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A Well-worn Question: Why Am I Havng Trouble Maintaining Fc?


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FC tested at 9.0 at 7:30PM per my Talyor K-2006. Normally acceptable, but I'd added 3lbs of lithium to my 16,500 Gal pool at lunch today after a heavy bather load yesterday. Expected FC level to be approx 20ppm.

CYA has consistently read 50-60ppm. Temps in the StL area have been 90F+ for the past several days and are forecast to remain so for the next week. Water temp has been reaching 90F all the while. I have a problem w/ my SWG (my IntelliChlor isn't communicating w/ my Easy Touch automation panel, so I've been controlling output directly through my IntelliChlor). That said, I have an IC40 - rated for 40,000Gal. Hard to imagine it's not producing enough chlorine - ran it at 100% for about 10 hours Sat night and tested 8ppm Sunday morning, before guests arrived.

I'm beginning to wonder if the communication problem is due to a fault in my IC40. No problems maintaining 4-7ppm last year at 5-15% output setting.

I haven't cleaned my cell since install in September 2007 - could scaling reduce output to this degree?

Thanks for any advice.

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Don't know anything about SWGs. But, there's a way to tell if there's a problem in the water (algae etc): In the evening, turn off the SWG. Test your chlorine. Don't add anything. In the morning before the sun hits the pool, test it again. If your FC dropped by 1ppm or less, and your CC is 0.5 or less, you're fine. Otherwise you have something eating the chlorine and you need to shock.

--paulr

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Don't know anything about SWGs. But, there's a way to tell if there's a problem in the water (algae etc): In the evening, turn off the SWG. Test your chlorine. Don't add anything. In the morning before the sun hits the pool, test it again. If your FC dropped by 1ppm or less, and your CC is 0.5 or less, you're fine. Otherwise you have something eating the chlorine and you need to shock.

--paulr

I really need to proof for typos before posting. Thanks for your reply - I'll do that tomorrow night/Thu AM. That said, I'm about 99% sure I have neither nascent algae nor phosphates in my pool. I haven't cleaned my IntelliChlor since installation in Sept 2007 and I'm thinking it's the problem and is due for an acid bath. A local Pentair service rep is due to make a call re: my com problem between my EasyTouch and IC40, and I'm going to instruct him to clean the cell when he comes out to troubleshoot. Needless to say, I've neglected that but I don't have much spare time between work, chores and a 3 and 1 year old.

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I would definitely inspect the cell. Scale buildup would reduce efficiency so would output less during the same on-time.

You can easily see if you've got any organic demand (including algae) in the pool by doing an overnight chlorine test. You should see a drop of <= 1 ppm FC overnight (from after the sun is off the pool to before the sun significantly hits the pool).

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Richard, I finally found my strap wrenches (my builder installed my IC40 flush to the equipment deck and overtightened the unions - it was impossible to remove w/o the strap wrench). Removed and inspected the cell - the blades were spotless, looked like new. I installed the cleaning cap and added acid anyway, no foaming action after a minute, so I rinsed and reconnected the cell. I'm guessing scaling on the blades is mainly a problem for pool owners who don't test and adjust their levels regularly. I've kept the CH in my vinyl-lined pool just below 150 since install in Sep 2007.

Anyway, I've concluded that the water is just so warm (90-92F at 8PM for the past week) that the chlorine is breaking down almost as rapidly as the IC40 can produce it - despite a CYA level of 60ppm. The spa section of the booklet that accompanied my Taylor K-2006 suggests exactly this.

Am I missing something?

Thanks!

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Your cell has probably failed. Here's a test to check. Crack the down stream union while the pump is running, enough for it to leak water. Make sure unit has been running for at least 5 min. take a sample of that water thats leaking from the cell union and test for Cl! Another check is look at the returns to see if you can see any "champagne" bubbles that are produced by the generator while it's producing Cl.

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The cell is producing clouds of hydrogen bubbles - they're readily visible at night and the return side wall is covered w/ fine bubbles. 3 friends of mine (1 has a SWG, the other 2 have inline chlorinators) are also having problems maintaining their chlorine levels in this heat. I know that high water temperature accelerates chlorine consumption and I really believe that's what I'm experiencing.

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Just keep in mind that you need an absolute minimum FC that is at least 4.5% of the CYA level in an SWG pool. So if your CYA level is 60 ppm, then that's a minimum FC at all times of 2.7 ppm, so 3 ppm FC would be safer. If it gets below that then algae can grow and consume chlorine, even if the nascent algae isn't yet visible (i.e. water not yet dull/cloudy, let alone green). It sounds like you normally keep it at least that high or higher so that's good.

You are right that warmer temperatures accelerate algae growth, but it also accelerates chlorine kill rates as well (all chemical reactions are sped up). So unless there was already enough algae growing and and too close to the chlorine kill rate, then it shouldn't have increased chlorine demand by so much. If there was a buildup of organics in the pool, then that could explain an increase in chlorine demand as the temperature warmed up, but not by that much. My pool temp varies from 86-90F and I don't see anywhere near the sort of chlorine demand you do. It's around 0.8 ppm FC per day with 3-4 ppm FC and 30 ppm CYA, but the pool is covered except for 1-2 hours. Pools exposed to sun all day do sometimes see a 50% drop in FC unless the CYA is kept higher -- at your CYA, one would expect more of a 30-40% drop in FC per day from sunlight (at a CYA of 70-80 ppm, it's down to around 15-30% so would require less SWG on-time, but the minimum FC would be 4 ppm).

The best way to distinguish between algae growth or organic load (say, from bathers) vs. breakdown from sunlight is to do an overnight FC test. If it drops by <= 1 ppm FC, then the FC consumption is likely due to breakdown from sunlight. If it drops by > 1 ppm FC, then it is more likely to be either algae growth or oxidation of organics in the pool. For the latter, shocking to an FC that is 40% of the CYA level (so 24 ppm FC in your case) and maintaining that until the overnight FC drop gets lower should fix the problem. After that, maintain the FC at no lower than around 3 ppm.

Chlorine does not break down that much faster at higher temperatures unless it is in concentrated form. The chart at the bottom of this link gives you an idea of the half-life of chlorine at various concentrations and temperatures. The concentration of chlorine in a pool or even a spa is so low that it is not breaking down (i.e. converting to oxygen gas and chloride ion) from temperature. What is true is that chlorine will outgas faster at higher temperatures (and with more aeration), but with CYA in the water this effect is far less since it is the hypochlorous acid (and a small equilibrium amount of chlorine gas) that outgasses. The chlorine bound to CYA is not volatile. It is also true that chlorine will oxidize organics (mostly ammonia/urea from sweat/urine) faster at higher temps, but such organics need to be present.

Consider that even in an unused spa at high temps, the daily chlorine demand is usually not more than 25% of the FC level.

I'm not saying that higher temps don't make all reactions go faster, including reactions of chlorine with algae or organics, but that unless you have such algae or organics in the pool, you shouldn't see such high consumption. You mentioned you had a high bather load, so that could explain some of the faster drop, especially if anyone urinated in the pool (don't laugh, more people do that than you might think, including adults). You might also check your filter (and skimmer and pump basket) to see if it needs cleaning/backwashing. Organics caught in the filter still get oxidized by chlorine (to the extent that they can).

Richard

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I backwashed the filter thoroughly after opening and all organic debris (leaves and bugs) I've vacuumed since then are caught in my skimmer sock. I also degreased the filter about a month ago and check/rinse the pump basket regularly. So I'm going to try nuking the pool tonight - come to think of it, I haven't tested to confirm I'd reached shock level (40% of CYA) since opening, so I'll be sure to test FC about an hour after shocking.

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I think my problem is resolved - I didn't test FC yesterday but tonight it measured 11, w/ no FC, after running the pump overnight w/ 20% output on the IC40. This is the first time that FC has increased over the past few days w/o either shocking or simultaneously increasing pump run time & boosting IC40 output to 60-80%.

As I mentioned, my FC only tested at 9 only 7 1/2 hours after shocking w/ lithium a few days ago. Assuming this was due to rapid oxidation of either nascent algae or organic bather load (probably mostly sweat given the temps the past several days - that might be the issue!), wouldn't I have been detecting CC before shocking? Or was I keeping FC high enough to outpace consumption such that the level never dropped low enough to allow CC to form?

Lugged 6 gallons of 6% generic bleach home tonight but I elected not to shock since my FC actually rose. Will test in a few mins and again tomorrow AM before turning the pump on.

I am pleased that my cell blades exhibited 0 scale when I examined yesterday. I attribute that to what I've learned here about maintaining proper TA and CH levels, so thanks for that, plus the feeback on this thread.

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As I mentioned, my FC only tested at 9 only 7 1/2 hours after shocking w/ lithium a few days ago. Assuming this was due to rapid oxidation of either nascent algae or organic bather load (probably mostly sweat given the temps the past several days - that might be the issue!), wouldn't I have been detecting CC before shocking? Or was I keeping FC high enough to outpace consumption such that the level never dropped low enough to allow CC to form?

Unless there is a larger rapid introduction of something like ammonia or urea into the water, the chlorine will continually oxidize the bather waste before the CC gets too high. In an outdoor pool, sunlight seems to help accelerate the process (especially for urea). Usually, higher CC is seen under high bather load or if there is significant algae growth (or in indoor pools where controlling CC is more difficult).

Anyway, it i certainly possible that you had something in the water that chlorine needed to get rid of. Hopefully, this fix by shocking did the trick and you won't have problems from this point forward.

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