FC, CC, CYA and shock question

Tfastle

Active member
Mar 17, 2014
38
Albq. NM, United States
#1
Until I came to this forum shocking had been presented to me as a "do it every couple of weeks" thing. Clearly that's not accurate but that's pool builders and suppliers for you.

I've read quite a bit here and in pool school and get bits but haven't yet seen it stated specifically (I'm sure it's here but I haven't found it), when to shock? It's clear that it's different for each pool and it depends on your FC, CC and CYA levels. I just tested mine and they are as follows - CYA=25, FC=5.5, CC=1 . I gather that CC should be near or at zero (mine has been between 0 and 1). Because it is not 0 should I shock it or, is there a point at which I should?

I have an 18000 gallon pool with a retractable cover and keeping the Chlorine levels in the 3-5 range is not difficult, I suspect because the cover is closed a good part of the day. I understand that my CYA levels are a bit low but I manage my Chlorine via Bleach and Tablets (tablets and fine tune with bleach). My understanding is that over time, if I am using tablets (or anything with a stabilizer in it) my CYA levels will increase. My thinking is that I will let that happen as I use the tablets until it gets to a level I don't want to go over and then quit the tablets. Because of the simplicity I do like having the tablets. Does this make sense and any rough idea of how long that might be? My guess is a loooong time but I don't know for sure. If it is a long time then it's not a major concern and maybe I should bump my CYA a bit?

My other levels are PH 7.2, CH 550 (high but we have very hard water here - Albuquerque NM), TA 100. From reading on here, with my CH at 550 and at PH around 7.2 I should be ok.

Any input would be appreciated.

Thanks!
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
#2
You're getting the hang of things here. :goodjob: While we typically don't advise using tabs, in your case (with a cover), you might be able to keep FC/CYA levels fairly low. That's good for now. As you have learned, the CYA will begin to climb, so keep a close eye on that to ensure your FC (recommended range) matches the current CYA as noted on the Chlorine/CYA Chart (link below).

Yes, CC should be 0.5 or less. When it goes over that, it indicates there may be more transitional organics than can adequately be removed by the sun. Hence, perhaps leave the cover off one day (or more of it) to see if that helps. Normally we don't just "shock" or increase FC unless we suspect a problem. So the day you decide to roll-back the cover for a little while longer to let the sun oxidize CCs, you might increase FC just a bit more to about 7-8.

As for CH, you are correct that a lower pH is advised when CH begins to climb. The Poolmath calculator will be a great tool for you. Load your numbers and then look near the lower part of the chart at the CSI row. That will tell you the water's ability to be corrosive or cause scale.

After all of that, if you still find CCs elevated, compare that to FC 24-hr usage (2-4 typical) and water clarity. If needed, you can do an overnight (OCLT) test just to ensure there is no algae trying to bloom.

Hope that helps.
 

pabeader

LifeTime Supporter
TFP Guide
May 14, 2015
4,349
Cartersville Ga
#3
A properly cared for pool, never needs to be "shocked".
To properly care for your pool, you first need to do your own testing. Pool $tore testing has one thing in mind; selling you stuff.
 
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Tfastle

Active member
Mar 17, 2014
38
Albq. NM, United States
#4
Thanks for the information TS! I have tried to use the pool calculator but I'm not 100% clear what I am doing but will play with it. I will up my FC tomorrow and let the sun do some work. By "FC 24 hour usage", do you mean see what the FC is one day and then the next (24 hours) check and the difference is the "usage". I don't know what an OCLT test is but I'll read on it.

Any rough guess on how long it might take for my CYA levels to go up to target levels (40-50 PPM). I know it depends on how many tablets are used (probably 2-3 per week) but is it something that takes a very long time? If so, should I bump the CYA levels now or might I be getting into the target range in a year or two by using the tablets?

Agreed Pabeader, I have always tested my own water but just recently got a good test kit (TF100).

Thanks again for the help!
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
#5
By "FC 24 hour usage", do you mean see what the FC is one day and then the next (24 hours) check and the difference is the "usage".
Exactly! If my FC at 6:00 PM is 5, and I increase it to 7.5, then tomorrow about the same time I hope to find my FC back to 5 or so (hence I used about 2.5 ppm).

Here is the page for the OCLT: Pool School - Perform the Overnight FC Loss Test (OCLT)

Any rough guess on how long it might take for my CYA levels to go up to target levels (40-50 PPM).
Each tab has just under 2 ppm of stabilizer, so it depends on how fast you use them. Remember though, once CYA is in ... it's in. The only way to remove CYA is water exchange. That's why we advocate bleach (or SWGs) so much to avoid that pitfall.
 
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Tfastle

Active member
Mar 17, 2014
38
Albq. NM, United States
#6
That's very helpful, thanks!

If that's 6-9 ppm of the volume of the tablet then, if my math is correct (approx 4,5 cubic inches per tablet, 51 tabs/gallon, 18000 gallon pool), it's going to take me 924000 tablets to raise my CYA 9ppm. At 3 tabs a week that's around only 5000 years. I better back off on the tablets a bit! :) I'll watch my CYA, my guess is I won't see it go up much but ... as I get older my guesses get worse and worse.

Thanks again!
 

wjr75

LifeTime Supporter
Jun 6, 2013
879
IL
#7
My pool is 20k gallons (close to yours) and I have mine drained every spring and power washed. That's how we roll up here in the Midwest with plaster pools because of winter staining. I bring my CYA up to 24 ppm initially using pellet CYA (I also have a auto cover). I then use the 3" tabs which in my case also adds 1.7ppm CYA per tab until I get up to around 45ppm of CYA. I usually use about 10 tabs to add the extra CYA to get to the 45ppm CYA. I then switch to 100% liquid 12% chlorine for the rest of the season.
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
#8
At 3 tabs a week that's around only 5000 years.
ha. I'm not so sure about the math there. :confused: What I like to do is refer back to the Poolmath calculator near the bottom where it shows "Effects of Adding Chemicals". There you can list what products you are adding from the drop-down and it will tell you how much effect(s) that product will have.

Tfastle, I need to correct my note about how much CYA is in each tab. I should've said it this way:
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
By checking the Poolmath calculator, it looks like one 8 ounce trichlor tab will increase CYA by 1.9. Five tabs will increase CYA by just over 9. So depending on your auto-feed setting, it can add-up before you know it. When you need CYA increased quickly, we recommend granular stabilizer. If time is not a problem, the tabs are okay. Also be aware they are a bit acidic and can lower pH as well. Hope this helps.
 
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Tfastle

Active member
Mar 17, 2014
38
Albq. NM, United States
#9
That would make a lot more sense. I was pretty sure it wouldn't take quite 5000 years but I am surprised the CYA would add up that quickly ... thanks WJR. I will stick with my original plan and let the tabs bring it up and then switch to bleach. All very helpful information and appreciated!! Thanks! This is a great forum.