Importance of CYA in hot tubs

[Mizu]

Assuming an average hot tub of 400gal, which is used for 1 hour three times a week and covered the remainder of the time, what is the importance of the cyanuric acid in the water chemistry? The tub is only exposed to UV rays for 3 hours a week, which is 1/56th of a week which is 1.7% of a week. With such a small exposure to UV, why is it important to have any stabilizer in a hot tub at all? Does cyanuric acid prevent the general degradation of chlorine as well?

And a follow up question, does cyanuric acid interfere with the efficiency of bromine the same way it does chlorine? In other words, if a spa operator were using bromine tabs/sodium bromide and oxidizing with dichlor, would the cyanuric acid buildup from the dichlor eventually lead to problems with the resulting hypobromous acid?

 

[chem geek]

Cyanuric Acid (CYA) is not just for protecting chlorine from breakdown from sunlight, but also significantly moderates chlorine's strength. 4 ppm Free Chlorine (FC) with 30 ppm CYA in a hot (104ºF) tub is roughly equivalent to around 0.6 ppm FC with no CYA. If you don't use any CYA, then the chlorine is too strong and will react more quickly with swimsuits, skin, hair, as well as outgas faster and oxidize hot tub covers more quickly and yes, this increases chlorine demand. You want some CYA in the water to act as an active chlorine (hypochlorous acid) buffer so you have plenty in reserve while keeping its strength to reasonable levels. With no CYA, the chlorine is too strong (unless you try and maintain very low chlorine levels, but that is generally hard to do) while with too much CYA the chlorine is too weak and takes too long to oxidize bather waste and can be too weak to kill some pathogens quickly enough.

CYA does not bind with bromine so has no effect in that case (ignoring some direct CYA shielding effect from UV in sunlight). Buildup of CYA from using Dichlor to shock a bromine tub can still occur (though isn't much of a problem), but it generally takes longer since some of the bromine comes from the tabs so you may end up replacing the water before the CYA builds up too much. One can use the equivalent of the Dichlor-then-bleach method when using bromine by initially using Dichlor for shocking or handling the extra bather load for a while and when one has added around 33 ppm FC cumulatively of Dichlor, then one can switch to using bleach. Or if one wants to use bleach initially, then this is OK in a bromine tub since it will get used up in minutes oxidizing bromide to bromine (assuming you have initially created a bromide bank by adding sodium bromide). You just don't want to use bleach initially in a chlorine tub since it will be too strong without any CYA to moderate it.

 

[Mizu]

Great, that answered my question perfectly. Thanks Chem Geek! I was aware of the negative effects of too much cyanuric, though I didn't stop to take into consideration just how potent chlorine is without any. Thanks again!