New System running at 90% but always low Chlorine levels - what "eats" up the Chlorine?

[Meichler]

11,000 gal pool, new Salt System with a T-9 cell rated for 25,000 gal, but can barely maintain 2.0ppm of free Chlorine with a system setting of 90%.
The running cell voltage is 24.3V, at 4.4A - which is perfectly normal according to the system manufacturer.
Salt level is at 3400ppm, Temp 89F. Pool water looks sparkling clean, with no indication for algae growth.
Chlorine <2.0, pH 7.6, Alkalinity 100, Calcium 400, CYA 90, LSI 0.02
The question is, why do I have to run this new cell so hard at 90+ %, for 8 hours, just to barely maintain a minimum Chlorine level?
I'm trying to achieve Chlorine levels of about 4.0ppm, to have adequate sanitation levels for the high CYA level.
I would expect 40-50% with a T-9 cell for an 11,000 gal pool, is this reasonable?
Can the cell be bad and not produce enough chlorine, but still display normal Voltage and Amp readings?
Can be something in the water that consumes all the Chlorine?
Looking for advice.

 

[JohnT]

You can have just enough algae to consume chlorine excessively without seeing it.

Did you test the CYA yourself or get the number from a pool store?

 

[pjt]

With a FC of 2 and CYA of 90, you probably have invisible contaminants in the water that are consuming your FC. How are you testing? Raise the FC level with liquid chlorine and perform an OCLT. There is a high probability you'll need to SLAM.

FC of 4 is the bare minimum with a CYA of 90. You should be targeting a FC of at least 6. FC-CYA Chart

 

[chlorinatorpro]

At first glance it appears the amperage level is too low for a salt reading of 3400. It should be closer to 6.0a with that high a water temperature. Are you absolutely sure the salt is 3400ppm?

Could you post ALL of the diagnostic readings from the display? Are there any lights on? Blinking or solid?

 

[Meichler]

You can have just enough algae to consume chlorine excessively without seeing it.

Did you test the CYA yourself or get the number from a pool store?

I testet the CYA myself with a Taylor kit and the pool store came up with the same. It’s high because of the prior use stabilized chlorine pucks. Don’t get any indication of algae when I brush and also had the pool shocked really good last week.

 

[pjt]

I testet the CYA myself with a Taylor kit and the pool store came up with the same. It’s high because of the prior use stabilized chlorine pucks. Don’t get any indication of algae when I brush and also had the pool shocked really good last week.

There are invisible contaminants that are consuming your FC. Did you perform an OCLT?

If you maintain FC at or above the target FC according to the FC-CYA Chart you normally never have to shock your pool.

 

[Meichler]

At first glance it appears the amperage level is too low for a salt reading of 3400. It should be closer to 6.0a with that high a water temperature. Are you absolutely sure the salt is 3400ppm?

Could you post ALL of the diagnostic readings from the display? Are there any lights on? Blinking or solid?

Tested the salt level again today and get 3400-3500 with a silver nitrate test, pool store also returned a 3400 reading. The T-9 cell is 2 weeks and the unit is set for this cell type. I agree the Amp reading should be over 5 Amps closer to 6 on a fresh cell at this salt level and 88F temperature, but the manufacturer claims 4.4 is within normal range for this cell type. Not sure if I can agree? All diagnostics tests on the Control unit are good. Display: 3500 for salt, 88.5F, 24.4 V, 4.41A, 90%, 3600, At -5, r611, Cell F-9 (the correct setting for this cell), E000=salt Display correction @0
Do you think it‘s possible the cell is questionable?

 

[Meichler]

With a FC of 2 and CYA of 90, you probably have invisible contaminants in the water that are consuming your FC. How are you testing? Raise the FC level with liquid chlorine and perform an OCLT. There is a high probability you'll need to SLAM.

FC of 4 is the bare minimum with a CYA of 90. You should be targeting a FC of at least 6. FC-CYA Chart

Agree that I should target a 6ppm chlorine level for the CYA at 90, and trying to get it there. Will follow your advise and go through a longer Slam cycle just to make sure and will do a OCLT test, something I have not done yet. Thanks.

 

[pjt]

Tested the salt level again today and get 3400-3500 with a silver nitrate test, pool store also returned a 3400 reading. The T-9 cell is 2 weeks and the unit is set for this cell type. I agree the Amp reading should be over 5 Amps closer to 6 on a fresh cell at this salt level and 88F temperature, but the manufacturer claims 4.4 is within normal range for this cell type. Not sure if I can agree? All diagnostics tests on the Control unit are good. Display: 3500 for salt, 88.5F, 24.4 V, 4.41A, 90%, 3600, At -5, r611, Cell F-9 (the correct setting for this cell), E000=salt Display correction @0
Do you think it‘s possible the cell is questionable?

Rule out contaminants first. If you're passing SLAM exit criteria, you can perform an overnight performance check of your SWG.

 

[reggiehammond]

Tested the salt level again today and get 3400-3500 with a silver nitrate test, pool store also returned a 3400 reading. The T-9 cell is 2 weeks and the unit is set for this cell type. I agree the Amp reading should be over 5 Amps closer to 6 on a fresh cell at this salt level and 88F temperature, but the manufacturer claims 4.4 is within normal range for this cell type. Not sure if I can agree? All diagnostics tests on the Control unit are good. Display: 3500 for salt, 88.5F, 24.4 V, 4.41A, 90%, 3600, At -5, r611, Cell F-9 (the correct setting for this cell), E000=salt Display correction @0
Do you think it‘s possible the cell is questionable?

While you troubleshoot the issue, don’t hesitate to get the FC up using liquid chlorine.

 

[Teald024]

11,000 gal pool, new Salt System with a T-9 cell rated for 25,000 gal, but can barely maintain 2.0ppm of free Chlorine with a system setting of 90%.
.
The question is, why do I have to run this new cell so hard at 90+ %, for 8 hours, just to barely maintain a minimum Chlorine level?

I would expect 40-50% with a T-9 cell for an 11,000 gal pool, is this reasonable?
Can be something in the water that consumes all the Chlorine?

The T-9 cell can produce 0.9 lbs of chlorine per day at 100% for 24 hours.
You are running 90% for 8 hours.
0.9 lbs in your pool is 9.8 ppm
9.8x 0.90/ 24x 8 = 2.94.
so the way your cell is set, it is making 3ppm per day.
A typical pool uses 2-4 ppm per day to the sun. So basically your SWG is making just enough to replace the daily sun tax.
If you increase your FC by using liquid chlorine up to the level you want, then you can use your SWG at your current settings to maintain it there.

 

[Meichler]

The T-9 cell can produce 0.9 lbs of chlorine per day at 100% for 24 hours.
You are running 90% for 8 hours.
0.9 lbs in your pool is 9.8 ppm
9.8x 0.90/ 24x 8 = 2.94.
so the way your cell is set, it is making 3ppm per day.
A typical pool uses 2-4 ppm per day to the sun. So basically your SWG is making just enough to replace the daily sun tax.
If you increase your FC by using liquid chlorine up to the level you want, then you can use your SWG at your current settings to maintain it there.

Thank you for the calculation.
Okay, so the pool has a more than sufficient CYA level which should result in an 70% Cl retention rate. This relates approximately to the 1.5ppm drop per day which I experienced last year when the pump went down. Similar pool. Same Florida Sunshine all day long.
Why would they call it a 25,000 gallon cell if it can produce just enough chlorine for maintaining a 10,000 gallon pool during a typical 8 hour run cycle?

 

[Teald024]

Technically, the pump is only running 8 hours to make that 3ppm. If you look back up, your cell can make 9.8 ppm in your pool. If you ratio that to 25k gallons, you get 4.3 ppm. In theory that should be able to replace what the sun burns off, but you need to operate the pump and cell 24/7 @ 100%.

 

[Meichler]

Technically, the pump is only running 8 hours to make that 3ppm. If you look back up, your cell can make 9.8 ppm in your pool. If you ratio that to 25k gallons, you get 4.3 ppm. In theory that should be able to replace what the sun burns off, but you need to operate the pump and cell 24/7 @ 100%.

Thank you for clarifying this. Makes total sense.
I love the PoolMath calculator. Great tool.
Now I do understand why my other 10,000 gal pool but with a T-15 cell (1.43lb per day) runs effortlessly at 23% at 8 hours pump time.
And when I needed more I just dialed it up a little bit and had quick results.
I just did not expect, that I have to run a T-9 (0.9lb per day) cell so much longer to get the same chlorine generated, plus maybe something is still in the water, consuming some extra chlorine, this can lead easily to what I'm experiencing.
The calculations provide proof. Learned a lot.
Great forum, thank you all for your help and tips. I learned a lot in a couple of days.