Losing 10-12 ppm FC daily?

[OneMom]

I have been a long time member here, and once we started using borates to control our constant yellow mustard algae problem, taking care of the pool has been a breeze. However, this month we have run into a problem that we have never seen before. Since the middle of this month, we have been going through huge amounts of FC daily. As soon as it started, we brushed & shocked until we passed the overnight shock test. Within days, we were back to losing a minimum of 8 ppm per day, so we shocked again until we passed the overnight shock test. Unlike in the past, it took three days of scrubbing & shocking before the FC held overnight. So, as of this past Monday morning (at 6am) , we had "officially" passed the test. We tested the pool this morning (just 48 hours later), and the FC was down by 20 ppm again! We have a brand new TF-100 kit, so our chemicals aren't old. We have re-tested our CYA, etc. many, many times...thinking perhaps we somehow messed up the testing process, but all of our values remained consistent. We have scrubbed EVERYWHERE, lights, pool sweep, skimmer basket, etc. Our CYA is 30, but we have been shocking to 30 FC in attempt to beat this. After all of the years of doing this, we are just baffled by this. What the heck is going on?

Recent Test Results: We use liquid chlorine, most of the time. We will be leaving on vacation (for 7 days) at the end of this week, and that is the only time we use our auto chlorinator (with the chlorine pucks). Our pool is vinyl, and partly shaded 1/2 of the day. It has been warm here in WI, but we've only had 5 days over 90 degrees so far this summer, so it isn't as warm as AZ or TX. We haven't used the solar cover at all.

CYA: 30
FC: 6.5
CC: 0
PH: 7.8
TA: 120
Borates: 50

 

[mpacheco72]

I lost 8.5 since yesterday but I've had full sun for 16 hours and it's been around 105 degrees here. It sounds like you might be losing too much.

 

[JasonLion]

Your CYA level is at the low end of the range, so you are going to lose something on the order of 2/3rds of your starting FC level to sunlight each day. If you start with a very high FC level, you will lose a lot of chlorine. Starting at 30, I would expect you to go down to somewhere around 10 after a sunny day.

Daytime chlorine loss is not a particularly interesting number. It depends on too many things, sun, temperature, CYA level, and so on. Overnight chlorine loss will tell you what is really going on.