So I have some updates everyone... My pool builder suggested that I didn't need to worry about CYA, but that seems like dumb advice. I watched the speed that Chlorine and ORP levels dropped without CYA, and it was pretty surprising, even in winter as it is now.
During the first few weeks, I'd add liquid Chlorine about once / week, but it would effectively be zero within a couple of days. As you all know, when I started adding salt to my pool, the SWG was having a tough time generating Chlorine to keep up with the loss during the day. I do think this was partially caused by a low salt level because I was slowly bringing it up to where it needs to be (it's at 3200 ppm now according to the Taylor test kit), and partially caused by the sun burning off what Chlorine could be generated... hence the 0.8 level I quoted last Wednesday.
Starting on Thursday, I added 4 lb of CYA in granular form. I put this in a sock in the skimmer, and after 24 hours squeezing it felt like there was only a paste in the sock; no granules. At this point I pulled the socks out, and slowly mixed the paste with buckets of pool water and poured the 'milk' down the skimmer. As you would expect, this resulted in a low pH initially, but by Saturday afternoon, I was getting just under 30 ppm CYA result with the Taylor test kit. Pool Math suggests that for my 18,000 G pool, I'll need 4lb 4oz to get to 30 ppm, and that after adding 4lb, I should be at 26.5 ppm, so this all does check out.
Anyhow, during all of this, the ORP reading obviously went low due to the CYA getting dissolved, and it's slowly recovering. The result of this is that the IntelliChem requested full Chlorine generation to try and raise ORP, and this took me from 1.2 ppm Free Chlorine on Friday night, to 6.4 ppm Free Chlorine by Sunday morning. The pool pump and SWG were only on from 7am - 4pm, so that's a lot of chlorine to be generated in such a short duration and in cold weather... good to see!
I've been monitoring the Free Chlorine level since then, letting it drop to < 4 ppm with no SWG and I'm seeing a drop of about 1.5 ppm every 24 hours. This is a lot lot slower than with no CYA. I think it would have been gone already if that had been the case.
I'm impressed that CYA works so well, and after such a short period of time.
ORP is still < 700 even though Free Chlorine is so high, but it's rising. I'm hoping that it will stabilise soon and I'll be able to find an ORP value that correlates with Free Chlorine for my pool.
Next step is to bring the Calcium Hardness up to a target of 300ppm.