Had 20+ people in the water two weekends in a row and my free chlorine is just getting eaten alive, even with the pump running 12 hours a day. I shocked to 20 ppm after each party and it drops back to near zero within 48 hours. CYA is sitting at 60 so I dont think that's the problem. Wondering if I'm dealing with a chlorine demand issue or if my phosphates have climbed enough to cause this - haven't tested those in a while.
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Chlorine demand is insane after back to back pool parties
- Thread starter NC_Nick
- Start date
Before assuming phosphates, I'd rule out combined chlorine first. Do you have a DPD test kit that shows you both free and total chlorine? If your total is running notably higher than free, you've got chloramines building up and shocking to 20 ppm may not have been enough to fully break through, especially after that much bather load across two consecutive weekends.
The other thing worth checking is whether you actually hit breakpoint chlorination. With CYA at 60, you need free chlorine to stay above roughly 7.5 ppm for any meaningful oxidation to happen, and if it's crashing back to near zero in 48 hours that suggests the demand never fully cleared. I'd try a sustained shock this time, meaning get it up to 30 ppm and test every 12 hours, keeping it from dropping below 15 until it finally holds overnight. Phosphates can slow algae management down but they don't typically cause chlorine to vanish that fast on their own. The bather load is the more likely culprit here.
The other thing worth checking is whether you actually hit breakpoint chlorination. With CYA at 60, you need free chlorine to stay above roughly 7.5 ppm for any meaningful oxidation to happen, and if it's crashing back to near zero in 48 hours that suggests the demand never fully cleared. I'd try a sustained shock this time, meaning get it up to 30 ppm and test every 12 hours, keeping it from dropping below 15 until it finally holds overnight. Phosphates can slow algae management down but they don't typically cause chlorine to vanish that fast on their own. The bather load is the more likely culprit here.
poolmom_sarah
New member
NC_Nick, with that kind of bather load two weekends running I'd be curious what your combined chlorine number actually is. That chloramine buildup can make it look like the chlorine just disappears when really it's getting consumed trying to oxidize all that waste. The break point calculation when CC is high can mean you need to shock quite a bit past 20 ppm to actually clear it out.
Same thing happened to me after a graduation party two summers ago. Probably 25 people in the water over a Saturday afternoon and I was chasing chlorine for almost two weeks straight. Turned out my combined chlorine was sitting around 1.5 ppm and every time I shocked I wasn't quite getting over the breakpoint, so I kept burning through product without actually fixing anything.
Once I got a decent test kit that showed me both free and total, the picture got a lot clearer. I had to push it up to around 30 ppm and hold it there for a bit to finally knock it out. The phosphate angle is worth checking at some point but in my experience the CC problem will mask everything else until you deal with it first.
Once I got a decent test kit that showed me both free and total, the picture got a lot clearer. I had to push it up to around 30 ppm and hold it there for a bit to finally knock it out. The phosphate angle is worth checking at some point but in my experience the CC problem will mask everything else until you deal with it first.
NC_Nick, the 48-hour drop back to near zero is the thing that stands out to me. When I had a similar situation a few summers back, I kept thinking I was shocking enough because the number looked high right after treatment, but the combined chlorine was the real culprit the whole time. My total was running maybe 2 ppm above my free and I just wasn't hitting the math right on breakpoint - you need roughly 7.6 times your CC to actually cross it, so if your combined is sitting at 1.5 like jdawg mentioned dealing with, you're looking at somewhere north of 10 ppm just to get over the hump, and then you still want buffer on top of that.
With a CYA of 60 the other thing I'd watch is whether you're actually holding that 20 ppm long enough for it to do the work. At that stabilizer level your chlorine is partially protected from UV but it also means you need a higher absolute level to drive the oxidation reaction. I'd get the full picture with a DPD test first so you know your actual CC number, then do the breakpoint math and shock accordingly, and honestly try to do it at dusk or overnight so the sun isn't competing with you. Phosphates can cause headaches but I've never seen them produce this kind of crash this fast - this really does read like an incomplete breakpoint situation that's just been repeating itself each weekend.
With a CYA of 60 the other thing I'd watch is whether you're actually holding that 20 ppm long enough for it to do the work. At that stabilizer level your chlorine is partially protected from UV but it also means you need a higher absolute level to drive the oxidation reaction. I'd get the full picture with a DPD test first so you know your actual CC number, then do the breakpoint math and shock accordingly, and honestly try to do it at dusk or overnight so the sun isn't competing with you. Phosphates can cause headaches but I've never seen them produce this kind of crash this fast - this really does read like an incomplete breakpoint situation that's just been repeating itself each weekend.
NC_Nick, I'm kind of in the same boat as you right now and watching this thread closely. One thing I didn't realize until recently is that the test strips I was using don't even show combined chlorine separately, so I had no idea how bad my chloramine situation actually was until I got a proper drop test kit.