lower chlorine consumption

Hi, new member. 17000 gal. plaster pool.
FC: 12 to 20
PH: 7.8
TA: 90
CH: 220
CYA: 75
Southern Ca. pool with a lot of sun exposure. Recently shocked out my Mustard Algea with high Chlorine doses of around 46. Cleaned the plaster (white) and behind the light real well. Everything looks good and is stable except for the chlorine usage is quite a lot per 24hrs. I try to keep my FC minimun at 12 or above so I build it to 20 or so when adding and it lasts about 2 days. I use the 10% 1 Gal. Walmart Chlorine. I read where the FC usage should settle to around 1ppm drop per 24hr period but i'm still adding around 4ppm per day. (if ppm is the correct term) The water has never been so clear and it looks awesome but will the FC consumption slow down or is it just the result of my high CYA?

What your chlorine consumption 'should' be -- and what it will be -- are not always as closely related as we'd like. Even more frustrating, we can't always figure out the reasons for the differences.

So, just keep in mind, you have to live with the pool you've got, and they all have idiosyncrasies.

However, in your case, you are keeping your chlorine higher than you need to do, unless some algae is still present. That will definitely increase chlorine consumption. I'd recommend trying to back down gradually, from a 10 - 20 ppm range, to a 5 - 10 ppm range. But, as you drop, watch for re-occurring mustard algae, which can be VERY hard to eradicate.

Also, have you tried testing for chlorine at night after sundown and in the morning before sunup to see if the chlorine consumption is only due to sunlight, (which would be the case if there's no chlorine drop at night but some during the daytime), or if there's still something in the pool that's consuming it, (which would be true if you're losing it at night when the sun isn't out)?

Keep in mind too, that the chlorine consumption will drop some as the water temp drops....

Janet