Can't get FC to drop in SWG pool

PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
Hello everyone,

New to the forum, new to pool ownership, not new to pool maintenance but not a hardened veteran either. For a couple of years many years ago, I was a manager at a large public pool, so I understand fundamentals and definitely know how to properly test chemistry. We recently bought a house that came with a 15K gallon fiberglass saltwater pool with a <1 year old SWG. When we moved in a week ago, I noticed the water was looking a bit cloudy, so thought I'd put my brand new K-2006 test kit to use and see what I had inherited. Here's my initial results:

FC 5.0
CC 5.0
PH 7.4
TA 30
CH 250
CYA 20
SALT 3500
Water Temp 84F
SWG was set at 70% (pump running 9 hours/day)

With such a high CC, I decided I wanted to get my CYA up a bit and then shock the pool. In hindsight I probably should have shocked first and then worried about the stabilizer, but what's done is done. Anyway, for starters I added 2 pounds of cyanuric acid and ran the pump for a full 24 hours (after which the water became crystal clear). Three days after the stabilizer was added, I decided to do another test and see what had happened, completely understanding that CYA may not fully register until up to a week after. Note that I had still not shocked the pool at this point. Here's my test results:

FC 20
CC 0
PH 7.8
TA 30
CH 250
CYA 50
SALT 3400
Water Temp 84F
SWG still at 70%

My thinking was that with CYA so low, the previous owner had to run the SWG at a higher output. Once CYA was boosted and the chlorine wasn't getting chewed up as much by UV, the 70% output was translating to more FC being retained in the pool. So, I flipped the SWG generator off that morning and let the sun do it's work. By the end of the day, FC had dropped to 15. By the end of the next day, it had dropped to 7. Yesterday at about noon, I took a FC reading of 5. So I turned the SWG back on and turned the dial down to 20% for the final 5 hours of pump run time just to see where that would put me. I ran a full test around 6PM yesterday not long after the pump shut down:

FC 18
CC 5
PH 7.8
TA 30
CH 250
CYA 60
SALT 3300
Water Temp 76F

Not believing it, I tested the FC and CC again and got the same result. Anyone have any idea what's going on? The T-CELL-5 isn't exactly a huge SWG. Considering CYA is only 60, the only possible thing I could think of is that the output control for the SWG isn't working; but it's less than a year old so I'd find that surprising but not impossible. Hoping someone can help me out here because I can't have the pool at one extreme or the other. On top of all this, my wife and kids are asking me when they can jump in, and I'm hoping they can do so before Labor Day. TIA.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,746
Northern NJ
Welcome to TFP.

Let your wife and kids jump in and enjoy your hard work. Any FC level below SLAM level for your cya is safe to swim. See FC/CYA Chart. For CYA 60 that is below 24.

Just to check your testing method - what ML water sample did you use and how many drops to clear on the FC test?

Also, your CC of 5 is concerning. What does your water look like?
 

PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
Welcome to TFP.

Let your wife and kids jump in and enjoy your hard work. Any FC level below SLAM level for your cya is safe to swim. See FC/CYA Chart. For CYA 60 that is below 24.

Just to check your testing method - what ML water sample did you use and how many drops to clear on the FC test?

Also, your CC of 5 is concerning. What does your water look like?
Yes, the CC of 5 is concerning. The water is actually pretty clear, maybe not as clear as the day after I added the CYA, but I would by no means call it cloudy. Regarding the FC test: I used the 10 ml sample and it was 36 drops to clear on the most recent test. I have no interest in obtaining a 0.2 ppm resolution for a pool.
 

JamesW

TFP Expert
Mar 2, 2011
16,589
It doesn't seem likely that the CC would go from 5 to 0 and back to 5.

Are you using any non-chlorine shock?
 

JamesW

TFP Expert
Mar 2, 2011
16,589
A T-5 produces about 12 oz chlorine gas per day at 100%.

That's 6 ppm per day at 100% or 0.25 per hour at 100%.

At 20%, the increase is 0.05 ppm per hour or 0.25 ppm in 5 hours and that's assuming no loss to the sun.

Overall, your numbers make no sense at all.

There has to be testing errors or some missing or incorrect information.

We don't "shock" a pool. We follow a process called SLAM.

 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
14,595
Bedford, TX
PT,

There are zero reasons to "shock" a saltwater pool, like you probably did with your old tablet pools.. It is just not required.

I have three saltwater pools and have not added a single bag of pool store "shock" for a combined total of about 19 years... and have never had algae. Just maintain your CYA and FC per this chart and you are home free... FC/CYA Chart

Thanks,

Jim R.
 
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PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
A T-5 produces about 12 oz chlorine gas per day at 100%.

That's 6 ppm per day at 100% or 0.25 per hour at 100%.

At 20%, the increase is 0.05 ppm per hour or 0.25 ppm in 5 hours and that's assuming no loss to the sun.

Overall, your numbers make no sense at all.

There has to be testing errors or some missing or incorrect information.

What test kit do you have?

We don't "shock" a pool. We follow a process called SLAM.

The numbers make no sense to me either. FWIW, pool volume is right off the manufacturer's paperwork that was provided to me when I bought the house. I'm using a brand new K-2006 from Taylor; I'm pretty familiar with it as a former pool manager. All the reagents have expiration dates in 2021. I've read up on SLAM and intend to follow it as needed. Regardless of terminology/methodology, do you think it's best at this point to bring the pool to shock level? I will do another test this evening when I get home and make sure I'm not missing a step or something, but I did two careful tests back to back and came up with the same results.

Since we're on the topic of SWG's, there's another question I have for the community since I'm new to saltwater pools. Everywhere I've read, this site included, says SWCG pools need to have a higher CYA level around 70-80. The inside cover of my SWCG control panel says to maintain CYA at 30-50. My inclination has always been to follow manufacturer specs. Which should I actually follow?
 

JamesW

TFP Expert
Mar 2, 2011
16,589
50 ppm CYA is fine. We find it beneficial for the CYA to be higher, but 50 is ok.

I would do a full SLAM.

Be careful about doing the tests correctly with the same test sample volume each time.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,746
Northern NJ
Are you sure you have a T-5 cell? That is for a 20,000 gallon pool and small for your pool.

Are you sure it is not a T-15 cell?
 
Last edited:

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,746
Northern NJ
Yes, certain of the cell model. T-5 is for up to 20K gallons but small for my 15K gal pool? Isn't the T-15 for like 40K gal?
We recommend you have a cell at least 2X your pool volume. If you don’t have an oversize cell you have to run it 24/7 at close to 100% to generate adequate chlorine.

Have you looked at the labels on the cell?
 

JamesW

TFP Expert
Mar 2, 2011
16,589
A T-5 will make 6 ppm per day for 15,000 gallons.

Assuming a 3 ppm per day chlorine demand, you would need to run the pump 12 hours to produce enough chlorine.

A T-15 would only need 6 hours to produce the 3 ppm. So, you could reduce pump runtime with a bigger cell and the cell would (theoretically) last twice as long.

If you have a variable speed pump, the pump runtime becomes less of an issue.

What are the diagnostic readings from the Aquarite?

Note: Chlorine doesn't just come out of thin air. The T-5 didn't produce the fc increase. So, either someone is adding chlorine without your knowledge or your testing is inaccurate.
 
Last edited:

PointeTaken

Well-known member
Aug 28, 2019
57
Hoover, AL
50 ppm CYA is fine. We find it beneficial for the CYA to be higher, but 50 is ok.

I would do a full SLAM.

Be careful about doing the tests correctly with the same test sample volume each time.
So I should sell the Cal-Hypo back to Wally World and pick up some jugs of 10% bleach instead?