do this) the tabs will dissolve quicker which means you'll use more
tabs which means the rate of the CYA will increase faster.
Another factor that you may want to consider is to know the starting point of your CYA when you do a refill. Fill water (tap or well) will not have any CYA in it. If you start with tabs when the water is 0CYA it will take a long time, perhaps years, to accumulate to 100ppm. If you shock with Dichlor and get the CYA to 50ppm, then start using tabs, the CYA will rapidly increase to 100ppm.
It was not uncommon for me to drain and refill a pool every 3 - 4 years. Again, as stated above, when the temperatures reach 110 - 115 for weeks on end, the chlorine tab use was tremendous, even if I was only going through 5 per week.
It sounds like you don't close your pool so the pool is getting dosed with tabs and CYA through the year. This will also cause a constant rise in the CYA. I would ask if you know what the CYA was when you started to add the tabs the last time?
Your CYA level keeps increasing because stabilized chlorine, and probably too much, is being used in the pool. The following are chemical rules of fact regardless of the concentration of product and of pool size:
For every 10 ppm free chlorine added by Trichlor, it also increases the CYA) by 6 ppm. For every 10 ppm free chlorine added by Dichlor, it also increases CYA by 9 ppm
The only way to reduce the CYA is to do a partial drain and refill. There's no chemical to reduce the CYA. Concerning why your pool has a faster rate of increase of CYA is dependent on several factors, many of which were stated above. Anytime you use Trichlor tabs consistently you're going to have an increase in the CYA.
My advice is to keep very accurate and weekly records. Once the CYA gets to 40 - 50ppm it may be time to stop using them and manually chlorinate the pool to keep the chlorine levels between 1.5 - 3.5ppm.
To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Cyanuric Acid (CYA)" category.
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Hope this helps and have a great Summer.