Can't Lower Cl Level

I have a 15K gallon in-ground pool and I can't get the Cl level to drop. It's been over 20ppm for about 2 months and I haven't added any Chlorine during that time.

I use 3" tabs in the skimmer and I alternate in with liquid CL about every 3 or 4 applications. I shock the pool with 68% Calcium Hypochlorite about every 2 or 3 weeks during the summer and about once per month in the winter (we're in Northern California, so the winter is short)

I have also been trying to lower my Cyanuric Acid level (I assume that it's tied in with the high Cl) - I've been over 100ppm since August and I've drained my pool down ~6" three times now without any significant reduction.

The rest of the readings are within normal range: pH = 7.6, Alkalinity = 70 - 80, Ca Hardness = 400 - 450.

Besides draining the pool a few more times, is there anything else I should be doing?

How are you testing? What test kit do you use? Around here it is generally recommended that you use the Taylor K2006.

I'm using the Taylor K2006 to test.

No tabs in the skimmer (not a good idea, anyway)... no liquid chlorine added... no cal hypo application... no SWG... for the last 2 months, and the FC continues to be 20 ppm?

Unless there is a mischievous neighbor or unruly teenager playing a joke on you (by adding chlorine at 3:00 AM), I would reexamine my test kit solutions or procedure.

Yes, very high levels of CYA will certainly slow down the process (more about CYA level, below) and the time of year means that the sun's UV rays are not reducing the chlorine very much but halogens dissipate over time. If you have a pool cover in place, remove it.

I, too, live in N. California and during late fall and winter chlorine levels take a long time to come down ... but not two months. My pool's temp is about 60 degrees F. right now. Chlorine demand is about 1/2 ppm per day. CYA = 40 ppm. No SWG operating, uncovered.

The CYA level in your pool probably started out to be much higher than 100 ppm (hard to accurately measure - have you tried diluting the sample with tap water 50/50 and multiplying result by 2?)

If you have a Taylor kit, read some of their articles about testing. If you're 100% sure your test products and process are correct and were desperate to reduce the FC, you could go the chemical route (thiosulfate) but I think you're mismeasuring and/or the CYA is crazy high. If this were my pool, I'd continue the controlled drains until CYA < 80 ppm.

Hope that helps.

Why do you want to drop the chlorine level? If you are done swimming for the season, I say leave it and see what it does. Don't put anything else in it, monitor your levels as you are doing, and only add what you need to maintain chlorine and pH. You don't need the tabs because they probably have CYA in them. When the weather starts getting better in the spring, start the drain and refill process until you CYA is down and go from there.

Swimdaddy has a great point which has not been brought up elsewhere for you. Your CYA may well lower in Spring, and with a plaster surface, the high FC is most likely not going to hurt as it will eventually come down.

Everyone has asked you about whether you use pucks or granular shock and I haven't seen your response. Both of those will raise/maintain CL (FC) levels.

Thank you for your responses. I am using pucks and I alternate liquid every few applications.

I took a sample to the pool supply store to get it tested (to confirm that my test kit was accurate). They confirmed that the readings were correct - Cl is about 10ppm and CYA is about 115ppm (down from 150-ish).

I am going to stop adjusting the chemistry and leave it as-is (as was recommended above). They CYA will eventually come back down to normal levels.

I think that I need to go away from using the pucks as often as we have been using...

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Originally Posted by matthewnadeau

I think that I need to go away from using the pucks as often as we have been using...

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Sorta right. You need to stop using the pucks altogether. You can save what you have for extended absences, but that's it. Every puck adds CYA and your CYA is already silly-high.

Agreed. Live by the motto "only put in your pool what it needs to be clear and comfortable". Your pool needs chlorine and a balanced pH, everything else is just to help maintain chlorine and pH. The exception is needing calcium in plaster pools.

Quote:

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Originally Posted by matthewnadeau

They CYA will eventually come back down to normal levels.

I think that I need to go away from using the pucks as often as we have been using...

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The CYA will not normally come down on its own but if you use unstabilised chlorine at a high level you will oxides the CYA down to a reasonable level. I had to do this last year when emptying was not possible and I used around 80-100 litres of liquid chlorine