Why A Minimum Cya For Bleach/dichlor Method?

[lagreca]

Why is there a minimum CYA level for the bleach/dichlor method?

If the spa is covered and primarily only used at night, is there a need for any dichlor, or can you start using bleach right away?

[chem geek]

CYA doesn't just protect chlorine from breakdown from the UV in sunlight, it also significantly moderates chlorine's strength. When CYA is present, most of the chlorine is bound to it so the active chlorine level (hypochlorous acid) level is lower. If you were to use bleach first without any CYA in the water, the chlorine would be too strong and would oxidize swimsuits, skin and hair too quickly and would outgas faster degrading hot tub covers faster and would be harsher on equipment. At 104ºF and 7.5 pH, 30 ppm CYA cuts down the active chlorine level by roughly a factor of 6. At lower temps, it's even a bigger difference (over a factor of 25 at 80ºF).

There is still plenty of active chlorine to prevent bacteria from growing and to oxidize bather waste (especially at the higher temp found in a spa). The FC should then be seen as mostly a measure of chlorine held in reserve (bound to CYA) released as needed to maintain a low but effective active chlorine level.

This is why one uses Dichlor initially to build up the CYA level. Alternatively, one could add some pure CYA to the water, but for spas it's usually easier just to use Dichlor to build up the CYA while adding chlorine at the same time. For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.