Why CYA in covered hot tub?

[MasterTinkerer]

Switching my hot tub over to saltwater. Bought a Saltron mini on eBay.

As I think about the swap over, I started to wonder about the need for stabilizer. The hot tub is covered at all times - except when I’m in it. So the exposure to sunlight is 30 minutes per day max. Generally it is used in the evening, so I’d say it’s closer to zero exposure.

 

[duraleigh]

While you don't need it to protect FC from the UV, CYA provides a nice buffer to ward off the harsh affect of the chlorine.

 

[Donldson]

CYA buffers the chlorine so it won't be as harsh. In a hot tub that is especially useful in keeping things comfortable. Since you don't need it for protection from solar degradation you can aim for a lower number than you normally would for saltwater, something like 20-30 ppm. Make sure to check it at least monthly as you can easily lose 5-10 ppm of CYA each month in a hot tub as it breaks down from the heat and chlorine.

 

[MasterTinkerer]

I follow what you guys are saying, but by adding CYA, I will need to increase FC in order to have the same effective sanitization. So I still don't understand the need to have CYA in a covered tub...

 

[JoyfulNoise]

I follow what you guys are saying, but by adding CYA, I will need to increase FC in order to have the same effective sanitization. So I still don't understand the need to have CYA in a covered tub...

With zero CYA in water and an FC of 1.0 ppm, you'd have almost 0.5ppm hypochlorous acid (HOCl) which is the sanitizing and oxidizing form of chlorine in water. That's about 5X the amount of hypochlorous acid when you add 30ppm CYA and 1.5-2ppm FC. So, based on the active chlorine levels, your tub water will be very harsh on skin, clothes and hair. Also, without CYA to buffer and moderate the amount of HOCl, you will likely generate higher concentrations of the nastier CCs (nitrogen trichloride will be more favorably produced at the expense of monochloramine) which will make the tub smell bad and lead to irritated noses and eyes.

You should be able to add just the minimal amount of CYA (20-30ppm) and the Saltron will keep up with 2-3ppm FC. You can then use manual bleach additions to raise the FC higher after soaking to take care of the bather waste. The Saltron is really there to keep detectable levels of FC in the water, it's not really there for oxidation control. I'm assuming the purposes of you buying it was to get away from having to use liquid chlorine but that's not really the best approach. SWG's add FC very slowly and are good at maintaining a certain level commensurate with their output, they are not good for shocking or generating higher levels of chlorine in a short period of time.

 

[MasterTinkerer]

ah ha... maybe it's just because I'm a big dork, but that explanation made a lot more sense to me! Thanks Matt!

I understand the need to add liquid each time, and will continue to do so. My previous calculations said 1/3 cup per person per 30 minutes of tub time.

The reason for the saltron is to keep a minimal level of FC there during non-usage times... and for the benefit of the salt water itself.