Is CYA needed?

[Curmudgeon]

My new hot tub is being used mostly in the early morning and at night. I have not added any CYA since it is covered except when in use, hence little UV exposure. And water temperature is depleting my chlorine anyway requiring several additions of HTH daily.

Am I correct in assuming I do not need CYA?


Thanks

Curmudgeon

350 gal at 103 *F.
HTH, muriatic acid, sodium bicarbonate, calcium chloride, and borax

 

[chem geek]

Welcome to TFP!

CYA isn't just for protecting chlorine from breakdown by the UV rays in sunlight. CYA binds to chlorine significantly reducing the "active" chlorine concentration that does the actual disinfection and oxidation. This is not a bad thing since it takes very low levels of active chlorine to kill most pathogens. So you do want at least some CYA in the water or else the chlorine will be too strong and will oxidize swimsuits, skin and hair too quickly and will also tend to outgas faster which may be what you are seeing.

I suggest you start off using Dichlor as your source of chlorine after an initial fill for a cumulative Free Chlorine (FC) dose of around 33 ppm. That will get you to around 30 ppm CYA. If you use the tub every day and add around 2 teaspoons of Dichlor after every soak in 350 gallons, then this is around 8 days of Dichlor usage. After that, switch to using bleach -- Clorox Regular unscented 6% bleach.

If you still find your chlorine usage to be high after doing the above, then shock the tub with bleach since you may have chemicals that need to be oxidized from the fill water. If your tub is new, then you'll need to replace the water much sooner on that first refill since there's often a lot of oil and contaminants in a new tub.

You say you are using HTH for your tub. Can you be more specific about the HTH sanitizing product you are using? Is it Dichlor? Is it bromine tabs? Is it Cal-Hypo? I can see that HTH Spa Shock is Cal-Hypo and it appears that HTH Spa Chlorinating Granules are also Cal-Hypo. It would not be good to use too much Cal-Hypo in a spa. If the Calcium Hardness (CH) gets too high, you can end up with scale. If you have a good test kit, such as the TF100 from tftestkits.com, then you can check your CH level and stop using the Cal-Hypo when the CH gets to around 120-150 ppm. So you'd start with some Dichlor, then use Cal-Hypo, then switch to bleach -- if you really want to use the Cal-Hypo (otherwise, just go from Dichlor to bleach).

Richard

 

[Curmudgeon]

Thanks for your response. This is the first filling of the hot tub.

The HTH I am using is just calcium hypochlorite. I chose to use HTH rather than bleach because a friend with a 35,000 gal. pool offered to let me dip into his chemicals since I will use so little compared to his pool. He has no CYA on hand yet this year.

I am making my new TF-100 test kit available to him in exchange. He has been relying on pool store tests with poor results. My gallon of HCl came from Home Depot and the borax from Walmart.

I am adding calcium hypochlorite to get 5 ppm Cl (~ 10 g) and my tests are usually 0.5 – 1 ppm Cl a few hours later. The HTH raises the pH to 7.8 so I add ~ 20 ml of HCl afterwards.

Current analysis:
pH = 7.5
Cl = 1 ppm
TA = 45
TC = 230
Borate est = 50

If I understand you are saying the CYA acts as a reservoir to buffer (so to speak) the "active" chlorine concentration (free chlorine). Thus I should shoot for the recommended 30 ppm CYA.

Thanks

Curmudgeon

350 gal at 103 *F.
HTH, muriatic acid, sodium bicarbonate, calcium chloride, and borax

 

[chem geek]

Yup, you've got it.

The rise in pH from the HTH or any other hypochlorite source of chlorine (e.g. bleach, chlorinating liquid, lithium hypochlorite powder) is not permanent. As the chlorine gets used up, the pH goes back down. The ongoing use of this source of chlorine is close to pH neutral. If the pH is rising when measuring the same FC level (i.e. the next time you need to add chlorine) then this is most likely due to carbon dioxide outgassing meaning the TA is probably too high.

So I wouldn't try adjusting the pH right after adding the chlorine. You should get to a point where you don't need to adjust the pH very much. You might need to have the TA much lower, even 60 ppm, if you have a lot of aeration from spa jets.

It looks like you've already got 50 ppm Borates in the spa so I'm surprised that the pH rose as much as it did when adding the Cal-Hypo -- that shouldn't have happened unless the pH was already high even before you added it.

If by "TC" you mean "Calcium Hardness (CH)", then that is already high at 230 ppm. You most definitely do not want to use the HTH Cal-Hypo product or you will risk getting scaling in your spa. Scaling occurs more at higher spa temperatures so you want a lower CH than in the pool -- usually not above 150 ppm in a spa. You can somewhat compensate for this by having a lower TA level which will also help keep the pH from rising as much. Is the 230 ppm the CH of your fill water? It sounds like you've already used some Cal-Hypo, but I do not know how much cumulatively. With Cal-Hypo, for every 10 ppm FC it also increases CH by 7 ppm.

Richard