Chlorine will not hold

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wposton84

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May 18, 2010
7
I've been taking care of a pool for about 5 years now and every time its opened it requires 4 pounds of shock twice per week for about a month to bring the chlorine to a stable level. I will shock it on a Monday and by the time I return on Thursday the Chlorine level will be hovering near 0, even with the addition of 6-8 chlorine tabs each visit. Here are the stats on the pool:

-25K gallons
-sand filter
-in-ground
-vinyl liner
-Do not know the flow rate of the filter or the size of the pump

And here are the test results from yesterday (May 17):

pH: 7.2
TA: 100 ppm
FC: 0 ppm
TC: .3 ppm (may have been a residue leftover from my personal pool, which has chlorine around 5 ppm)
CH: Unknown
CYA: 100 ppm

-Clarity of the water is great, it has been used every season for the past 5 years. It was in fairly good shape when it was opened April 30.

This year, the owner wanted the pool open early, so I have attempted to expedite the chlorination process by using 8 pounds of shock instead of 4 the past couple visits. I've put in nearly 40 pounds of shock since the pool opened and have been consistently adding chlorine tabs but the level will still not hold in the ideal range. Anyone have any theories as to why this is happening or how I could rectify it?
 
Welcome to TFP!

High CYA level greatly reduce the effectiveness of chlorine. To make real progress you need to lower your CYA level. Also, the CYA level might actually be much higher than 100, since the most common CYA tests report levels higher than 100 as if they were 100.

You also need to look at where the CYA has been coming from to get your CYA level that high. Trichlor tablets contain CYA, as do granulated "shock" products bases on dichlor (which is the most common "shock" product). If you keep using those products your CYA level will continue to go up and your problems will get worse.

When you have a chance, you might want to do some reading at Pool School. There is lots of good background information there, including much more on the various choices of chlorine products and their various pros and cons.
 
Thanks for the advise, Jason and Rob, I greatly appreciate it.

The pool uses Calcium Hypochlorite for shock (so I don't think that is where the CYA is coming from), but it does use Trichlor tablets (which I believe is the source of the high CYA level).

If you two are correct that my problem is the CYA level, why will the pool begin holding chlorine after a month or so?? Wouldn't my CYA level be as high or higher after a month of Trichlor tabs, if that is the source of the extra CYA?

Thanks again for the help, guys

Will
 
Will

It probably takes a month for you to kill off all the organic matter in the pool when the CYA is so high. At that point the chlorine will begin to hold.

Like Jason pointed out, most test kits aren't reliable above 100. Your CYA may very well be higher, the test just can't tell you that it is.

Rob
 
Alright, I think I'm getting a handle on this, could you please tell me if this is right?

The CYA is too high in the pool, which decreases the efficacy of the chlorine. If the CYA were down to 30-50, the chlorine I am adding would be much more effective which would kill any remaining organic material at a faster rate and the pool would begin to hold chlorine much sooner than the 1 month to which I keep referring.

I guess my only other question would be why is the water so clear? I would think that it would appear at least a little cloudy if there was an excess of organic material waiting to be cleaned up by the chlorine. Is it possible that there is simply something microscopic that needs to be cleaned out, but is not affecting water clarity?

Again, I can't thank you enough for your help.

Will
 
You can have clear water and still have organics consuming the chlorine. Until you get your clients CYA down into reasonable range you can't effectively fight it because you have no idea what your shock level should be. And without a good test kit you won't know for sure when you're done shocking.

I'll reiterate Jason's suggestion that you read Pool School thouroughly and apply the principles there. I'll also suggest that the best thing you can do for your client is to suggest that they install a SWCG or a liquid chlorine feeder of some type. That would be something you could maintain for them and not have a repeat of the issues you're seeing now.
 
wposton84 said:
Alright, I think I'm getting a handle on this, could you please tell me if this is right?

The CYA is too high in the pool, which decreases the efficacy of the chlorine. If the CYA were down to 30-50, the chlorine I am adding would be much more effective which would kill any remaining organic material at a faster rate and the pool would begin to hold chlorine much sooner than the 1 month to which I keep referring.
Click to expand...

Exactly...and clear water isn't always clean water.

Rob
 
Thanks, Rob. You rock.

I'm going to have the water checked professionally before I drain it (to be absolutely sure the CYA is where I think it is), then read up on Pool School and find an alternative way to chlorinate that will not yield these problems.

Thanks again

Will
 

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my suggestion...order a tf100 test kit from Tftestkits.net before spending $ on having a pro do it for you. $68 and you will be able to test your water in the convenience of your backyard and get realiable/consistent results. Having a good test kit is the key to BBB and keeps you from spending $ on items that are just not required to maintain pool water chemistry.
 
Can I recommend a Taylor K-2006 C or a TF 100 XL, when you go to replace reagents. You will love having an FAS-DPD test, believe me. :)

IMHO Test strips are especially worthless for CYA tests... back when I had an overstabilized pool, my strips always read 30-50 when it was actually over 100.
 
I've heard that test kit (TF-100) mentioned over and over on a number of forums and I'm in the process of finding the cheapest price for it and getting it ordered. Thanks for the recommendation, guys.
 
The TF-100 is only offered at tftestkits.net while the Taylor K-2006 is available at many different sites with the lowest priced one found to date being at Amato Industries. Note that the TF-100 has more volume of reagents (and uses Taylor reagents) so is more comparably priced per test and is also packaged in more reasonable quantities for tests you use the most. You can read about the differences between these two test kits in Test Kits Compared in the Pool School.
 
Sorry if I'm beating a dead horse, but I had one more issue with Cl and CYA levels that I cannot figure out.

My personal pool holds chlorine like no other. I shocked it once and the CL level has not come down at all since then (3 weeks ago). I have been adding Trichlor tablets (about 2-3 per week) and today, the CYA reading is 150 ppm. So I'm a little confused as to why Chlorine holds so well in my own pool (with a CYA reading of 150), but not in the other pool I care for (CYA 100 ppm)? Obviously every pool is different, but it would seem that the high CYA level would render the chlorine virtually useless.

Here's the info on my pool if that helps:

-15K gallons
-sand filter
-in-ground
-vinyl liner

Today's Readings:

TA: 160
pH: 7.4
FC: 10
TC: 10
CH: Unknown (Vinyl liner)
CYA: 150


Thanks in advance for the help, you guys are an incredible resource.
 
I'm not an expert... but I might suggest...

I would expect your CL to hold a long time with a high CYA as long as your FC is high enough and you are not dealing with organics as you would be in opening a pool with a high CYA.

Remember, that is what CYA does: It stabalizes your CL, so you do not lose as much to UV. Your trade off is that your CL is less effective.

If you are dealing with a lot of organics, your CL will continuously be eaten because it will be very difficult and impractical to get to an effective shock level.

Your present FC levels are not unreasonably low for such a high CYA level... Though I personally would like to see it closer to 15, though at levels that high, I wouldn't call it a recomendation. I'd more like to see your CYA come down.

Also, it appears that you are not testing your CH because of vinyl liners.

You are shocking with Calcium Hypochlorite, and particularly with the high TA levels that I am seeing, I'd want to get a CSI. You could be exposing yourself to some scaling, particularly if you see your pH start to drift upwards.
 
Assuming that the test readings are correct (and you really can't know that unless you are using a good test kit yourself, such as the TFTestkits.net TF-100 or Taylor K-2006), then a low FC/CYA ratio does not mean you will definitely get algae or a higher chlorine demand. It just means it is more likely. The reason there is no guarantee about this is that the pool may be poor in algal nutrients (phosphates or nitrates or limited sunlight) or have some algae inhibition products in it (borates, copper metal ions, algaecide) or could just be lucky enough not to get algae spores (very unusual).

It's like Clint Eastwood saying, “You've got to ask yourself one question: 'Do I feel lucky?' ...”

On the other hand, if you maintain the appropriate FC for the CYA level, you can almost always prevent algae growth and have a reasonable chlorine demand without the need for algaecides or phosphate removers or weekly shocking. In the very rare case where more is needed, then you always have the option of spending more to use an additional product, but you needn't start there.
 

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