Do you add chlorine at night or during the day?

[Warren Dale]

Curious what most people do. I’ve always added chlorine in the evening because someone told me sunlight burns it off faster during the day. Not sure how much that actually matters. Does timing really make a difference or am I overthinking it?

 

[Danny Meerman]

Curious what most people do. I’ve always added chlorine in the evening because someone told me sunlight burns it off faster during the day. Not sure how much that actually matters. Does timing really make a difference or am I overthinking it?

Evening is generally better. Sunlight does reduce chlorine levels so adding it at night gives it more time to work before UV starts breaking it down.

 

[Joe Smith]

Curious what most people do. I’ve always added chlorine in the evening because someone told me sunlight burns it off faster during the day. Not sure how much that actually matters. Does timing really make a difference or am I overthinking it?

I’ve done both and haven’t noticed a massive difference as long as levels are consistent. The bigger factor for me was just sticking to a routine so I didn’t forget.

 

[John Knight]

Curious what most people do. I’ve always added chlorine in the evening because someone told me sunlight burns it off faster during the day. Not sure how much that actually matters. Does timing really make a difference or am I overthinking it?

If you run your pump mostly during the day just make sure there’s enough circulation after adding it. I usually add mine at dusk and let the pump run a couple extra hours.

 

[Warren Dale]

Good to know I wasn’t totally off. I’ll stick with evenings since that seems to make sense with how I run the pump anyway. Thanks for the input.

 

[Liam Brooks]

You’re not overthinking it, timing does matter a little, just not in a dramatic way.

I add mine in the evening most of the time, mainly because it’s more efficient. UV absolutely breaks down free chlorine, especially if your CYA is on the lower side. If I add 3 to 4 ppm at noon in July, I can watch a chunk of that disappear by late afternoon. Add the same dose after sunset and it gets a full night to oxidize organics before the sun starts chewing on it.

That said, daytime dosing isn’t “wrong.” If your stabilizer is in range and your daily chlorine loss is predictable, it’ll still work. I’ve added during the day plenty of times when I’m adjusting after testing. The key is knowing your pool’s demand. My pool in 85 degree water with a decent bather load will lose more than 2 ppm on a sunny day no matter what.

What made it click for me was tracking daily loss for a week. Once I knew roughly how much I lose to UV versus actual demand, the timing question felt less mysterious. Evening just stretches your chlorine’s efficiency a bit. It’s not magic, just less sun fighting it right away.

 

[Robert Smith]

I used to think evening was the “rule” too, but after a while I realized it mostly comes down to how stable the pool already is. When my stabilizer was low one summer, chlorine I added during the day would drop way faster. Once I got that dialed in, the timing mattered a lot less.

These days I usually add it late afternoon or early evening just because it fits my routine. I’m already out there emptying skimmer baskets and glancing at the system. I’ll check chlorine and pH, then add what’s needed while the pump is running strong. One thing I noticed is circulation makes a bigger difference than the clock. If the pump runs a couple hours after dosing, it mixes way better.

I also keep an eye on filter pressure before adjusting anything. If it’s creeping 4 to 5 psi above my clean baseline, I rinse the cartridge first. When the filter is dirty the water movement isn’t great, and chemicals don’t distribute evenly. Once I started paying attention to that, the pool stopped having those weird swings where chlorine seemed to disappear faster than expected.

 

[Nathan Parker]

I used to think this was a strict rule too, but after a few seasons I realized it’s more about efficiency than right vs wrong.

I usually add chlorine in the evening if I can. My pool gets full sun most of the afternoon and in mid summer the water temp sits around 84 to 86. If I dose at noon, the level looks fine for a bit but by late afternoon a good chunk of it is already gone. That’s mostly UV working on the free chlorine, especially if your stabilizer isn’t very high.

When I add it after sunset it just seems to hold better because the sanitizer has all night to work on whatever organics are in the water before the sun hits it again. My pump is a Pentair SuperFlo VS and I normally let it run a couple hours after dosing so everything mixes well and the chlorine spreads evenly through the pool.

That said, I’ve definitely added it during the day when I’m testing and see it’s low. As long as the pump is running and circulation is good it still works. For me the bigger factor ended up being water temp and bather load. On really hot weeks with a lot of swimming the pool just burns through chlorine faster no matter what time I add it.

Anyone else notice their daily chlorine loss jump once the water creeps past the mid 80s? Mine always seems to change around that point.