First, some history: We have a 23K gallon pool in southern CA that came with the house we bought a year ago. The SWG is an ESC48, and it was not working when we moved in. After getting the pool chemistry right, and checking things out, we found that the cell was clogged with calcium. I cleaned the cell in an acid bath, probably around 1:6 - 1:8 acid:water. I had to soak it overnight to clean all the deposits off, but afterward, it worked beautifully, producing Chlorine with about a 60% SWG on-time for a 10-14 hour pump time (I don't recall how long we ran the pump back in August/September 2013). A few weeks later, it quit working. Cleaning didn't help. So, we switched to regular chlorine (Leslie's Chlor-Brite) for the winter.
Fast forward to late April 2014. We bit the bullet and bought a $400 CompuPool GRC/EM replacement cell, calculating we would save at least that much in Chlorine over the next few years. We plugged it in, turned it on, and the ESC control panel was very happy... it said that it was at +/- 100, and all green lights. But, we couldn't keep the chlorine level up, even with the dial set to 100% SWG time. We tested the pool chemistry, which showed high alkalinity (~180), slightly high pH (~7.8), high calcium hardness (~500... this is normal for our area... the tap water isn't much softer, so it doesn't do any good to drain and refill), high phosphates (~200). CYA was a little high at 100, and the salt level was good at 5000 (everyone tells me this is high, but the ESC48 manual recommends 4500-5000, and higher isn't a problem for Chlorine production). We also had an algae problem. We added muriatic acid to the deep end overnight to bring down the alkalinity, which is now around 120. The pH had also improved to 7.6, and CYA was still good at 80. CH didn't change, as expected. First, we treated the phosphorus with Pool Magic + Phos Free, then a few days later, we shocked the pool with Chlor-Brite, and brought the chlorine level up over 10ppm, which got rid of all the algae (I still don't know the new phosphate level). We figured we had it all worked out.
Problem is, we still aren't maintaining a Chlorine level, even with a 10.5 hour filter time, and 100% SWG time. I know the cell works, because I put it in a 5gal bucket of pool water, and after 2 minutes, the 5gal of water was over 20ppm of Chlorine. Also, the other night, we had just the spa running for a few hours, and the Chlorine in the spa spiked up to around 5ppm.
Note that we also have a solar water heater for the pool, and the pool water is up around 85 degrees F.
Any ideas on what can be going wrong? Has anyone else had underperforming ESC48 cells? What can I try, what can I check?