Okay. I put in too much Dichlor over time using the "Vermont/Norhtman" method (as recommended by the person who sold me my spa) and plan to drain my water since my CYA is around 150 ppm. I've learned my lesson. Until I replace my water, however, TFP recommends a level of 11 to 18 ppm FC which is what would normally be a shock level at 20 CYA levels! Am I in danger of "bleaching" myself or being overcome by Cl2 gas at these levels (18 ppm or greater)? Put another way, my test strips read over 10 PPM FC; doesn't CYA "buffer" the Cl, so the effective readings should show lower levels anyway? Or do the FC readings from my chem tests some how "ignore" the binding of Cl to the CYA. It seems to me if, if I'm reading 10 ppm of FC, then that is 10 ppm of FC, regardless of CYA levels, including effectiveness.

Thanks for all the great help! Your web site is one of the few that seems to be consistent and in line with my background in chemistry and engineering.

Welcome to the forum The excessive CYA effectively buffers the FC so that it is the equivalent of far, far less chlorine.

More importantly, why not simply go ahead now and correct it with a drain (perhaps partial) and get your CYA to a manageable level? Managing the excessive CYA is doable but seems virtually always to be more trouble than it's worth.

And go a day or two without using the spa waiting for it to heat up? Never! Actually, we are going on a trip very soon and I plan to replace the water when we get back. My question is more academic. Does CYA buffered FC show up at the same concentrations as non-buffered FC? I thought the test kits work by detecting available free chlorine, and if it's buffered by the CYA, then it is essentially not free. For example, I can add bleach to the water based on the calculator to get my levels up to 20 ppm FC, but the test strips should just show up as 3 ppm FC due to all the buffering going on.

Hi, welcome to TFP! I myself cannot answer your question, but I have located a few threads you may find helpful. If you go to the forum section labeled "the deep end" that is where you can read some of the more advanced discussions on CYA and other pool chemicals.
Here is a sample:
chlorine upper limit for a given level of CYA
debunking the chlorine lock myth
chlorine/CYA chart comments

The chlorine tests show the chlorine that is bound to CYA because chlorine gets released from CYA quickly well within the time of the test (about half is released every 0.25 seconds so 94% in 1 second). The indicator dye reacts with active chlorine (hypochlorous acid) and the FAS-DPD titrant drops do as well (and reverse the dye reaction near the end of the test which is what makes the sample go clear). So you can't use the FC test to determine the active chlorine level. It only tells you the total amount of chlorine available to become active -- that is, the amount that is in reserve plus the small amount that is already active. Even with no CYA in the water the FC test isn't measuring active chlorine since hypochlorite ion gets converted to hypochlorous acid very quickly so gets measured in the FC test. So just as without CYA to know the active chlorine level one must know the pH, similarly with CYA in the water to know the active chlorine level one must know the CYA level.

At hot spa temperatures, there is more active chlorine at the same FC/CYA ratio than at typical pool temperatures. Nevertheless, 11-18 ppm FC with 150 ppm CYA at 104ºF is equivalent in active chlorine (hypochlorous acid) concentration to 0.4-0.7 ppm FC with no CYA so still not very high. At 80ºF in a pool, it's more like 0.08-0.14 ppm FC with no CYA. Nevertheless, we don't lower the recommended FC/CYA ratios for spas because bacteria can grow fairly quickly and biofilm formation is a bigger problem in spas so one needs to kill the bacteria more quickly to ensure proper disinfection and spas aren't always at 104ºF anyway.

As described in the "Chlorine/CYA Relationship" section in this post, most of the chlorine is bound to CYA and is essentially not a disinfectant and is a much slower oxidizer (roughly 1/100th the rate at the same concentration). It is also not volatile so won't outgas. It is active chlorine (hypochlorous acid) that is the fast-acting disinfectant, is a primary oxidizer (though hypochorite ion can also oxidize some chemicals) and is also somewhat volatile (though it is in equilibrium with chlorine gas, the amount of chlorine gas is very small and what is outgassed is mostly hypochlorous acid), but the amount of active chlorine is much lower when CYA is present.

Some additional technical posts that may be helpful for you to look at include this one, especially this one and this one.

Thank you Zea3 and Chem Geek. I'll check out the sites you provided now. This web site is simply AWESOME! I feel like it's almost a "privledge" to get an answer from "Chem Geek" himself...I'm glad you use your powers for good. Although I still on occasion branch out and read what some folks are saying on the "other" web sites, I still can't believe the number of contraditions and simply false information that is out there...that is until I found this site. Thanks again for your time.