Chlorine Level dropped to zero overnight

[Surf NH]

Hi folks, looking for some opinions here. The pool stats:

Located in Southern NH, two year old 39k gallon Gunite pool.
Opened again last week of April, it's been a cold spring, water temps past month hovering around low to mid 70's in June though it did reach 80F with some water heater help a week ago.
Chlorine and UV/Ozone system (yes I know opinions on that)
We have a chlorine disk feeder system, I've been keeping it loaded every day to the top. It uses roughly 2 disks per day in order to maintain 2 to 3 ppm free chlorine level.
My test kit is a Taylor K-1005.
I've been using the pool service of our installer, every two weeks, since install. I watch everything thing they do, read this forum, and test daily because I'm an anal nut.....trying to learn enough to eliminate the service cost.
We've hardly used the pool since opening in April, so very little contamination. It's located in an open area, gets all day sun, no trees, nothing besides pollen and the occasional frog in there, pool is spotless and the water perfectly clear.

The following situation happened this week, and it happened once last month two weeks after opening. The pool is perfectly clear, chlorine at 2 to 3ppm, pH at 7.5 maintained with muriatic acid periodically, total alkalinity at 90, calcium hardness at 400. Everything looks perfect, I'm able to maintain this with little variation over a month. The pool service guy comes this past Tuesday, I watch him test (more expensive Taylor kit), he has roughly the exact same results as mine. He doesn't add any chemicals, checks also clarity says it's perfect. Pool looks great that evening, no rain overnight so nothing changes, I go out to test the pool mid morning Wednesday and the chlorine is zero, I mean absolute zero ppm. I look at the disc feeder, it's down two discs like normal which means it's feeding chlorine into the water.

First thing I do is put the disc feeder on maximum flow, and I add a single disc in each of the two filter baskets in the pool. I retest mid day yesterday, it's still zero. At days end I pour in two gallons of super shock. According to the label that's may half of a normal shock treatment for my size pool, but I only had two gallons. I test this morning (Friday), now I get a chlorine reading over 5 ppm....so I'm back with some chlorine in water.

Also to mention, my first thought was my test liquids from Taylor need to be replaced. They all have 2024 usage dates on them, but I bought replacements last month from Taylor just in case....plus I test a lot so figured I'd use them up. I parallel path tested with the new test liquids yesterday, same results....the new liquids had 2025 usage dates on them.....so I think I eliminated the test kit as the issue.

What I need help with.....I'm able to maintain very consistent chlorine ppm for numerous weeks, with extremely consistent chlorine disc usage, and then in the bat of an eye, the level goes to zero.....not even gradually, it just disappears. This has happened twice in a month.

Any idea what could cause this, for sure I'm missing something.

 

[PoolGate]

Honestly you can either use the TFP method or your pool service method. The 2 are usually incompatible. Right off the bat I can see your chlorine level is I assume too low (need CYA to know for sure). You are always on the verge of algae with a really low chlorine level. We need to know your CYA level to know for sure though.

Can you post a full set of tests? CL, CC, CYA, TA, CH?

 

[drsipe]

First, you need to get either the TF-100 test kit of the Taylor K-2006 test kit,,,, I know they are expensive, but in the end, it will save you money and anxieties.

With the proper test kit, you can get more accurate readings and including the CYA levels in your pool. The CYA level is important to determine the proper free chlorine level that you need to keep your pool properly sanitized. Without knowing your CYA level it is not possible to determine if your chlorine level is adequate, but I would suspect that 2 - 3 ppm is too low and zero is defiantly too low. Once your chlorine level is low, algae gets a foothold and just raising the chlorine level to a few ppm will not kill the algae.

Since you have been down to zero chlorine (or at least suspect you have been down to zero) you need to perform a SLAM. For a SLAM you will need a proper test kit and a supply of liquid chlorine.

Swimming Pool Test Kits Compared

What pool test kit should I buy? Why not use test strips? Our recommended test kits will pay for themselves in no time!

www.troublefreepool.com

SLAM - Shock Level and Maintain

We’ve all had to deal with a green pool. Clear algae from your pool with a SLAM! Read here to use shock level and maintain to clear your pool.

www.troublefreepool.com

Until you get a proper test kit and you can begin your SLAM, you should use pool math to determine the amount of liquid chlorine to add 5 ppm to your pool daily, this will not solve your problem, but it will keep it from getting out of hand. The only real cure is a SLAM.

 

[Surf NH]

thanks....CLA is 80-90.....no algae in pool since restart late April, ....I've got the Taylor test kid, but it's the smaller version....intend to get the better one also, but this is testing extremely close to the expensive Taylor kit my service guy uses. Again, I used liquid chlorine to start a burn out last night, got it up over 5ppm now. What I don't understand is how I can be consistently at 2-3 ppm for weeks, then overnight it goes to zero, while my chlorine discs are still be utilized up?

 

[Decoy205]

If your cya is too high and you’ve been running chlorine at low levels it’s not enough fc to keep up with sanitizing. My guess is you have an algae bloom about to happen.

If you follow TFP you have to ditch the solid chlorine tabs and move to liquid chlorine or better yet get a Salt system.

Your low level of chlorine plus most likely a high stabilizer level is causing an outbreak of organics to grow. Keep adding liquid chlorine no more pucks. Get the right test kit and follow slam procedure as mentioned above.

 

[PoolGate]

thanks....CLA is 80-90.....no algae in pool since restart late April, ....I've got the Taylor test kid, but it's the smaller version....intend to get the better one also, but this is testing extremely close to the expensive Taylor kit my service guy uses. Again, I used liquid chlorine to start a burn out last night, got it up over 5ppm now. What I don't understand is how I can be consistently at 2-3 ppm for weeks, then overnight it goes to zero, while my chlorine discs are still be utilized up?

Easy. 2-3 PPM is too low for your CYA level. You are playing with fire trying to skirt the bare minimum for chlorine levels. And now you see why you shouldn't do that.

 

[tikoh]

thanks....CLA is 80-90.....no algae in pool since restart late April, ....I've got the Taylor test kid, but it's the smaller version....intend to get the better one also, but this is testing extremely close to the expensive Taylor kit my service guy uses. Again, I used liquid chlorine to start a burn out last night, got it up over 5ppm now. What I don't understand is how I can be consistently at 2-3 ppm for weeks, then overnight it goes to zero, while my chlorine discs are still be utilized up?

What test did you use for CYA? 3ppm FC is way too low for 80 CYA...but I'm surprised you're dropping to zero if you're using a tab feeder and the water is clear...

Has the feeder ever run out of tabs?

In any case, do the Taylor CYA test, get rid of the tabs, and go from there.

 

[Surf NH]

Hey guys, thanks for all feedback, I'm using a Taylor test kit, the K1005......using DPD reagents, and a turbidimetric CYA test, same test methods as the Taylor K2005 kit. I'm not sure of the exact difference of the kits, they seem to use identical test methods and reagents? Chemical reads the day before it went to zero, per our service guy who was using a Taylor commercial test kit (I watched him test, my test kit close numbers by slightly different):

Chlorine 3ppm
pH 7.5
Alkalinity 100
Hardness 300
CLA 80

the feeder has never run out of tabs, I've been running 2 to 3 ppm free chlorine level for two years, never had a spec of algae since pool was installed, the water is pristine clear as as are the Gunite walls....they're white, easy to see buildup.

I'm sorry, I missed the issue on the chlorine tabs, exactly why are they bad again? To me it's logical that they feed at a consistent rate 24/7, going through 2 tabs per 24 hr and refill the feeder each day. Again, from a chemistry standpoint, what would cause the chlorine test to go to zero overnight when the pool water is continuously taking on chlorine from the tabs and testing had been dead nuts consistent for a three weeks?

 

[tikoh]

Hey guys, thanks for all feedback, I'm using a Taylor test kit, the K1005......using DPD reagents, and a turbidimetric CYA test, same test methods as the Taylor K2005 kit. I'm not sure of the exact difference of the kits, they seem to use identical test methods and reagents? Chemical reads the day before it went to zero, per our service guy who was using a Taylor commercial test kit (I watched him test, my test kit close numbers by slightly different):

Chlorine 3ppm
pH 7.5
Alkalinity 100
Hardness 300
CLA 80

the feeder has never run out of tabs, I've been running 2 to 3 ppm free chlorine level for two years, never had a spec of algae since pool was installed, the water is pristine clear as as are the Gunite walls....they're white, easy to see buildup.

I'm sorry, I missed the issue on the chlorine tabs, exactly why are they bad again? To me it's logical that they feed at a consistent rate 24/7, going through 2 tabs per 24 hr and refill the feeder each day. Again, from a chemistry standpoint, what would cause the chlorine test to go to zero overnight when the pool water is continuously taking on chlorine from the tabs and testing had been dead nuts consistent for a three weeks?

The issue is that the tabs contain CYA along with chlorine. As you keep using them, your CYA will continue to rise. While moderate levels of CYA protect your free chlorine levels from being burned off by sunlight, CYA also reduces the effectiveness of chlorine. These articles give a pretty good explanation, but feel free to ask any questions!

The Free Chlorine and Cyanuric Acid Relationship

Managing your cyanuric acid and Free Chlorine levels is key to stabilizing your pool. Learn how these two items affect each other.

www.troublefreepool.com

CYA Chlorine Relationship - Further Reading

edit: added 2nd article

 

[HermanTX]

To me it's logical that they feed at a consistent rate 24/7, going through 2 tabs per 24 hr and refill the feeder each day. Again, from a chemistry standpoint, what would cause the chlorine test to go to zero overnight when the pool water is continuously taking on chlorine from the tabs and testing had been dead nuts consistent for a three weeks?

In reading your original post and the one above - have not read how long are you running your pump? Is is 24/7 or a shorter time?
The chlorinator only provides chlorine if water is moving through it. same for pucks in your skimmer. Furthermore, we do not recommend putting pucks in your skimmer as it builds up acid in the skimmer and when the pump kicks on, you have that slug of acid hitting your pump directly.

 

[Surf NH]

In reading your original post and the one above - have not read how long are you running your pump? Is is 24/7 or a shorter time?
The chlorinator only provides chlorine if water is moving through it. same for pucks in your skimmer. Furthermore, we do not recommend putting pucks in your skimmer as it builds up acid in the skimmer and when the pump kicks on, you have that slug of acid hitting your pump directly.

Sorry, didn't mention, it's a variable speed pump, runs 12 hrs high speed during the day, 12 hrs 1000 rpm less at night. Correct, normally I never add chlorine tabs in the skimmer, just did it Wednesday to see if it helped to bring up the ppm level. It didn't.

 

[HermanTX]

Sorry, didn't mention, it's a variable speed pump, runs 12 hrs high speed during the day, 12 hrs 1000 rpm less at night. Correct, normally I never add chlorine tabs in the skimmer, just did it Wednesday to see if it helped to bring up the ppm level. It didn't.

One idea is that the chlorinator valve is clogged which is not allowing flow through it to dissolve the pucks.
Also, with pucks, even with ones you put in the skimmer basket, take time to dissolve, so you only get a minor increase in FC over a 24 hr period. Furthermore you only have half your time at 1000rpm so that may not be high enough to really dissolve much.
These on only ideas because you mention that this has worked in the past but recently had a significant drop in FC

 

[Surf NH]

thanks all for feedback, .....let me read up on pucks a bit more, appreciate the links.....still learning.

 

[Bperry]

Hey guys, thanks for all feedback, I'm using a Taylor test kit, the K1005......using DPD reagents, and a turbidimetric CYA test, same test methods as the Taylor K2005 kit. I'm not sure of the exact difference of the kits, they seem to use identical test methods and reagents? Chemical reads the day before it went to zero, per our service guy who was using a Taylor commercial test kit (I watched him test, my test kit close numbers by slightly different):

Chlorine 3ppm
pH 7.5
Alkalinity 100
Hardness 300
CLA 80

the feeder has never run out of tabs, I've been running 2 to 3 ppm free chlorine level for two years, never had a spec of algae since pool was installed, the water is pristine clear as as are the Gunite walls....they're white, easy to see buildup.

I'm sorry, I missed the issue on the chlorine tabs, exactly why are they bad again? To me it's logical that they feed at a consistent rate 24/7, going through 2 tabs per 24 hr and refill the feeder each day. Again, from a chemistry standpoint, what would cause the chlorine test to go to zero overnight when the pool water is continuously taking on chlorine from the tabs and testing had been dead nuts consistent for a three weeks?

The Taylor K2005 is missing the more accurate FAS-DPD chlorine test reagents. I think you can get an add on to make it a K2006 though.

 

[RLP]

Never put tabs in the skimmer. Use a floater instead.

 

[Surf NH]

The Taylor K2005 is missing the more accurate FAS-DPD chlorine test reagents. I think you can get an add on to make it a K2006 though.

Thanks....actually I re-read what I wrote, and it appears I missed this.....you folks recommend the K2006, I was comparing K2005 to the one I have, K1005. It appears my K1005 is virtually identical to the K2006, except the K2006 has the FAS-DPD Chlorine test....it appears K2006 can also do the regular DPD test (same as K1005).....what's the difference between FAS-DPD and straight DPD?

Dang, should have purchased the K2006 originally but whatever....

 

[dfwnoob]

You can buy the FAS-DPD test separately. Taylor K-1515-A.

DPD-only tests generally max out at 5 ppm and do not go high enough for our purposes.

 

[Saturn94]

Keep in mind algae grows in a pool well before you actually can see it. By the time you can see the algae, it’s well out of control.

This is a classic case of FC being too low for the CYA level. If you keep adding pucks (which continuously raises CYA level) without also raising the maintained FC level, you will eventually experience an out of control algae outbreak (not to mention an unsanitary pool).

IMO, pucks are a trap that traps the pool owner into an unsustainable cycle. Add more pucks to increase FC means CYA goes up, which requires more FC, so add more pucks and CYA goes even higher which means more FC needed, and so on and so on (not to mention pucks effects on driving down pH and TA, so need to add more stuff to offset that!). It’s quite the (expensive) juggling act that eventually results in having to drain the pool and start over.

 

[Surf NH]

Never put tabs in the skimmer. Use a floater instead.

Thanks, understood. Quick update, pool has been stable, but my CYA is too high and I'll have to do a drain/fill process. I'm just gathering facts, learning more.

I stopped by my local supply shop this weekend, to challenge the local store expert on the pucks containing stabilizer. She tells me they also sell pucks without stabilizer, but they can't be used with the feeding system the pool came with. To me this makes no sense, might as well use liquid chlorine as many of you have recommended.

I mentioned before, I'm using a pool service (from the company that installed the pool) every other week since the pool opened up two years ago...basically to learn how they think also....but I've been putting my own chemicals in for the most part. The service guy is coming tomorrow, I'm quite sure he's testing with a commercial Taylor kit, not positive if it's the k2006. I want to watch his test and see what's different from mine, I also want to watch him test the CYA.....as many mentioned, there's some technique involved there. I want to verify I'm high, though I'll be surprised if he gets a lower result.

One thing I'm trying to understand.....we used what I researched was a very high end pool installer in New England....South Shore Gunite, they're installing from Martha's Vineyard to the Canadian border, they only do Gunite, they aren't cheap. I'm anal, I monitored everything they did during the installation, I filmed them every day they were here, I took pictures of all the underground piping connections and grounding of the metal frame, all the wiring, you name it. They did a great installation, I was constantly impressed. These guys don't even offer a salt option for chlorine, they do 100% pucks....and at the time, they told me every installation they do is UV/Ozone. This all made sense to me, I'm an engineer, I've been exposed to textile water treatment systems using UV/Ozone since the mid 1980's, this is really common in my industry. But, I'm no chemist and it's killing me that I missed the conversation regarding stabilizer in the pucks.

So now I'm perplexed, why does South Shore Gunite build their pools this way? There must be a reason beyond they want to make us buy more chemicals as they're mainly an installation company. I'm wondering if CYA isn't normally as big a problem in the northeast, considering our cooler water (slower puck degradation) and shorter pool season. Our pools are drained quite a bit each off season, multiple times during the winter as the snow melts and keeps filling up the pool. There has to be roughly 40% to 50% water turnover between seasons, and this would reduce the CYA every year. It's my theory anyhow, but live and learn.

 

[Saturn94]

She tells me they also sell pucks without stabilizer, but they can't be used with the feeding system the pool came with.

The store happens to be right in this case! The pucks without CYA have calcium instead. If you put the calcium pucks in a feeder that was used for the CYA pucks, you risk an explosion! I think there is also an issue of the CYA puck dispenser design being different than the calcium puck dispenser (something to do with they difference in the way the dissolve).

Also, generally speaking pool builders understand how to build pools, but don’t understand pool chemistry. They basically just repeat what pool stores teach, and pool stores teach what the pool chemical sales reps teach them, which is essentially how to keep the money flowing in. If a pool store taught the methods taught here (which are based on chemical science, not sales reps), they would soon go out of business.