CYA Depletion

[DT835]

How quickly does CYA deplete in a hot tub that is not exposed to UV?

What are the consequences of CYA falling below 20? I have seen in the forums that using bleach with CYA at low levels is too harsh? What exactly does harsh mean? Is it unsafe?

Thanks in advance

 

[mknauss]

The water temperature has the greatest effect on CYA degradation, not UV.

If the CYA falls below 20 ppm you may find the chlorine more harsh to skin, eyes, and swim clothes. It is not deadly, just not as pleasant. May cause a rash, itchy eyes and skin, etc.

 

[duganderson]

Regardng low CYA.......someone on this site posted this. Sorry I don't know the author. Do folks agree with this? .........

" TFP stresses the dichlor-then-bleach method because it is imperative to have a minimum of 30ppm CYA in a chlorinated hot tub. Why? Because the presence of CYA modifies the rates of reaction of chlorine with different organic compounds and reduces the likelihood of forming some of the nastier THM and DBP compounds. For example, when there is no CYA in water and chlorine is allowed to react with nitrogen compounds, the far more dangerous nitrogen trichloride CC will be favored (it will have the faster reaction kinetics). When CYA is present, monochloramine is much more likely to form and it is far less reactive and dangerous than nitrogen trichloride. So again, circumstances matter A LOT when we talk about chlorine as a source of potentially harmful chemical reactions."

 

[Mdragger88]

I know this thread is older but I’m chiming in to explain part of the thinking about cya & how it plays out in real life - as for the chemistry aspects @duganderson mentioned, I have no clue lol.
Here it goes…
If you have no cya any fc over 3 or 4ppm will begin to be quite harsh on people & equipment (just ask my stainless steel jet trim). Momentarily this isn’t a major concern but for long term it is.
The other problem presents itself once the bather load is introduced, 2-4ppm of fc can be consumed quite quickly depending upon how many people there are, how dirty they are, & how long they stay in the tub. This leaves no wiggle room before going to zero if you only started with say a comfortable 2-3ppm. So the need for cya is twofold, people/equipment comfort/protection & allowing you to start with a high enough fc level that you don’t broach zero because nasties proliferate rapidly in a heated spa without fc.
person to person transmission of pathogens is a very real risk.
The rate of cya degradation differs for every tub but in general the hotter the water the faster the degradation. I find I need to use a little dichlor every week or so to maintain my cya level. I keep my tub at 104 degrees. Ymmv.