huge clorine demand

[chem geek]

Since you said you checked your filter, I assume you checked other places such as your skimmer and pump basket. You might also check under any removable ladders and perhaps behind light niches. Something is seriously consuming chlorine in your pool, and losing it overnight means it's not the UV in sunlight.

I also assume you test your chlorine level within an hour after you've added it just to make sure your chlorine isn't weaker than you think. So I too am at a loss. This is very unusual. Even if it were some conversion of CYA to ammonia and/or partially oxidized CYA, you should see your chlorine demand dropping and you would see at least some CC.

[sabres07]

Since you said you checked your filter, I assume you checked other places such as your skimmer and pump basket. You might also check under any removable ladders and perhaps behind light niches. Something is seriously consuming chlorine in your pool, and losing it overnight means it's not the UV in sunlight.

I also assume you test your chlorine level within an hour after you've added it just to make sure your chlorine isn't weaker than you think. So I too am at a loss. This is very unusual. Even if it were some conversion of CYA to ammonia and/or partially oxidized CYA, you should see your chlorine demand dropping and you would see at least some CC.

Yes, I do check chlorine levels after adding. For example, last night at 9 PM I added 8 gallons of 6% bleach and raised my FC level from 19.5 ppm to 42-43 ppm which it tested at 11 PM. This morning at 7 AM, my FC level was 37 ppm, for a loss of 4-5 overnight. I had a similar loss the night before of 5-6 ppm. No cc reading this morning, either. I will check behind my light housing today. Pump basket is clean. Under ladder looks clean, too.

I am at a loss here and it is getting very expensiive!!

The reason I took it up to high 30's for FC is that keeping it at the shock level recommended by the BBB method wasn't working. The chart recommends 20 ppm for shock level for me.

[BigDave]

Is it at all possible that you're testing CC and FC out of order? I'm sorry for asking, but I've been puzzling over this issue. The FAS-DPD test method is:
Measure pool water sample(I'm presuming 10ml)
Add a scoop or two of R-0870 DPD powder and swirl until solution turns pink.
Add the R-0871 titrating solution counting each drop and mixing.
Multiply drop count by 0.5ppm to get FC reading.
Add 5 drops of the R-0003 DPD Reagent #3.
If the solution turns pink again:
Add R-0871 solution counting each drop and mixing.
Multiply drop count by 0.5ppm to get CC reading.

I hope you're not offended, this is a real headscratcher.

[sabres07]

Is it at all possible that you're testing CC and FC out of order? I'm sorry for asking, but I've been puzzling over this issue. The FAS-DPD test method is:
Measure pool water sample(I'm presuming 10ml)
Add a scoop or two of R-0870 DPD powder and swirl until solution turns pink.
Add the R-0871 titrating solution counting each drop and mixing.
Multiply drop count by 0.5ppm to get FC reading.
Add 5 drops of the R-0003 DPD Reagent #3.
If the solution turns pink again:
Add R-0871 solution counting each drop and mixing.
Multiply drop count by 0.5ppm to get CC reading.

I hope you're not offended, this is a real headscratcher.

Ha...not offended at all. In fact my son asked me the same thing. I am testing properly I believe.

I just tested again at 10 AM. My FC is now down to 28-29 ppm (last nite at 11 PM it was 43!). I didnt believe those results so I tested it a second time and got the same number. Also, I did get a slight reading on cc this time, too, of 0.5<. Also, the water is crystal clear.

I am starting to think that draining the pool down by half and re-filling might be a cheaper way to go.

Any thoughts??

[BigDave]

Maybe try a Chlorine Demand Bucket Test to help figure out what's going to help. I'd start tonight and do an overnight test of just plain pool water to see if it's something in the water or something else causing such high chlorine consumption. After tomorrow morning, start dosing the bucket to see what it will take to get chlorine to hold - if you can at all. Then figure out if the chlorine or the water will be cheaper. Also maybe also do a bucket test with half pool water and half fill water to see if that's going to help.

[sabres07]

UPDATE: Last nite was the first nite my FC held from sundown to sunrise. I pulled my light niche and found a whole lot of crud back there. Very disgusting. So I scrubbed it all out and left the light out for two days so that shocked pool water could circulate in there. Maybe that was the problem with my huge chlorine losses. To re-cap, I had my pool at shock level (above 20 ppm) for at least 12-13 days now. It took that long to defeat whatever was in my pool. Hopefully I am done now. I will keep it up above shoock for at least one more day to be sure.

[JimK]

UPDATE: Last nite was the first nite my FC held from sundown to sunrise. I pulled my light niche and found a whole lot of crud back there. Very disgusting. So I scrubbed it all out and left the light out for two days so that shocked pool water could circulate in there. Maybe that was the problem with my huge chlorine losses. To re-cap, I had my pool at shock level (above 20 ppm) for at least 12-13 days now. It took that long to defeat whatever was in my pool. Hopefully I am done now. I will keep it up above shoock for at least one more day to be sure.

Good to hear! Thanks for the update.