Low FC - can't figure out why

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alphadog

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Jul 17, 2013
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I thought i was an expert - built and serviced pools in my younger days, and have owned 2 in the past 10 years. My current pool is a SWG system (Hayward) and everything was dialed in perfect last year - pump ran about 7 hours at night and 2 hours during the day, chlorinator at 65%.

This year, all was good in may with the same setup, but for the last 6 weeks Free Chlorine (measured by OTO or DPD) is reading close to zero - even with chlorinator at 100% for 12 hours. Even running for 36 hours straight and i only get up to 1ppm. I thought it was the cholorinator as i pulled water of the return line and saw low readings - but the chlorinator numbers (V/A) look good and bucket test shows it is putting out chlorine. I am using the Taylor DPD test kit - and i have run out of ideas. I can keep throwing in a few cups of granular every few days, but that is irritating.

CYA ~ 80
FC = 0 (slight pink from above so maybe 0.2)
TC = 0 - (unless i pull water right off the return line - then it shows FC 1.0, TC 1.5)
PH = 8 - yeah a little high, but shouldn't be a big deal
Phosphates - 0
TA= 135
CH = 250

I am still leaning towards the SWG not putting out enough CL - but no one (even Hayward) wants to tell me what it should put out.

I have a T-15 cell on a ~20K gallon pool. I wound up paying for a service call because the cell made bubbles.

Any ideas to rule in or out the water or cell are appreciated...
 
:wave: Welcome to TFP!!!

How old is the cell? Is it reporting the correct salt level?

The SWG are good at maintaining FC levels, but are not very good at raising it quickly. The FC is only lost to 2 sources, the sun and organics in the water. Your CYA is good, so sun not likely a factor. Now since the FC has dropped too low, likely something is in the water that is consuming the FC as fast as the SWG can make it. If you see the FC getting too low, you should be using bleach to supplement the FC.

Solution: you need to go through the SLAM process to ensure that there is nothing in the water. [slam:147fegyf][/slam:147fegyf]

Once you ensure that the pool is sanitized (using bleach in the SLAM process), then you can see if the SWG is able to maintain the FC levels.
 
Thanks for the advice.... sorry i left out a few details.

The SWG is one year old - or in service for a year. The pool was plastered, filled and opened April of 2012. Salt is showing as 3100 ppm and Volts/amps look good. Inspected and light acid wash just because i was not sure what a dirty one looked like. Always used tabs in the past, not SWG.

Can i use granular chlorine or is the added CYA going to be an issue, and if so, is there a conversion chart to indicate how much each time? I see my target FC should be 9 and shock 31.

I just noticed you mentioned bleach - I guess liquid without the CYA is the preferred method? I guess i am lucky i never had to to do this in the past.

I assume i can leave the SWG going. but i may have to pick up the other DPD test that measures more then 10PPM for CL.
 
Well, you tested the SWG and it's putting out chlorine. Therefore, it's going somewhere. Two things use chlorine: sunlight, and organics. If you do an overnight loss test, and it fails, you have something growing in there. My guess is that it will fail.

If you have something growing in there, you need to kill it. Kill it, not stun it. That means the conventional pool store dump-a-bag-of-this-powder-in shock won't do it. It needs to be sustained. We call it SLAM. Your CYA level is already at the upper limits for a SWG, and CH is good. But you don;t want CH to get higher, or it may promote scaling on the SWG plates, or the pool walls, particularly with your high pH. That leaves you with bleach as the only viable option.

Instructions on the overnight test, the SLAM process, even a discussion on sources of chlorine can all be found in Pool School.
 
If the granular is dichlor or trichlor it is adding CYA and you are at a good level now. If it is cal-hypo, then it is adding calcium and your level for that is good already as well.

So, yes liquid chlorine is the preferred way to add FC. You can use poolcalulator.com to determine the amount of bleach required to achieve a certain target.
 
Well, you would need 900 oz of 8.25% bleach ... which is 7 gallons ( although the bottles are not gallons).
And you need to keep the FC at that level until you pass the 3 criteria to stop.

Do you have the FAS-DPD chlorine test to measure these levels? What test kit do you have?
 
I have the normal DPD test kit, but I just found the instructions for the drop by drop method and it looks like i just need to buy R-0870 and R-0871 to complete my kit.

I had defaulted to 6% for bleach.... thanks for correcting.
 
Final question. I get the idea of holding the cl level super high, but how long does it take to come down to a safe level. 30 ppm until my overnight drop is only 1ppm is OK, but won't this take forever to get back down to normal?
 

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Thanks for the help everyone. I got the chlorine in the pool, and everything i need for the FAS-DPD test - but reading the taylor book, it says the 25ml test can detect as little as 0.2ppm, and the 10ml test can detect up to 20ppm - my target is 30ppm according to the chart (due to my 80ppm CYA) - so am i going to have a problem detecting a 1ppm change overnite? I just put the liquid in, and even though i have a Paramount in-floor cleaning system, i thought i would give it 30 minutes before i test. Although for a 10ml sample @ 0.5ppm per drop, i am going to need 60 drops if my CL level is at 30ppm.

My pool was much lower cost last year :( - i knew there was a reason i swore i would never own one :)

By the way, in the midwest, Menards (like Home Depot) sells 12.5% Sodium Hypochlorite for $3/gal and on sale now for $2.50.
 
Your FAD-DPD test will test as high as you need it, just make sure to put a heaping spoon of powder in. If you have little crystals left at the bottom of the vial after you mix then you added enough. Yes, it will take 60 drops to test 30 ppm, but that's what it takes to do it right. Frankly it will save you on a lot of bleach doing it blind. Once you pass the OCLT and get you SWG keeping you on target you shouldn't need to SLAM again anytime soon. Oh, you can run the SWG while you SLAM to help keep the level up but you must turn it off overnight while you do the OCLT, it will skew the results.
 
Use the 10ml sample so each drop is 0.5ppm ... you will have no problem reading up to 50ppm.
Or you could use 5ml to start so each drop is 1ppm ... then use more accuracy when you are reading for the OCLT and to verify CC.

Yes, Menards has some great deals on chlorine :goodjob:
 
Ok, got it set at about 27 ppm FC. It is dark so will check tomorrow. Once I have the FC so it passes the overnight or CC level, should I just let it drop back to to 5 ppm before kicking on the chlorinator?
 
If you are doing the OCLT tonight, make sure you let your last chlorine addition circulate for an hour then do your night FC test, then make sure you test the FC in morning before the sun hits the pool. You are not done with the SLAM process until you pass all three criteria simultaneously (also in my sig).
 
I think I saw a question about the CYA test and your 30ppm target. The Taylor large tube CYA range is 20-100 ppm. It cannot detect CYA below 20 and it is inaccurate if you are above 100ppm. You should be fine.
 
Overnight i dropped down to about 13ppm - so I based on my addition, i figure a gallon of 12.5% gets me about 6ppm - so I am back up to around 25, but i need more bleach, I underestimated.

But I am happy to see such a large drop in FC - something in is living in there. Hopefully I can add enough bleach and test to get it ready for overnight test tonight.
 
Was you water clear before the OCLT? If so, I would look for hiding places. These could include night, the backside of ladders, inside ladders. I would also think a good sweep of the floors and walls could help.
 
The pool has been clear since I opened it but the SWG has not kept up and every time the cl level drops to zero the water gets cloudy.

When we opened the pool it was full of worms and the pool had filled the 18" of water I had drained. During may it was cool and the pool did fine, June got hot and rainy here in OH.

The pool water smelled terrible when we opened but was clear. I guess there was a lot of invisible organic material. Filters were washed once this year - after the cloudy water started.

Got my fingers crossed that this will fix it all.

BTw, no ladders or anything in the pool. Only thing to wipe down is skimmer throats and walls
 

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