Can anyone spot what I'm doing wrong?

Donna's Poolboy

LifeTime Supporter
May 29, 2007
29
Iowa
Hi gang--
I just opened about 2 weeks ago. I've run the vac several times, brushed the walls and run the sand filter 24/7. Here are the numbers (18'x36' 23,000 IG vinyl pool):
FC 17
cc 0
ph 7.5
alk 120
CA 200
cya 70

The water temp is 60 and the pool is crystal clear. Here's the problem: I've been losing chlorine at a rate of about 10 ppm per day. The air temperature hit 75 today, but has been in the high 60's for the last week or so. I've been trying to keep the chlorine at around shock level (25-27 ppm) until the cl holds overnight. Here are the cl numbers from the last 24 hours:
yesterday 8 pm: 23.5
today 4:30 AM: 23.0
noon: 21
2:30 pm 20
5:00 pm 18
6:00 pm 16.5

I thought I was out of the woods when I saw the cl remain fairly constant over night. But then I lost 7 ppm before the sun went down today. In the past, a 2-4ppm drop was normal--higher if we were in the pool all day. 7ppm is a big drop for a cool day, cold water and a bather load of 0. I hate to think how much I'll be using when it's sunny, 85 and full of 9 year old kids with questionable potty habits.
I just added 4 jugs of 6% bleach which will kick the cl back up to around 25. Should I stay there or go even higher? Again, the water looks perfect (I can tell the main drains are fastened down with a philips head screw!). Am I missing anything obvious? Got any suggestions?

One other thing--does the cooler water temperature effect the reliability of the cl test? Should I be warming the water sample up before I test it?
 

duraleigh

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Apr 1, 2007
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Well, chlorine is consumed by two things.....the Sun and organics in the pool. You're CYA of 70 offers adequate protection from the Sun so there's only one other logical answer.....you have organics (almost surely algae) in your pool. Even tho your water appears clear there is simply no other reason for the chlorine to go away at that rate,

The Answer? Add more chlorine up to shock level (about 20ppm in your case) and hold it there until your consumption drops back to the 2ppm daily you should expect. That'll stop the cl loss and you can then let your cl drop back to around 5-8ppm
 

JohnT

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Apr 4, 2007
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SW Indiana
Pool Size
21000
Surface
Vinyl
Chlorine
Liquid Chlorine
Donna's Poolboy said:
I thought I was out of the woods when I saw the cl remain fairly constant over night. But then I lost 7 ppm before the sun went down today.

I'm going to suggest another possibility. Everything you posted says inadequate CYA, so I wonder if your CYA reading is wrong. There are things that can affect the reading, and make it read high.
 

JasonLion

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May 7, 2007
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Silver Spring, MD
With a CYA of 50 you should lose about 50% on a sunny day and you lost less, perhaps 30%. If your CYA level was actually say 60 your results would make sense to me. The CYA test is around +-15, so I don't think there is anything surprising here.

No significant overnight loss means no organics.
 

duraleigh

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But then I lost 7 ppm before the sun went down today. In the past, a 2-4ppm drop was normal--higher if we were in the pool all day. 7ppm is a big drop for a cool day, cold water and a bather load of 0.

Your prior 2-4ppm drop daily in Cl I would consider normal. 7ppm drop with a CYA of 70 is excessive, IMO, and I think you are killing algae. You are on the right track by adding more chlorine and I think that, given the condition of your water, you will see the chlorine consumption slow back to normal very shortly.
 

Donna's Poolboy

LifeTime Supporter
May 29, 2007
29
Iowa
I too questioned my cya result and I've tested it 4 or 5 different times. Between 70 and 80 is what I come up with. My cya level was really high last season (the last test I did according to my log was in July and it was over 100). I drained around two feet of water at closing time (early Oct.) taking the water level to just above the returns. I don't know how many times I backwashed between July and Oct but I'm sure I did at least a few.

I just tested the water this morning and the Cl was 20.5, which is a drop of 1 ppm from 8:30 last night. I'll test it again a bunch of times throughout the day and note the amount of sunlight hitting the pool.

Thanks for your help with this!

Bill
 

Donna's Poolboy

LifeTime Supporter
May 29, 2007
29
Iowa
I should have posted an update earlier, but I think it's mission accomplished. The cl consumption is a little under 2 ppm per day. I think in total, it took 8 days at 20 ppm or higher before the massive chlorine decline started to slow down.

One thing I did was change the bulbs in the pool light. When I pulled the light fixture out, there was all kinds of nasty stuff in there (I hadn't taken the light out since the pool was built 3 years ago). I scrubbed and vacuumed out the opening for the light and left the fixture off for 24 hours so clean water could circulate into it freely. I don't know if that had anything to do with the large cl consumption or not, but it needed to be cleaned and the bulbs needed changing too.

Thanks for the help!
 
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