High Cyanuric Acid In Spa

[burt4750]

I have a perplexing problem concerning cyanuric acid in my spa. I have been using the chlorine method for several years, but just in the last 10 months or so, my cyanuric acid always runs high. The pool store folks told me it was due to my use of stabilized chlorine granules. So, with that being said, I started using plain household bleach. That went fine for a while. Now the cyanuric acid levels are again high. I have done partial drains and total drains but the level keeps on creeping up. I change my spa water quarterly. I use non-chlorine shock every other day, bleach on the off days, and baking soda for Ta and Ph up or down when those levels are off. If my calcium hardness is off, I adjust that with calcium hardness increaser.

I have taken water samples to two different pool stores and am told the same thing.

Can anyone give me an explanation as to why this is happening?

[chem geek]

For every 10 ppm Free Chlorine (FC) added by Dichlor (chlorinating granules), it also increases Cyanuric Acid (CYA) by 9 ppm. This is why we have the Dichlor-then-bleach method that avoids the CYA buildup. You use Dichlor initially until you'e cumulatively added around 33-44 ppm FC which should give you around 30-40 ppm CYA at which point you switch to using bleach. You still use Dichlor about once a month since CYA slowly gets oxidized by chlorine in spas at the rate of around 5 ppm per month.

If you are using bleach, you should initially have some CYA in the water either using pure CYA or using Dichlor. Otherwise the active chlorine level will be too high and can damage equipment, spa covers, oxidize swimsuits skin and hair faster, etc.

Once you are using bleach, the CYA level should not be climbing. Bleach has no CYA nor does non-chlorine shock. Double check your non-chlorine shock to make sure the ingredients don't list Dichlor since some "shock" products are a combination of Dichlor and MPS.

I suspect the pool store testing is wrong. Get your own good test kit -- the Taylor K-2006 -- to know what is truly going on.

[burt4750]

The non-chlorine shock I use is "Fesh 'N Clear" I purchase from L _ _ _ _ _'s Pools. The ingredients are listed as Potassium peroxymonosulfate - 38%; other ingredients - 62% with available oxygen of 4%. I asked the store clerk what the "other ingredients" were and he could not tell me. Could this maybe be the problem and they don't want to divulge the rest of the ingredients?

[chem geek]

This MSDS shows the product to be Oxone which is the triple salt that includes potassium monopersulfate, though it also indicates that the product may contain up to 20% sodium carbonate, most likely to balance the acidity of the Oxone/MPS.

So that would not increase the CYA level. However, if you were to use Leslie's Chlor-Brite, then that is Dichlor that would increase the CYA.

[mrwrick]

My 1989/90 Hot Spring Classic was used for the first 22 years exclusively with Dichlor. After it was refurbished in 2013 I switched to the

Dichlor/then bleach method. Water quality and balance have been fine until the last month when pH has been high (close to the purple color described in the Taylor guide.) When I recently tested for CYA it is off the scale, way off. Could the years of Dichlor use have built up aresidue of CYA that has not dissipated and causing the extreme high CYA (ppm above the Taylor test scale) and high pH readings?

[chem geek]

Though there might be some residue of CYA leftover in the piping, there shouldn't be that much. Next time you do a water change and before you start using Dichlor initially, measure the CYA level of the water. If you measure some then it's leftover. Strange though. Could it be that you started out using Dichlor for too long at the start of this refill?

When you say the CYA is above the Taylor test scale, the scale is inverted so 100 ppm is near the bottom of the tube. Are you saying that only a small amount of sample water obscures the black dot in the tube? If instead you fill the water to the top of the tube and the black dot is still there then that means you have < 30 ppm or < 20 ppm depending on the tube you are using. CYA is slowly oxidized by chlorine at around 5 ppm per month in a hot spa so you need to use Dichlor for a day a month to replenish it. If the CYA is too low than chlorine can outgas and have the pH rise faster than if only carbon dioxide outgases.

[mrwrick]

Again, you hit the nail on its head for me. I have not read the scale correctly, so when seeing the dot at the top of the tube I assumed the CYA was too high, off the scale. Now I can check it for real and based on what you have said I expect have the lower figure. It is entirely logical that since I have not added Dichlor in some time, only straight bleach, my pH has risen due to the excess of chlorine without the CYA buffer. The AhhSome clean, drain and refill is looming closer to get this spa's water back to known balance as it used to be. Thanks Chem Geek.