isipwater - Clearly you are wise enough to know TFPC is the way to go. You joined in 2015, you have a TF-100, and you know high CYA doesn't just go away on its own because you started threads in 2016 and 2017 and 2018 all about high CYA. You have fought this enough. As described above, drain from one end, fill at the other. Get your CYA down. Live the good life. Your plaster will never be dry doing it this way. Whatever the inefficiently compared to some perfect drain/refill doesn't matter if you can avoid three more years of struggle, isn't that true?
For anyone that reads this in the future and doesn't understand why all of us are so steadfast in our encouraging isipwater to start exchanging water as soon as scheduling and logistics allow, please consider this little hypothetical:
Reminder that the
SLAM Process process is an aggressive and efficient assault on algae and contaminants - while remaining responsible to the longevity of the pool itself. Another concern with performing a
SLAM Process process on a pool with CYA over 100 is the extreme nature of it. For example, let's say you have a CYA 200 pool and you wanted to do something inspired by the shock level and maintain process. Let's pretend you calculated your shock FC to be CYA 200*40% = shock FC 80 and somehow that didn't scare you straight right then and there. So first you would have to let your FC fall under 10 to perform a PH test - which would be way below your best guess minimum FC value of CYA 200*7.5% = min FC 15. Each time you wished to check PH you'd be going 5 or more FC below your min, and even lower below your target, whatever it would be. In any case, you move forward with it. So you close the pool to swimmers since your FC will be too low for your CYA and the water unsafe. You wait however many days it takes for FC to fall under 10. You then test and adjust PH to 7.2. You wait 30 mins and then you're ready to raise your FC from 10 to 80. Thank goodness your pool is only 20,000 or this might get ridiculous. Let's assume that you are lucky and live somewhere that has a pool store/grocery/hardware store that has fresh 10% trade bleach liquid pool chlorine shock, and it's only $3.33/gal. So for your very first addition to hopefully reach your ludicrous goal of FC 80 you slowly pour 1,791 oz (aka 14 freaking gallons) in front of the return facing the deep end. As you spend the better portion of 15 minutes doing this, you do the mental math and realize you're pouring a nickle and a half into the pool each second, totaling You realize that just this one initial dose is at least three times what you would use in an entire week combined if you'd just do the water exchange and move on. Oh well, you know better than the largest pool forum on the internet. Then you brush the pool if the fumes aren't hazardous. I bet at this point you're really looking forward to testing the FC after it circulates for 20-30 minutes, right? Think about all those approximately 160 drops of R-0871, one drop at a time, as you pray you don't lose count and have to start over, having wasted a massive % of the bottle on just one of many tests to come. Hope you ordered the XXXXXXL option! Just kidding! Luckily for you, you remember that TFP taught you the Taylor FAS-DPD test begins to bleach out around 20-25ppm. That means you have to very carefully dilute your pool water test sample 1:4 and multiply the final FC by 5. You know how important it is to be exact in the dilution because since the less precise you are here, the more off your calculated FC will be. So while you wait for the water to circulate so you can verify you reached your goal of 80ppm FC, you remember how corrosive strong bleach solutions can be and wonder what this nearly-nuclear level of bleach will do to your equipment that has surfaces that can rust exposed to the water - especially the internal components. Hopefully the spa, heater, and solar will all make it out of this okay and not fail years before they would have otherwise, and potentially in a way that the warranty wouldn't cover. You snap back in the moment and remember that you need to haul off your empty 14 gallon jugs for disposal/recycle. You look over at your stockpile of 23 more gallon jugs and hope it is enough to complete the process, since you bought out all the stores in town and you will have to start using regular bleach, which means even more jugs, and this empty pile of 14 already seems like a lot, and you've only just begun day 1 of who knows how many. You realize that you're frowning as you remember the concerned looks you got when you were buying up the initial load of 37 gallons and how people assumed you had nefarious plans, but they had no proof. Oh well, what's done is done. Back in the moment. Time to get the sample so you can dilute it and run your test ... but wait ... what's going on? It looks like you're hooking up the manual vac so you manually vac to waste/pump down the deep end while adding water in the shallow end! You've seen the light! Oh you're smiling! You're so at peace and excited to get your trouble free pool that you don't even regret the $46 in bleach you already put in because you know it helped you find truth and get on a better path.
You are so grateful to TFP that you become a Supporter! You shed a tear of joy over the epiphany that you are living the happy ending to inheriting the high CYA pool, and your redemption story was beautiful.
I missed isipwater's confusion and thank you for posting the answer.