What in the heck is eating my chlorine!

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doncaruana

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Aug 25, 2011
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Northville, Mi
Pool Size
15500
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Pool is at about 30cya. I always run pretty consistent, SWG running at 40% 10 hours a day (enough to put roughly 2ppm of chlorine a day in the pool). After the pool store guys who did some work left the SWG unplugged for a week, I got something growing.

I've been SLAMing it for a few days now and I've never had issues like this before. My water is absolutely crystal clear right now, but whatever is going on is chewing through chlorine something fierce.

Now here's the weird part...
I thought I had this licked...yesterday evening it was at 9. I put a gallon of 12.5 in and ran the pool for an hour to mix it up good (I actually just noticed the pump 24/7 thing in the slam article - never had to do that before). Anyway, that should have got me to about 17, but I measured 15. Figured okay - we'll just see. This morning I got 15.5. A little odd, but maybe I miscounted last night. Sounds like we're home.

I left the SWG up at 60% and let it run today - that should put me at an extra 3ppm for the day. Now it was a hot sunny day today and we swam this evening. Sun goes down and I test the water. It's at 7ppm!! Holy cow! Even if I lost everything for the day to the sun, that's still another 8 gone.

I didn't get any more bleach because I thought we were done. So at this point...
Should I run the pump overnight with SWG to see where we end up?
This seems pretty aggressive to me - certainly I've never had anything chewing it up like this - is this not unusual for others?
 
Time to start looking with a toothbrush in your hands. Do you have a light? steps that stick out? Have you looked around your skimmer face and behind the door? around the returns?

Have you been brushing your pool? That is very important to get it all up and in the water for the FC and filter to work on.

:kim:
 
Let it run all night (with SWG on) and FC was right at calculated values and CC was only 0.2. I'll stop for more chlorine on the way home and keep my fingers crossed.
 
Definitely been fighting an issue similar to yours. Had a bad algae bloom that just wouldn't go away. Finally got the algae killed but my chlorine levels are still in the tank. I finally ordered a decent test kit and realized I had no CYA in the pool (could see the black dot clear as crystal with the tube full). If you're getting a lot of rain or anything to dilute your water. I'd keep an eye on your CYA just in case.
 
I've seen my pool use upwards of 8 ppm FC on hot, sunny days when my CYA got down in the 20-30 range (unknowingly) and I was still trying to keep target FC in the 50-60 range. The higher the FC gets, the more is exposed to UV degradation and not protected by CYA.

I'm not saying that you don't have something growing. But with a CYA of only 30, FC in the 15 range will get chewed up very quickly by the sun.
 

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I normally run really well at around 60-70 cya. Then after a week with my SWG off, my chlorine was gone and my cya was half what it was - I never actually paid close attention to CC, but I figure there was some bacteria in there and it chewed up my CYA Anyhow, I figured I'd shock at that level anyway (easier to get to shock levels and too long or expensive to get CYA up).
 
CYA normally doesn't reduce on its own and is reduced whenever your replace water like during backwashing, large amount of rain with overflow or splash out. I believe it usually degrades at the rate around 1 ppm per month.

When a large amount of CYA disappears it is converted into ammonia and happens during the winter months with really cold water. This typically happens when the pool is close in northern climates.

CYA should be tested once a month or more frequently if you've had to replace a large amount of water.
 
kcindc said:
CYA normally doesn't reduce on its own and is reduced whenever your replace water like during backwashing, large amount of rain with overflow or splash out. I believe it usually degrades at the rate around 1 ppm per month.

When a large amount of CYA disappears it is converted into ammonia and happens during the winter months with really cold water. This typically happens when the pool is close in northern climates.

CYA should be tested once a month or more frequently if you've had to replace a large amount of water.
Click to expand...

I measured it about a week or two before the SWG was left off. It was at 60. And I sure didn't replace half the water in the pool. So something ate it pretty darned quick.
 
Can you verify if you did the CYA test correctly? It is very finicky and light sensitive. Full sunlight, back to the sun, vial waste level with two fingers. Pour to 100, check for the dot, pour to 90 check for the dot, etc.

The only way to go from 60 to 30 is by replacing water. Did you have some heavy rain storms? Do you have an autofill? Could you have a leak?
 
kcindc said:
Can you verify if you did the CYA test correctly? It is very finicky and light sensitive. Full sunlight, back to the sun, vial waste level with two fingers. Pour to 100, check for the dot, pour to 90 check for the dot, etc.

The only way to go from 60 to 30 is by replacing water. Did you have some heavy rain storms? Do you have an autofill? Could you have a leak?
Click to expand...

Been testing it consistently the same way for years. No autofill, no leak. Had some rain, yes, but not enough to lose half my CYA. Bacteria can eat it faster than just over the winter, given the right conditions...I reference chem geek's post and my own experience...
 
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