Do you target FC level at the start or the end of a peak season day?

[truepoolblues]

First year with a pool. I have 20ppm CYA so I have been targeting 2-3ppm FC. We have SWG and during peak season this looks like starting the day around 3.5ppm and ending the day around 2ppm (yes, I test 2x a day lol).


A pool store just told me that's too much chlorine and I should aim to start the day at my 2-3ppm target and it's totally fine to end the day with zero and then have the cell replenish overnight. Assuming my SWG could actually generate 2-3ppm overnight (it can't) is there any logic to what she's saying?


I always assumed I should be trying to hit the 2-3ppm target as consistently as possible, at all times, but is this overkill? Are TFP adherents content to start the day at their target FC and have it drop to zero at the end of the day as long as it starts the next day at target again?

 

[ajw22]

Target TFP recommendations…

What Are My Ideal Pool Levels?

Use our tool to find ideal chemical levels for any swimming pool. Check TFP recommended levels on pH, free chlorine, cyanuric acid, and more.

www.troublefreepool.com

 

[PoolStored]

A pool store just told me

Ignore the pool store. Sincerely, PoolStored.

I always assumed I should be trying to hit the 2-3ppm target as consistently as possible, at all times, but is this overkill?

Always follow this found here. Link-->FC/CYA Levels

At your peak, keep it toward the high end of the range...then, when you have demand during the day, you don't go below minimum.

 

[truepoolblues]

Ok thanks for the validation, I'll keep doing what I'm doing! Pool has been crystal clear all summer.

 

[Mdragger88]

20 ppm cya is awfully low for a swcg pool unless its covered.
It can mean that you must run the cell more which depletes its finite 8-10k hour lifespan more quickly in people years.

Letting fc fall below minimum risks algae.

 

[alvindo]

I used to target the bottom end of the recommended rang, but that caused a problem. I have between 50 and 60 CYA.So I would shoot for 3-4 ppm FC it was fine, but then we had the Fourth of July weekend and I had too many days of depleting it out of range. Boom, dead algae showing up. I now run at the higher end of the range and if we get heavy use or sun I’m still in the safe range.

The big thing for me was figuring out how much chlorine my SWG produces. Luckily for me, 10% adds 1 ppm over 24 hours, so the math is easy. I test my FC in the evening and adjust my SWG percentage so that I’m back to 8-9 ppm the next day.

I was nervous about keeping FC that high because you see 1-3 ppm recommendations in so many places. But for me, and my CYA, 7-9 is the place to be. At 3, I’d be fighting my pool all the time. At 8, it’s really a trouble free Pool. No algae, no chlorine smell, and just clear water.

 

[truepoolblues]

Warning: Long winded post with lots of number crunching.

TLDR: I'm not convinced I should take my CYA up beyond 20 ppm. Tell me what I'm missing (with the math to back it up!)

Understanding Cyanuric Acid (CYA)

Cyanuric acid (CYA) protects chlorine from UV degradation from the sun, buffers pH and contributes to total alkalinity, and slows chlorine. Learn more.

blog.orendatech.com

Based on the charts from Orenda I get 90-95pct protection of my chlorine somewhere between 15-20ppm of CYA. Now these are hourly numbers so if I have my pool uncovered during peak UV from 10 to 2 the loss would be somewhere between 1-(0.95)^4=0.19 and 1-(0.9)^4=0.34. Which is right in line with what I observe empirically. My demand spreadsheet assumes I will lose 27% of my starting chlorine on any day I uncover the pool.

This is approximately 0.27*3.5=0.95 ppm of chlorine.

If I take CA up to 60 I should get closer to 0.99 hourly protection ie. (0.99)^4=0.04 daily loss from UV (vs 0.27), a significant difference I agree!

Now if we want to be apples to apples on sanitizing power, at 7.5 ph and 20ppm CYA, the concentration of HOCL on my pool is about 3%, and above 50ppm CYA it would be sub 1%.

https://pulsarsystems.net/pool-science/cya-effects-on-disinfection/

0.03*3.5=0.105 pct HOCL in my pool, more than double what is required to keep algae at bay.

With >50ppm CYA you would need .105/0.01=10.5 ppm chlorine to match my HOCL concentration. If we relax that down to accepting 0.05 HOCL we still get a minimum of 0.05/0.01=5 ppm FC to keep algae at bay.

If I take CYA up to 60ppm and target 5ppm I am looking at 0.04*5=0.2 ppm daily loss from UV. So now we are looking at a daily 'savings' of 0.95-0.2=0.75ppm, which still sounds material on the surface but keep in mind I have a 2x sanitizing/algae fighting power in the 20 CYA/3.5FC world vs the 60CYA/5FC world.

The final piece of the puzzle for me is that the swim season is short here. I only ran the cell for 700 hours in July and August. It's 10 degrees here today, the cell has basically been off for the last week. We'll maybe get a couple more weekend swim days in before we close the pool in October.

Our SWG's capacity is approximately 0.185 ppm/hr so over those 700 hours the SWG should have produced approximately 0.185*700=130 ppm of chlorine.

Over July and August I had the pool uncovered on 27 days so this is approximately 0.75*27=20ppm 'wasted' chlorine that I could have theoretically saved by taking CYA up to 60ppm.

This is a 20/130=0.15 pct reduction in my cell life. A replacement T15 cell costs about $1300.

$1300*0.15=$195 'extra' I am spending over the cell's life by having CYA at 20 vs 60ppm.

We didn't finish our pool until Jul 1 this year so let's assume if add in May and June on a go-forward basis that just doubles everything to 2*700=1,400 cell hours and 2*27=54 uncovered days (unlikely as May/June can still be quite chilly around here)

If we are again conservative and assume a cell life of only 8000 hours then: 8000/1400=5.7 year cell life at 20ppm CYA and
8000/(1400-54*0.75/0.185)=6.7 year cell life at 60ppm CYA.

Let's again be conservative and say I only get 5 years instead of 6.

$195/5=$39 per year 'insurance' I am paying to have 2x the algae fighting power in my pool. This is a trade I would take all day long (to put into perspective it costs me well over $250 to obtain the TFP recommended Taylor test kit in Canada, which apparently needs to be replaced yearly, which I gladly do).

Ok let's hear it. . . where is my logic wrong?

 

[truepoolblues]

Re the CYA comments: I do keep the pool covered when not in use. I also just rechecked all my calculations and posted the logic here: Not convinced of the high CYA recos for a short-season pool

I love TFP overall but I did a lot of number crunching and made a very deliberate decision to stick at 20ppm CYA. I would love to have someone point out any holes in my logic!

 

[SoDel]

I’ll poke an empirical hole. If I let CYA fall below 50 my FC demand increases noticeably. So the 90% or 95% protection at 20 ppm doesn’t sound right based on my experimental sample size of one lol. I also take a different approach to pool management, like a pool is expensive, so a few bucks here and there if it makes my life a little easier is money well spent. So my calculation is I keep my CYA somewhere around 50-60 ppm, I let my FC range between 7.5 and 10 ppm, I have happy hour every evening at 5:00 pm. Q.E.D.

 

[Pauls234]

The science behind higher cya for swcg is around here somewhere and the discussion is well traveled ground. In simple terms, higher CYA means less chlorine loss due to UV so your swcg can be run at lower %, extending the life of the cell.

I was LC jug lugger for 10 years before getting an swcg in 2022 so was always afraid of high CYA. I finally decided to experiment midway through the 2022 season and slowly bumped my CYA from 40 to 70. I just went back and here is my note from then. Many many others on this forum have experienced the same.

 

[mknauss]

I do keep the pool covered when not in use.

With the pool covered and as you are in a very northern climate, you may keep the CYA much lower if you wish. You may have an issue in mid summer if you leave the cover off for a few days, so be sure to monitor that.

If it works for you -- go for it. TroubleFree comes in many forms.

 

[Newdude]

Being so far north, you may not even need the 60 CYA in the first place. 40 might be the best of both worlds for you.

And you're splitting too many hairs. You can have a general idea of what kind of loss to expect, but on any given day, the UV demand (when open) will be higher or lower than your equation.

 

[Saturn94]

I do keep the pool covered when not in use.

This makes a big difference in reducing FC loss and likely explains your success with lower CYA levels. By covering or using higher CYA levels, either way FC is being protected from UV.

Somewhere around here it was discussed how that chart showing FC loss in relation to CYA level doesn’t seem to jive with real world experience. It certainly doesn’t jive with my real world experience. I suspect this means there are real world variables not accounted for in that graph.

 

[JoyfulNoise]

Just an FYI - 0.05ppm (50ppb) is what is needed for disinfection of simple BACTERIA, not algae. Algae is a very diverse and complex class of aquatic microorganisms that can live with chlorine exposures well above 1ppm FC with no CYA, so about 0.5ppm HOCl or 10X higher than your assumed protective value. Once algae starts to reproduce, it can quickly and easily overwhelm even middling levels of chlorine.

Your pool configuration, locations, and local environment have much more to do with successful low CYA levels than most pools. Without UV loss (opening the cover for a few hours is minimal UV exposure at best) and with lower overall temperatures as well as a short season, it’s much easier to manage a pool on the low side of recommended. Most people don’t have auto-covers and most pools exist in warmer climates than yours. So using lower levels of CYA would increase the risk of water fouling greatly.

TFP Recommended Levels are designed for maximum benefit to the most number of users possible. There will always be side-cases and exceptions to general recommendations.

 

[AUSpool]

I think the latest graph for UV protection of FC by concentration of CyA is this one from Wahman, 2018. But again, it is deceptive or ambiguous depending on what outcome the reader is searching for. In a real world application I’ve found that increasing CyA from 50ppm to 70ppm increased my FC by ~1-2ppm over a week with no other change other than the increase in CyA. This means the chlorinator can be turned down to maintain the same FC level which significantly increases the life of the SWG. In terms of bather comfort I’m not sure there is any benefit of the higher CyA.

I use the Taylor 2000 series comparator for CyA tests and think 20ppm would be difficult to manage as it’s below the lower range of the scale. You run the risk of actually being at or below 10ppm. I would consider increasing your target range to 30-40ppm CyA.

 

[Mdragger88]

A mostly covered pool is similar to an indoor pool- for those the recommended cya level is around 30.
For more reasons than just that cya is sunscreen for fc.
Having adequate fc to account for the bather load is important so you don’t fall below min. This can be harder to do without fc being too harsh at low cya levels. At a cya of 20ppm you must constantly walk a fine line between minimum and adequate but not too harsh. Increasing cya a tad gives you the wiggle room to increase fc without it being harsh on people or equipment so you can stay free & clear from minimum without walking a tightrope.
While I am glad you test the pool frequently it isn’t necessary to micromanage twice a day unless you’re making changes or something.

 

[dereksanders]

Or unless you’re walking the tightrope.

 

[hexabc]

From reading ChemGeek’s complex analysis of interactions between CYA and chlorine in the pool (I’m not a chemist) I have taken one important lesson: it’s safer for the swimmers and the pool to maintain a higher level of CYA. Measuring CYA is error prone - it’s hard to be certain if the true level is 20 or just 10. Keeping it at 40 at least, and maintaining the recommended range of free chlorine, ensures that all active chlorine is bound to CYA and released only as needed. That’s why it’s safe to swim in the water with free chlorine up to the SLAM level.

 

[truepoolblues]

Thanks everyone for all the thoughtful replies! I've got lot's to consider and some chemistry rabbit holes to go farther down before the start of our next swim season!