Had about 20 people over Friday and Saturday, and my free chlorine tanked from 3 ppm down to basically zero by Sunday morning despite keeping the pump running the whole time. I shocked Sunday night with a good amount of granular and retested Monday - still sitting around 0.5 ppm, which tells me there's some combined chlorine buildup I need to burn through. Going to do a proper breakpoint calculation and hit it again tonight, but curious if anyone else is dealing with this kind of demand spike after a big party weekend or if my CYA might be creeping up and masking things. Tested CYA at 60, which should be fine, but maybe worth another look.
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Chlorine demand through the roof after 4th of July weekend
- Thread starter NC_Nick
- Start date
poolmom_sarah
New member
Same thing happened to us after a big cookout last summer. The combined chlorine just kept eating through shock until I held it at a high enough level for several hours straight - took more product than I expected, but once it finally broke through the demand evened out pretty quickly after that.
Nick, that 0.5 reading after shocking is pretty telling. When free chlorine just refuses to climb despite adding a decent amount of product, you're almost always fighting a combined chlorine issue, not just depletion. The breakpoint calculation is the right move - just make sure you're accounting for total chlorine minus free chlorine to get your combined number, then shooting for roughly 10x that combined value in free chlorine to actually break through it. A lot of people underestimate how much product that actually works out to be, especially after a holiday weekend with heavy swimmer load.
The other thing I'd check is whether you had any sunscreen and sweat in the water, because that stuff creates chloramines fast and it compounds quickly when you have 20 people over two days. I had a similar situation a couple summers ago after a big gathering and I kept adding shock thinking I was getting somewhere, but I was basically just treading water until I held FC up in the 10-12 ppm range for a few hours and retested to confirm I'd actually crossed the breakpoint. Once I did that, the chlorine started holding like normal again. Give it time after you dose, don't retest too soon, and run your pump the whole time to keep things circulating. You'll get there.
The other thing I'd check is whether you had any sunscreen and sweat in the water, because that stuff creates chloramines fast and it compounds quickly when you have 20 people over two days. I had a similar situation a couple summers ago after a big gathering and I kept adding shock thinking I was getting somewhere, but I was basically just treading water until I held FC up in the 10-12 ppm range for a few hours and retested to confirm I'd actually crossed the breakpoint. Once I did that, the chlorine started holding like normal again. Give it time after you dose, don't retest too soon, and run your pump the whole time to keep things circulating. You'll get there.
Nick, the 20 people over two days is a heavy load and honestly the combined chlorine buildup after that kind of bather load can be stubborn. One thing that helped me when I was in a similar situation - I added my shock at night after the sun went down and kept the pump running through the morning, then tested before anyone got in. Giving it that longer contact time without UV burning off the chlorine made a noticeable difference in how far I could push the free chlorine before it stabilized.
Also worth checking your cyanuric acid if you haven't already. If it's crept up over the summer from stabilized chlorine use, it can slow down how effectively the chlorine works against combined chlorine, and you basically need more product to accomplish the same thing. Pool calculator sites can help you dial in the math once you have that number.
Also worth checking your cyanuric acid if you haven't already. If it's crept up over the summer from stabilized chlorine use, it can slow down how effectively the chlorine works against combined chlorine, and you basically need more product to accomplish the same thing. Pool calculator sites can help you dial in the math once you have that number.
NC_Nick, this is almost the exact same situation I ended up in last summer after a pool party and I had no idea what was happening at the time. I kept throwing shock at it and checking the next morning and the number just stayed stubbornly low, and I didn't understand why until someone here explained the combined chlorine piece to me. What finally helped in my case was testing at night before I added anything so I had a solid baseline on what my total chlorine actually was versus the free reading, because I was basically guessing before that. Once I did the math properly and dosed for breakpoint instead of just adding what I thought "seemed like enough," the chlorine finally held the next day. If you don't already have a decent test kit that reads both free and total separately, that would be my first step before adding any more product.
Coming at this from the hot tub side of things so take it for what it's worth, but breakpoint chlorination is something I've had to deal with repeatedly in my tub after heavy use weekends and the principle is the same. The thing that tripped me up early on was not realizing how much the combined chlorine number can balloon after a big bather load - I'd see a low free chlorine reading and just keep adding product without actually calculating what I needed to get above that breakpoint threshold. Once I started doing the math first and then committing to holding the level high for a few hours instead of just shocking and hoping, it clicked. Nick, sounds like you're already on the right track with the breakpoint calculation, just don't shortchange yourself on the dose and give it enough time to actually work through the demand before you test again.