Question about FC needing to get higher with SWG running at 90%

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philipaby

Bronze Supporter
Feb 5, 2020
28
Katy TX
Hello!
I'd recently had an issue with my pool where the SWG (Hayward T-15) was not working. Free chlorine readings were always on the low end at 1 or less ppm. Replaced the same and it started generating chlorine fine. Pool tech would take readings and adjust balance as needed once a week. He does mention that he does not maintain a lot of salt water pools as most of his clients have the normal ones.

I do have the occasional green/black spots showing up randomly on the walls of the pool that I would brush off with a steel brush. However, had this nagging doubt and based on recommendations from the folks here, got a TF Pro kit from the test kit website. Did my first set of tests and it was eye opening for me. Here's the first set of readings (did not do a calcium test yet)
FC - 4 ppm
Alkalinity - 200
CYA - 80
ph - 8.2 at least (highest level of pink)

Looking at the pool math charts for SWG pools for CYA 80, it is saying that my FC should be between 6 and 11 ppm. I also have a variable speed pump that runs at least 12 hours every day (3450 ppm for 2 hrs, 2750 for the next 2 and 1750 ppm for the remaining 8). The calculator is saying that I need to keep my SWG at 45% for 12 hrs run.

So three questions,
1) Do I need to add liquid chlorine every week (in addition to the SWG generating chlorine) to maintain to 6-11 FC range? Is the SWG not able to do that on it's own?
2) Or should I set the SWG to a lower percentage for that run time? (was thinking higher percentage would be more chlorine)
3) Or is the pH, alkalinity causing some kind of an issue with this?

Apologies if this was asked before in the forum, but did some scanning through prior posts and could not get to the right thread.
Thanks in advance
Aby
 
Are you ditching the pool service? He's doing you no favors. None of the test results are very good indications of the service you've received, and your pool pump is running higher electrical usage than you really need. All of this can be fixed, but I'd tell you to ditch the service and take care of it yourself.

The indications of algae show you need to SLAM Process but that's not going to be possible with CYA 80. What type of pool? Plaster, fiberglass or vinyl? Go ahead and test CH and report back. But I think you likely need to drain some water off (50% or so) and replace to reduce CYA to make a SLAM cheaper and easier.

Why do you have algae? Simple - your FC (free chlorine) is below minimum for the higher CYA levels per FC/CYA Levels.

With a SWCG, you should not be having to add liquid chlorine during swim season. The SWCG can handle that; but what it cannot do is outcompete algae. We need to eliminate the algae via SLAM first.

SWCG runtime is just a function of how much FC (free chlorine) the pool consumes on a daily basis. Generally I'd say you only need the SWCG to produce 3-4ppm to keep up. PoolMath can tell you the amount of time your SWCG needs to run to create 3-4ppm.

High TA causes pH rise; adding muriatic acid to lower pH to 7.2 will reduce TA and eventually around 50-60 TA, the pH rise will stop.

Lastly, pump speed and run time are based on:
1) How many RPMs are needed to keep SWCG flow switch activated
2) How long pump needs to run to skim properly.

I'd recommend doing what most of us do; run the pump 24/7 at the lowest possible RPMs. I run ~1300 rpm 24/7 for about $25 a month in electrical cost. By running 24/7, my SWCG can make chlorine all the time (based on % setpoint), skim constantly, and filter.
 
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reggiehammond said:
Are you ditching the pool service? He's doing you no favors. None of the test results are very good indications of the service you've received, and your pool pump is running higher electrical usage than you really need. All of this can be fixed, but I'd tell you to ditch the service and take care of it yourself.

The indications of algae show you need to SLAM Process but that's not going to be possible with CYA 80. What type of pool? Plaster, fiberglass or vinyl? Go ahead and test CH and report back. But I think you likely need to drain some water off (50% or so) and replace to reduce CYA to make a SLAM cheaper and easier.

Why do you have algae? Simple - your FC (free chlorine) is below minimum for the higher CYA levels per FC/CYA Levels.

With a SWCG, you should not be having to add liquid chlorine during swim season. The SWCG can handle that; but what it cannot do is outcompete algae. We need to eliminate the algae via SLAM first.

SWCG runtime is just a function of how much FC (free chlorine) the pool consumes on a daily basis. Generally I'd say you only need the SWCG to produce 3-4ppm to keep up. PoolMath can tell you the amount of time your SWCG needs to run to create 3-4ppm.

High TA causes pH rise; adding muriatic acid to lower pH to 7.2 will reduce TA and eventually around 50-60 TA, the pH rise will stop.

Lastly, pump speed and run time are based on:
1) How many RPMs are needed to keep SWCG flow switch activated
2) How long pump needs to run to skim properly.

I'd recommend doing what most of us do; run the pump 24/7 at the lowest possible RPMs. I run ~1300 rpm 24/7 for about $25 a month in electrical cost. By running 24/7, my SWCG can make chlorine all the time (based on % setpoint), skim constantly, and filter.
Click to expand...
Thank you for the information. After I posted, I tried looking for material on why SWG would not generate enough FC and saw that point you just raised, i.e. even if not visible, there could be enough algae that is consuming a lot of the FC generated by the SWG, hence cannot keep up. Totally understand the need to SLAM. I'm also wondering if I should do the overnight chlorine test just so see how much is being consumed without the SWG running through the night. That will tell me how bad the algae issue is. Apologies for being green on the algae issue as the pool has been looking really clear all through so I did not think along those lines.

The pool is plaster. I was under the impression that for high sunlight regions, CYA for SWG pools were supposed to be in the 60-80 range. However, I understand that for SLAM, lower CYA will not require as much chlorine etc.

As far as the pool service, he's pretty good with maintaining or repairing the pool equipment etc but I did have questions on pool chemistry. So really appreciate all the steer coming from folks like yourself on this forum.

Using the pool math calculator, it's saying that I need to get the FC to 31 for the SLAM. I've never done this myself before, hence wanted to make sure that kind of a number is not unusual for slamming.

Will work along those lines and report back depending on how it goes.

Thanks again
Regards
Aby
 
Last edited:
reggiehammond said:
The indications of algae show you need to SLAM Process but that's not going to be possible with CYA 80.
Click to expand...

SLAM Process with CYA 80 is doable. Slam FC will be 32ppm.

It will take a lot more chlorine then if the pool was half drained to about CYA 40 do the SLAM FC is 16 ppm.

Cost of draining and refilling and chemicals needs to be weighed against the cost if chlorine with a CYA 80 SLAM.
 
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ajw22 said:
SLAM Process with CYA 80 is doable. Slam FC will be 32ppm.

It will take a lot more chlorine then if the pool was half drained to about CYA 40 do the SLAM FC is 16 ppm.

Cost of draining and refilling and chemicals needs to be weighed against the cost if chlorine with a CYA 80 SLAM.
Click to expand...
Thank you ajw22 and indeed fair point that I will need to consider drain/refill vs chlorine cost. BTW, for SLAM, I understand that liquid chlorine is better. Would a product at the link here work?


Or should I be getting it from some other place?

Also should I be getting pH to the right value before the SLAM?
 
Under normal circumstances (clean, algae free) you should expect to lose roughly 3-5 ppm of FC every day in the summer. (In the winter this may be 1 per day or less.)

Your T-15 set to run at 100% over 24 hrs would produce 8.8 ppm FC per day in your 20K gal pool. You can run your SWG at 50% for 24 hours to produce 4.4 ppm. (SWG at 100% for 12 hours would also produce 4.4 FC). There should be no reason to regularly add liquid chlorine.

In summers I keep my CYA between 70-80 and FC about 7-9. (The pH test is not valid when the FC is above 10. This is the only reason I don’t target the 10-11 range normally)

Your algae issue is 100% due to low chlorine.
 
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Not to add to the puzzle but I did the overnight test today and noticed that the FC was still at 4 ppm (same as the reading I took yesterday evening)! On a side note, I have to tell you, this TF Pro kit makes FC testing way easier and I really like the fact that it is granular to the 0.5 ppm (not to mention watching the magnetic stirrer do it's thing is pretty cool :)).

So I would still want to SLAM the pool I guess, but if there was a lot of algae that was not visible, wouldn't there be a sizable drop in FC overnight? Bit confused now.
 
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philipaby said:
Not to add to the puzzle but I did the overnight test today and noticed that the FC was still at 4 ppm (same as the reading I took yesterday evening)! On a side note, I have to tell you, this TF Pro kit makes FC testing way easier and I really like the fact that it is granular to the 0.5 ppm (not to mention watching the magnetic stirrer do it's thing is pretty cool :)).

So I would still want to SLAM the pool I guess, but if there was a lot of algae that was not visible, wouldn't there be a sizable drop in FC overnight? Bit confused now.
Click to expand...

Sounds like you passed the OCLT and do not have algae.
 
philipaby said:
Would there then be another reason why my FC is only 4 ppm when the T-15 should be generating more? It was set to 90
%. Cause the Pool Math is saying that for CYA 80, my FC should be above 6.
Click to expand...

SWG's are good at maintaining a FC level. They often don't have the generating capacity to raise the FC level.

If you raise your FC to 6-8 using liquid chlorine will the SWG maintain it?

This is the basic information we need to know in diagnosing a problem:
  • Report all diagnostic readings when you..
    • Move the switch from auto to off and check all of the readings.
    • Move the switch back to auto and recheck the readings.
    • Move the switch to off for a minute and then back to auto and recheck the readings.
  • What are the first seven characters of the cell and box serial numbers?
  • What is the actual salinity and how are you measuring it?
 
Understood especially on the point that SWG by itself cannot raise the FC to high levels but the question was more on is 4 ppm the limit. And the Pool Math calculator says that it should be above 6 for a SWG based pool, hence the question.

I did take the readings like you mentioned and here they are.

When running normally it was
2900, 92, 25.5, 6.62, 81P, 2900, AL-0, r1.59, t-15

Switched to off
2900, 92, 31.1, 0, 81P, 0, AL-0, r1.59, t-15

Switched back to auto
2900, 92, 25.4, 6.73, 81P, 3000, AL-0, r1.59, t015

Switched off, waited a min and then to auto
2900, 92, 25.4, 6.68, 81P, 3000, AL-0, r1.59, t-15

The cell I had replaced was a Hayward Turbo Cell TCELL940 (think it's like an extended life t-15), that's also what I see on the side of the cell. Serial number is 3E23187 (this was under the bar code of the cell)

Salt readings taken via strips is in the 2800-2900 area.


Thanks
Regards
Aby
 
Your cell is generating and the diagnostics look fine.

Run your pump longer to get more chlorine.
 
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Raise your FC to about 7-8 quickly using liquid chlorine. Measure your FC 30-60 min after adding LC to verify (with pump running)

Set your SWG to produce about 4 ppm. Use pool math but 90% at 12 hours is about correct.
(Remember that the sun will consume about 4ppm every day this time of year)

Test your FC 24 hours later. If your FC remains the same then your SWG is producing, and is set correctly. If FC is lower or much higher, adjust your SWG to make more or less FC.

Continue to test FC every 24 hours (and make adjustments if necessary, aiming for 7-9 measured FC) until you get the hang of it. Eventually you can test every 2 or 3 days, or longer when you really understand your pool.
 
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.ben said:
Raise your FC to about 7-8 quickly using liquid chlorine. Measure your FC 30-60 min after adding LC to verify (with pump running)

Set your SWG to produce about 4 ppm. Use pool math but 90% at 12 hours is about correct.
(Remember that the sun will consume about 4ppm every day this time of year)

Test your FC 24 hours later. If your FC remains the same then your SWG is producing, and is set correctly. If FC is lower or much higher, adjust your SWG to make more or less FC.

Continue to test FC every 24 hours (and make adjustments if necessary, aiming for 7-9 measured FC) until you get the hang of it. Eventually you can test every 2 or 3 days, or longer when you really understand your pool.
Click to expand...
Thanks Ben, will do.
 
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