Whast Happens If Cya Get High?

[navsav]

Just got a taylor test kit and measured the CYA - it came out as 60-70 ppm. The book that came with the test said 30-50 was ideal. Whats happens if CYA get high? what is too much?

[countrygal]

As I understand it from reading another post, if the CYA gets to high it make the chlorine ineffective. I don't know at what level this will happen and I don't know what can be done to bring it down, I hope someone responds to this post as I also think my CYA may be to high.

[jadi]

As I understand it from reading another post, if the CYA gets to high it make the chlorine ineffective. I don't know at what level this will happen and I don't know what can be done to bring it down, I hope someone responds to this post as I also think my CYA may be to high.

Mine got to 150- 300ppm 3 weeks ago. First, I tried to drain 1/2 the tub then refill but it stays at 150+ppm. Then after reading some of Richard " chem geek " thread, I relize it was from too much dichlor I had use ( 1-2 tsp after every soak ). Last week I drain out and refill the whole tub. CYA is now 0ppm.

jadi

[chem geek]

The chlorine becomes less effective at higher Cyanuric Acid (CYA) levels. Essentially, the ratio of FC to CYA roughly corresponds to the amount of disinfecting chlorine (hypochlorous acid) and therefore chlorine's effectiveness. For most bacteria that are easy-to-kill, a higher CYA level is not a problem. The main concern is with the bacteria Pseudomonas aeruginosa that causes hot tub itch since it is more resistant to chlorine. We do not know for certain, however, exactly at what point this becomes a problem.

My very conservative recommendation is to use Dichlor for about a week or maybe two (to get to around 20 ppm CYA) and then to switch to using bleach, but again, that's being conservative. The Vermont and Northman methods both use Dichlor after each usage of the spa so the CYA level will build up over time rather quickly. This is because for every 1 ppm FC added by Dichlor, it also adds 0.9 ppm to CYA.

A relatively small number of people get hot tub itch and yet many hot tub owners use Dichlor as their only source of chlorine so this bacteria is either not in most spas and/or it doesn't affect most people (i.e. they aren't sensitive to it) so not keeping the CYA at lower levels is not taking a huge risk, at least not for spas. For pools, that's a whole other matter, since algae develops if the FC level gets below 7.5% of the CYA level and the risk increases the lower it goes (unless an algaecide or phosphate remover is used). Spas generally don't have that problem since they are covered and don't get exposed to sunlight, and are too hot for most algae.

Richard

[wireman]

I'm very unclear on if CYA reduces the effectiveness of chlorine? Does a higher CYA prevent a rash or? Does CYA do anything useful at all? Thanks!

[chem geek]

Cyanuric Acid (CYA) combines with the disinfecting form of chlorine, hypochlorous acid, to from a series of compounds collectively called chlorinated cyanurates. The Free Chlorine (FC) level measures both the hypochlorous acid, hypochlorite ion, and all the chlorinated cyanurates as a whole. When the Free Chlorine (FC) level is less than the CYA level, the vast majority of chlorine is in the form of the chlorinated cyanurates and these are not effective as sanitizers (or not nearly to the same degree -- somewhat like hypochlorite ion). So measuring FC by itself without knowing the CYA level does not tell you how well your water is being disinfected.

Specifically, at a pH of 7.5, the ratio of FC to CYA roughly gives the equivalent FC, in terms of disinfection capability, if there were no CYA. So, an FC of 4 ppm with a CYA of 20 ppm is roughly equivalent to 0.2 ppm FC with no CYA. Fortunately, it takes very little disinfecting chlorine to kill most bacteria, though the bacteria that causes hot tub itch takes more as it it more chlorine resistant.

The Department of Health in Pennsylvania in this link recommends against using any CYA in hot tubs and spas, but that is too extreme in my opinion. If you don't have any CYA in the water, then the water will be over-chlorinated because it is virtually impossible to consistently maintain an FC level of 0.2 ppm. So having some CYA in the water is a good thing as it essentially acts as a chlorine buffer, keeping the disinfecting chlorine level at a reasonably low value but releasing it quickly as needed.

Traditionally, CYA was only seen to be needed for outdoor pools because it protects chlorine from the breakdown from the UV in sunlight. It does this in two ways. CYA directly absorbs UV so shields lower depths of the pool directly and it also combines with chlorine (as mentioned above) to from compounds that breakdown more slowly from UV. The fact that chlorine effectiveness is reduced by CYA has not been made clear by the industry for reason you can probably guess (hint: Trichlor and Dichlor add to CYA levels and are very profitable sources of chlorine compared to chlorinating liquid, not to mention that Trichlor is highly acidic so needs pH Up to compensate and that high CYA without higher FC leads to algae in pools requiring algaecide treatments, etc.).

Technical details about all of this (the equilibrium chemistry has been known since at least 1973) is described in this post. The industry doesn't dispute this chemistry, but they claim that in "real pools" the level of CYA doesn't matter and they point to the Pinellas County study as proof. I talk about this in this thread. See also this link and this link and this link among many others describing the reduction in chlorine effectiveness when CYA is present.

Richard