Chlorine Demand

Chlorinating, maintaining the right chlorine levels,
chlorine problems. Dichlor, trichlor, cal hypo, bleach,
granules, chlorine pucks and chlorine sticks.
blackadder
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My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Chlorine Demand

Postby blackadder » Thu 14 Oct, 2021 16:42

Firstly, my pool is 30 years old, and I have been maintaining it for all that time.
Last year I drained the pool to repair a rusty patch in the pebblecrete, and since then I have had issues with getting the pool to hold a free chlorine level.
My current cya is 25, but it is spring and water temperature is still less than 20C.
I had a Saltmate chlorinator that appeared to be working when the cell was in a bucket of pool water, but due to its' age I had it replaced. I now have a K Chlor Digital RP30 which should be more than adequate for my pool of 50,000lt.
The problem started last summer with the pool going green while I was away. Normally the pool required little maintenance other than occasional salt top ups after rain, and pH balancing.
The local pool shop diagnosed low CYA, which strangely failed to register after being added. I had to add it twice more over the summer to get a reasonable level.
The pool has been tested for phosphates.
This week, I decided to 'shock' the chlorine level, and added 10 litres of liquid chlorine, and left the chlorinator running for 24 hours at 100%
Within 48 hours my FC level was zero.
I then tried sodium hypochlorite. Two kilos of that pushed the FC high, but again within 48 hours I have zero.
The pool is normally covered, and the water is clear. There is no chloramine smell.
I am struggling to understand what is going on. I have never experienced anything like this before and need help. (The local pool shop just want to sell me more chemicals).


Denniswiseman
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My Pool: 10k inground fibreglass, Telescopic Cover, Hayward Powerline pump, Quality filter with glass media, 27kw output heat pump, K-2006C test kit
Location: United Kingdom

Re: Chlorine Demand

Postby Denniswiseman » Thu 14 Oct, 2021 22:44

Some thing is killing your chlorine, you may have to Slam (Shock Level and Maintain) with relation to Chlorine / CYA Chart and Recommended Pool Levels

Let's have your numbers (CYA is very important) Use a pool store if you don't have a FAS/DPD kit
FC:
TC:
pH:
TA:
CH:
CYA:

Liquid chlorine and sodium hypochlorite are the same thing apart from the fact that sodium hypochlorite is a liquid not a powder. Did you mean calcium hypochlorite
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Fri 15 Oct, 2021 02:36

Thank you. Yes, I meant calcium hypochlorite. I'll get a sample analysed over the weekend and post them when to hand.
What would be likely to be killing the chlorine?
Denniswiseman
Pool Industry Leader
Pool Industry Leader
Posts: 2592
Joined: Tue 06 Sep, 2011 05:48
My Pool: 10k inground fibreglass, Telescopic Cover, Hayward Powerline pump, Quality filter with glass media, 27kw output heat pump, K-2006C test kit
Location: United Kingdom

Re: Chlorine Demand

Postby Denniswiseman » Fri 15 Oct, 2021 05:49

blackadder wrote:Thank you. Yes, I meant calcium hypochlorite. I'll get a sample analysed over the weekend and post them when to hand.
What would be likely to be killing the chlorine?

Your combined chlorine, the difference between total chlorine and free chlorine
Teapot1
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My Pool: 12000 gallons vinyl liner,

Re: Chlorine Demand

Postby Teapot1 » Fri 15 Oct, 2021 06:33

Without CYA the chlorine is pretty much all gone after a few hours.
I may not give you the answer you want to hear, but I will give an honest opinion of your situation as you decribe it.
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Fri 15 Oct, 2021 21:02

Ok Dennis, here are the figures:

FC 15.55
TC 16.61
pH 7.9
TA 83
CH 278
CYA 5

Bear in mind that I added 2kg calcium hypochlorite 24 hours before.
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Fri 15 Oct, 2021 23:25

Since my last post at 1pm, I have balanced the pH, added 2kg of cyanuric acid, added 2 litres of chlorine, and set the chlorinator to run at 100% for 24 hours.
Denniswiseman
Pool Industry Leader
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Posts: 2592
Joined: Tue 06 Sep, 2011 05:48
My Pool: 10k inground fibreglass, Telescopic Cover, Hayward Powerline pump, Quality filter with glass media, 27kw output heat pump, K-2006C test kit
Location: United Kingdom

Re: Chlorine Demand

Postby Denniswiseman » Sat 16 Oct, 2021 02:31

You really need to get another set of figures to determine your CYA to get the correct slam level to maintain
The initial figure shows you have CC in excess of 1
When you have new figures follow the procedure in my previous post where I asked for your numbers
Teapot1
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My Pool: 12000 gallons vinyl liner,

Re: Chlorine Demand

Postby Teapot1 » Sat 16 Oct, 2021 05:06

blackadder wrote:Since my last post at 1pm, I have balanced the pH.

Thats an error when chlorine is high as the pH will fall as the chlorine reduces. That means aerating to raise the pH (your TA is fine so use aeration to raise pH).
I may not give you the answer you want to hear, but I will give an honest opinion of your situation as you decribe it.
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Mon 18 Oct, 2021 18:30

I thank the two respondents for their help.
Ultimately, I found a product that removes chloramines in a matter of minutes. It is Potassium Peroxy Monopersulphate in a product called "" "Oxyfresh".
Since adding that, my free chlorine level has been holding for several days without the chlorinator turned on.
Obviously adding the CYA also helped.
Teapot1
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Joined: Thu 29 Apr, 2021 00:43
My Pool: 12000 gallons vinyl liner,

Re: Chlorine Demand

Postby Teapot1 » Tue 19 Oct, 2021 02:00

Using Oxyfresh will remove chloramines but will come up as combined chlorine when you test the water for free chlorine and total chlorine. It lasts around 2 weeks in the water.
I still use it when a customer has a bit of an emergency and cannot shock with chlorine because they have paying clients arriving in a day (why they dont call me earlier??)
I may not give you the answer you want to hear, but I will give an honest opinion of your situation as you decribe it.
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Tue 19 Oct, 2021 13:40

I gather from your last comments, that you think my problem will recur once the Oxyfresh has dissipated, and that the SLAM technique is still required.
I used the Oxyfresh on Sunday morning but other than that I haven't added anything to the pool at all.
My figures at that time were:
FC 14.4
TC 17.56
pH 7.0
TA 50
CH 277
CYA 47
Denniswiseman
Pool Industry Leader
Pool Industry Leader
Posts: 2592
Joined: Tue 06 Sep, 2011 05:48
My Pool: 10k inground fibreglass, Telescopic Cover, Hayward Powerline pump, Quality filter with glass media, 27kw output heat pump, K-2006C test kit
Location: United Kingdom

Re: Chlorine Demand

Postby Denniswiseman » Tue 19 Oct, 2021 13:49

With a CYA of 47 according to the Chlorine/CYA chart you require a slam level of chlorine about 19
I would give it a while as Teapot1 says to check just how much CC you have
Teapot1
Pool Industry Leader
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Posts: 1176
Joined: Thu 29 Apr, 2021 00:43
My Pool: 12000 gallons vinyl liner,

Re: Chlorine Demand

Postby Teapot1 » Tue 19 Oct, 2021 16:55

blackadder wrote:I gather from your last comments, that you think my problem will recur once the Oxyfresh has dissipated, and that the SLAM technique is still required.

No, it could well be the boost you need, I was just explaining a little on how it affects testing and its life in a pool.
As I described, used it myself on numerous occasions.
Price is its only real drawback.
I may not give you the answer you want to hear, but I will give an honest opinion of your situation as you decribe it.
blackadder
Pool Enthusiast
Pool Enthusiast
Posts: 16
Joined: Thu 14 Oct, 2021 15:26
My Pool: Inground concrete pool with pebblecrete finish. 10m x 5m and approx 50,000 litres.
Sand filter with salt water chlorinator.

Re: Chlorine Demand

Postby blackadder » Sat 06 Nov, 2021 18:17

Gentlemen, I need your help again. After using Oxyfresh, everything was running well, until Friday when my test strip suddenly read zero free chlorine. I gave it a good dose of liquid chlorine with a view to 'Slam' ing as previously suggested, and then had the water tested.
FC 10.97
TC 12.68
pH 7.2
TA 75
CH 302
CYA 51

With combined chlorine reading 1.71, I ran the chlorinator all night, added about 7 litres of chlorine, and had the water tested again this morning
FC 13.89
TC 22.92
pH 8.5
TA 0
CH 294
CYA 37

Combined chlorine is now 9.03? pH difference is obviously due to the added chlorine, but the CYA and TA changes I don't understand. They can easily be balanced later, but what I want to know now, is how to proceed with the Slam? How much more chlorine to add, and over what period?

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