Can't Hold Chlorine With High Ca Level

[Rgirard]

Hello all:

I know the answer to the question, I just don't know why. I need to drain the pool and start over.

My pool has not been drained for 4 years and the CA level is WAY over 100ppm. I'm sure the TDS are way over as well.

The story: At the beginning of the season I didn't bother measuring the CA level. I just added chlorine and pH balance to get within the correct levels. All was well. Beautiful clear blue water. Then the weather got warmer and all of a sudden the chlorine crashed and the algae started in. I did a CA test and found it to be way way past 100ppm.

Everything I have read indicates that my chlorine levels should be off the scale in "Chlorine lock" but my testing indicates 0 chlorine. Why is that?

Also, is 4 years a reasonable amount of time to have to replace water. Again, from all I have read it seems to take a few years longer before the CA or TDS get to the drain and refill point.

Thanks in advance for your help.

[AZMermaid]

Do you mean CYA (Cyanuric Acid) levels are over 100ppm, or CA (Calcium). And when you say "way over", what is the reading. Depending on your area, you can go a little higher (up to about 150) with some tolerance. Whether 4 years is reasonable for a pool drain is also depends on your area.

Before you dump the water, I would test for TDS, Free and Total Chlorine levels, current pH and Alk. It's not unusual for the chlorine levels to take a dive when the weather heats up, particularly once the water starts getting warmer.

[chem geek]

What you are seeing is pretty typical from continued use of stabilized chlorine such as Trichlor pucks/tabs or Dichlor powder/granular. The following are chemical rules/facts:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

So even with a low daily FC usage of only 1 ppm per day, this would build up 18 ppm CYA per month or over 100 ppm CYA in just 6 months. This assumes there is no dilution from splash-out, backwashing or rain overflow. Those with cartridge filters don't backwash so see the CYA climb the fastest, but even those who backwash will see the CYA climb pretty quickly unless their pools are fairly small (so that backwash volume represents a larger percentage of pool volume).

The active chlorine (hypochlorous acid) concentration is proportional to the FC/CYA ratio which means that as the CYA level climbs, the chlorine becomes less effective and eventually algae is able to grow more quickly than chlorine can kill it. At first this may appear as a higher chlorine demand, but eventually the pool will become cloudy and then green with algae. If your pool is rich in algae nutrients (phosphates and nitrates), then if your FC drops below 5% of the CYA level, then your risk of algae growing becomes very high. For manually dosed pools, a target minimum FC that is 7.5% of the CYA level is reasonable and safe.

To prevent the CYA from climbing you would need to use unstabilized chlorine such as chlorinating liquid or bleach, but you'd have to add that every day or two and that is less convenient. If you want to continue to use stabilized chlorine and not worry about CYA, then you need to use a supplemental algaecide (weekly PolyQuat 60) or a phosphate remover, both at extra cost. It's a cost/convenience trade-off.

You can learn much more at the Pool School.

Richard

[Rgirard]

Sorry. Yes I meant CYA. I am in Las Vegas and we do have that hard water. By way over I mean when I use my test kit with the small black dot at the bottom, I get about 1/3 way to the beginning of the scale before I can't see the dot.

So even though I'm putting chlorine in with a high CYA it will show 0 chlorine.

Thanks for the help with the Pool School. Very informative. I used to have a pool company maintin the chemicals, but they got out of hand price wise with constant fee increases. I get to do it myself now.

[chem geek]

Sorry. Yes I meant CYA. I am in Las Vegas and we do have that hard water. By way over I mean when I use my test kit with the small black dot at the bottom, I get about 1/3 way to the beginning of the scale before I can't see the dot.

So even though I'm putting chlorine in with a high CYA it will show 0 chlorine.

Thanks for the help with the Pool School. Very informative. I used to have a pool company maintin the chemicals, but they got out of hand price wise with constant fee increases. I get to do it myself now.

You've got algae consuming the chlorine faster than the chlorine can kill it which is why you add chlorine and it doesn't seem to hold. With the black dot disappearing around 1/3rd of the way to the 100 ppm, that means your CYA is very high -- probably around 300 ppm or so. It is not practical to shock your pool with chlorine at such a high CYA level. I suggest multiple partial drain/refill to get the CYA lower, to at least 80 ppm or below.

[PaulR]

By way over I mean when I use my test kit with the small black dot at the bottom, I get about 1/3 way to the beginning of the scale before I can't see the dot.

You can try diluting your pool water sample half-and-half with tap water, then try the CYA test again. (The tap water will have no CYA in it.) Double the result, of course. If that's still over 100, do it again. The dilution loses accuracy but it's not all that precise a test to begin with.

--paulr