Jandy SWCG's "optimal" water conditions: why raise CYA?

[famousdavis]

So I've reading and re-reading all the different user manuals I got with my new pool, plus all the info available in this forum. I haven't yet tried to grasp the math that creates correspondence between free chlorine and CYA levels, but I do understand that as CYA levels go up, there is the need for higher chlorine level, too.

My Jandy-made SWCG's user manual gives the following info for "optimal pool water conditions":

Free Chlorine: 1-3ppm (Jandy adds, "Continuous exposure to levels above 3.0 PPM may cause corrosion of pool metals")
Combined Chlorine: None
pH: 7.4 - 7.6
Chlorine Stabilizer (CYA): 50-75 PPM
Total Alkalinity: 80 - 120 PPM
Calcium Hardness: 175 - 400 PPM

Then there's a place, in small print, that adjust the above for water temps above 85 degrees: "...increase the stabilizer to 75-85 PPM, and super chlorinate with other chlorine agents other than the chlorine generator, to reach break-point chlorination."

Questions:

1) Why do SWGC pools generally require higher CYA levels than non-SWCG pools?
2) Why are higher still CYA levels required for warm water (our pool is 89 or 90 degrees right now)
3) If the CYA levels are supposed to be so high, why is the "optimum" level for free chlorine still listed at the low level of 1 - 3 PPM?
4) Is it true that continuous exposure of FC that is above 4 PPM really do cause pool metals corrosion (I have a heat pump, so this point interests me)
5) Do other SWGC shock their pools with liquid chlorine / bleach instead of just using the Boost button on their SWGC?

For the techies and math geeks only: I'm only in my 8th week of pool ownership, but thus far, I've only shocked our pool using the Boost button alone (that runs the SWGC for 24 hours continuously at a 100% rate). Our model of SWGC says that it produces 567gm of chlorine in 24 hours. I've got a 16,755 gallon pool/spa, which is equivalent to about 64,000 liters. So, one 64,000 milliliters is one PPM. If my SWGC generates 567gm in 24 hours, that's nearly 9 PPM of chlorine added to the pool in 24 hours (567,000ml / 64,000ml). My Taylor K2006 kit says my CC level is 0.5 (perhaps less, but no more than), and break-point chlorination is 10 times the CC level, I'm thinking my SWGC creates enough chlorine to superchlorinate my pool once a week without me having to do anything extra.

I only just got my Taylor kit a few days ago, and I use the Boost button starting Sunday morning, so I'll be curious to see if, on Monday morning, my FC level really is 7 or 8 PPM higher than it was on Sunday morning.

Your thoughts?

[waterbear]

To answers your first question:

Higher CYA levels translate into lower cell on time which translates into less hydrogen gas generation in the cell which translates into less aeration of the water which translates into less outgassing of CO2 which is the primary cause of pH rise in salt pools (barring new curing plaster).
NOT dropping the pH below 7.6 and lowering it back down to 7.6 and not lowers (and not letting it rise above 7.8-7.9 and always keeping it below 8.0) slows pH rise (the lower you put pH the faster it rises) and helps minimize the possiblity of scaling condtitons.

Running the TA low (70 ppm seems to be optimal) translates into less carbonation n the water which translates into less outgassing of CO2 which translates into better pH stablility and slower pH rise.



For the second:
It's not warm water, it's sun exposure. If the pool temp is that high then it is assumed the pool is getting a lot of sun and higher CYA is beneficial in ANY pool, salt or not. BEST place for CYA is at the manufacturers recommended maximum so get yours up to about 80-90 ppm givin that you are in Florida (even in my N. Fl pool the temps stay in the high 80's during the summer and the pool is in a screen room!
Today my pool is at 89 degrees and it's before noon!)

for the third:
Optimum level for FC is about 5% of the CYA level, generally 4-5 ppm. This way you do not need to worry about algae outbreaks or the need to superchlorinate at all!

for #4:
It depends on a lot of factors. First, the type of metal involved (titanium and ferro cupric alloys in heat exchangers are more corrosion resistant). It depends on other ions in the water (sulfates from dry acid and non chlorine shock for example). It depends on salt level (depending on manufacturer SWCGs operate anywhere from less than 3000 ppm to sea water salt levels with most falling somewhere in the 3000-6000 ppm salt range. Even at 6000 ppm the water is not considered particularly corrosive). It depends on whether there is a sacrificial anode in the pool. It depends on pH (low pH is probably one of the biggest factors in predicting metal corrosion.)

for #5:
Shocking with the boost or superchlorinate function:
1) shortens cell life
2) causes FC level to slowly rise over the time period the boost function is on and this is not the most efficient way to superchlorinate and can actually cause some undesirable oxidation byproducts to form.
3) it is slower since you need to wait for the cell to generate enough chlorine to reach the desired level which can take as long as 24 hours just to reach shock level and then you need to wait for it to drop down after, which can take up a day or two longer.

Shocking with chlorine or bleach:
1) does not shorten cell life
2) brings the FC level up to your target shock value immediately and is therefore much more effective chemically in oxidizing and killing algae
3) the entire process is much faster since you do not have to wait for FC level to rise, it is instantaneous. You just need to wait for it to drop.

[famousdavis]

Thanks Waterbear for all your input!

I clearly don't have a handle on the chemistry that's going on in my pool. I'm a "poser" -- posing like I know something about all these chemicals when I try explaining them to my kids or my wife, but the details are still way over my head. But I'm getting there, I'm getting there.

It seems like I have a choice to make with regards to how (or to what extent) I superchlorinate my pool. I can either A) use the Boost button my SWCG, or B) go to either my nearby pool store or supermarket to buy a few jugs of liquid chlorine / bleach, and pour liquid chlorine / bleach into my pool.

The Boost / SWCG option is nice because it's dumb-easy. And since I have a variable-speed pump, I can set the pump speed to a very low setting but high enough for the flow switch in the SWCG to allow chlorine production to occur. It saves the cost of not having to buy liquid chlorine and/or bleach, and it avoids hazards that can occur with transporting and pouring those chemicals. Biggest downside, it seems, is that it may not truly achieve break-point chlorination, and it may not oxidize organics the way you'd expect. Another downside is that shortens the lifespan of the cell, so the cell would have to be replaced more quickly.

The liquid chlorine / bleach option is nice because it's very effective in superchlorinating (is that the same as "shocking"?). You can easily control how many PPMs you want in your pool by controlling the amount poured into the pool. This method extends the life of the salt cell, since I wouldn't have to Boost at all. Downsides are it costs a few bucks each week to buy the liquid chlorine / bleach, it involves trips to the local pool store or grocery on a regular basis, and it creates a chance for a spill somewhere.

Does that sound like a pretty good summation of these two options for superchlorinating?

[waterbear]

You pretty much have it down. Superchlorinating and shocking are the same. The point you are missing is that if you get your water balanced properly, get the CYA up around 80 ppm and run your FC at 5% of your CYA (for 80 ppm that would be 4 ppm FC) you will probably never need to shock the pool. Only shock if you have persistent (longer than 48 hours) combined chlorine over .5 ppm when testing with a GOOD test kit (such as the Taylor K-2006). Normally, there should be no need to shock and with the FC at 5% of the CYA you should not have algae outbreaks either (and for more algae protection and also to help stabilize pH even better the addition of 50 ppm borates to the water is highly recommended for salt pools.)

If you want to post a current set of test results I can suggest any changes you should make to the levels.

[famousdavis]

Thanks again, Waterbear.

I'll wait until the end of this week when I can post side-by-side test results from the two pool companies I've been using, plus I'll use my new K-2006 kit to give you its results, too. I'm comparing everyone's test results right now before I figure out what I'll do on an ongoing basis with regards to water testing by the pool stores (I need to do that to keep the warranty on my Diamond Brite interior). Until then, though, I'll give you some info from this weekend's readings using my K-2006.

I can tell you that the SWCG is able to boost the FC from 1.5 (yesterday morning) to 12.5 (this Monday morning) in 24 hours. And whatever trace of CC I had (during yesterday's test, the pink color for the CC test would disappear after adding just 1 drop of reagent, so I supposed the CC level was 0.5 or less), is now almost entirely gone (eg, during this morning's test, I hardly can detect any pink color at all for the CC test, but it does get very faintly more clear after adding 1 drop of reagent). Based upon my math above, I would have predicted that the FC level would have reached about 10.5 (though I rounded the liters in my pool up a little, so perhaps that accounted for some of my error. (Note: I was testing using 0.5 increments, not 0.2 increments).

Other tests from my K-2006:

TA - 80 PPM (I subsequently added 4lbs of baking soda afterwards, which should have boosted it up to about 100 PPM)
CYA - 60 PPM
pH - ranges from 7.2 to 7.8; seems happiest at 7.6. I add acid when it reaches 7.8, which lowers it to 7.2 or 7.4. This morning's reading was 7.8, so I added 4 cups of acid.

Calcium Hardness doesn't seem to be an issue -- both pool companies say I'm fine, reading between 300 and 375 depending on the week.

I am pleased that I'm getting feedback from other people who note how crystal clear our pool water is, as compared to others' pools. I attribute a lot of that to our DE filter (my buddy who supervises the City of Fort Lauderdale's public pools urged me to get a DE filter for it water-clarity benefits), but I'm sure the right pool chemicals have a lot to do with that, too. In the sunshine, the water just glistens like an aquamarine jewel. It's just gorgeous.

[waterbear]

Since you have new plaster you will have a big acid demand from the curing plaster and will go through a lot of acid for about the next year. Nature of the beast but it is very important to keep tabs on pH, TA, and CH during this time.

As I said before, the lower you put the pH the faster it will rise (and the faster you will lower TA) so you might want to rethink dropping the pH to 7.2 unless it is part of the plaster curing instructions you got from the contractor. However, SGM's Diamond Brite warranty does state that pH should be kept between 7.4-7.6. Also TA lower limit is 80 ppm on the warranty so you might want to keep it there instead of 70 ppm even though 70 ppm will give a bit better pH stability in a salt pool.
Not sure why you added baking soda to raise the TA when it was at 80 ppm. Like I said before the higher the TA the faster pH will rise and you want to slow down pH rise in your pool, particularly with curing plaster! (bringing your CYA up to 80 ppm wil also hellp as I explained).

You are lucky that your CH is not too high since that is a common problem in pools with new plaster.

Also, with a CYA of 60 ppm 12.5 ppm FC is not high enough for shocking. If you have CC of .5 greater than .5 ppm then add bleach to bring the FC up to 20 ppm. Your boost function would need closer to 48 hours to do this from what you have posted. I would not recommend that.

Also, 1.5 ppm FC is way too low and that is why you are showing CC! Your FC should be 5% of your CYA for a salt pool so for a CYa of 60 ppm your FC should be AT LEAST 3 PPM AT ALL TIMES! If you do that you will find that you do not have CC not will you be prone to algae outbreaks.