Several weeks ago, began having difficulties sustaining appropriate FC levels (I target 5-7% of CYE). Weather here in St Louis was rainy, then hot, lots of swimmers-- I figured those factors were driving higher chlorine demand, so I turned up the SWG...eventually to 100% and then extended hours until it was running 24 hours. Still had trouble keeping FC in the pool. Except for FC, water balance was fine (see below). Water was and is perfectly clear.
Became concerned it was the SWG, had the tech run a diagnostic-- it seemed fine. I turned the SWG off and adopted manual dosing-- pool was consuming about 5 gallons of 10% chlorine/day to maintain a ~5.0 FC. Tried an OCLT, dropped from 7.0 to less than 2.0.
CYA is 70-80 (I'd say 70, pool store says 80), so I went with a target SLAM FC level of 32. I'm 5 days into the SLAM, about 100 gallons of 10% chlorine and a replacement brush with no reduction in chlorine demand. The OCL last night was about 8ppm (from 32ppm to 24ppm). CC throughout has been between 0 and .5 (using a 10ml sample, but it has generally been clear or faintly pink). Filter is clear, light niches are clear, skimmers are clear. Water is still perfetly clear.
The one thing is this: my phosphates are high: about 2800. I've been stalwart in my belief that the phosphates don't matter if I maintain sufficient FC...and that this situation will respond to the SLAM...but I'm starting to wonder if there's something else going on here.
i) Does it make sense to augment the SLAM with polyquat-60?
ii) Should I do a significant partial drain to reduce my CYA and phosphates?
iii) Should I pursue some other phosphate removal approach? (I don't know how I'd vacuum-to-waste without hiring a pool company)
iv) Any other ideas about this chlorine demand?
Pre-SLAM water balance
FC: dropped below 2.0 (we generally target 5-6)