FC occasionally starts dropping and can't be controlled

BennytheDog

Bronze Supporter
Nov 26, 2020
7
Sarasota, Florida
Pool Size
10000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-9)
I’m a newbie to pool maintenance, 3.5 months in, and I think I’m missing something because I’ve had two occasions where the FC starts dropping and my SWG with an oversized cell cannot keep up even when I crank it to 100% and extend the run time by 20%. I recently SLAMed because of this.

The first time the FC had only dropped to about 4 ppm. I started cranking up the duty cycle and it kept dropping. After a few days I got a CC reading of 0.5 ppm and tried to correct the problem with a couple of good doses of left over chlorine from my first SLAM. Things seemed fine for a week or so.

About two weeks later I had not tested the pool for a week and FC had dropped to 1.5 ppm with CC at 0.5 ppm. I cranked the SWG all the way up and it could not keep up making me think the SWG was not working correctly. After this SLAM my SWG is keeping up nicely and can drive the FC quite high.

What am I missing?

Thanks for your help.

Currently the pool is fine though the FC is a bit higher than my target.
FC: 9.5 (targeting 6-7)
pH: 7.3 (Dropped TA with MA addition in an attempt to slow the rise in pH. Aerating to get it in the 7.6-7.8 range)
TA: 60 (targeting 60-70. Had been 80)
CH: 500 (Seems to have creeped up from 400 after adding borates)
CYA: 80 (Had somehow dropped to 35-40 so I added 2 lb dry CYA)
Salt: 3000
Borates: ~45 (Added mid January 2021 to try to slow the pH rise. This seems to have helped but it still seems fast so I'm trying the lower TA as well.)
CSI: -0.58 (should fix itself as the pH comes back up)
Temp: 80 F
SWG: Currently set to 20%, 7 hours/day

I've attached a write up of my history to provide more details.
 

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cowboycasey

Gold Supporter
LifeTime Supporter
TFP Guide
Jul 3, 2013
5,926
Southern OK
You have a couple things going on... and you have caught both of them but missed the signs... As you get more of this down you will start to see...

1. In Florida you always get rain and therefore drain or overflow your water and your FC and CYA will go lower... Lower CYA is less protection for your chlorine and your cell can not keep up..

2. when your FC got low the algae started, you could not see it but once it starts you have to hit it hard with a SLAM till its dead, your cell will not keep up with an algae bloom..

3. I keep my FC above 7 at all times, I do not want to get to the minimum, I want to stay above the recommended... If you are testing once a week keep it above 7fc at all times.. if you want to keep it lower you have to test everyday because it can slip into the min category and you have a bloom... I travel most of the time so I know mine will be good, yes I use a little more chlorine/cell life but I am ok with that :)
 
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kellyfair

Gold Supporter
Bronze Supporter
Jun 29, 2016
4,248
Tampa, FL
Pool Size
7000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
I do exactly the same as Casey - I shoot for a point or two above target, and I don’t expect my SWG to do anything other than maintain. Gnarly weather? Throw in some chlorine. Sunscreen slathered swimmers all day? Throw in some chlorine.
 
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BennytheDog

Bronze Supporter
Nov 26, 2020
7
Sarasota, Florida
Pool Size
10000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-9)
Thanks for your responses. I think the FC target of 7 sounds like a good idea and also to stay away from the recommended minimums.

I'm guessing that I may have missed a heavy rain storm that caused dilution and/or a sudden increase in organics. Last night we had a fairly heavy rainstorm that added 2 inches or so to the pool. I'll check the chemistry in the am and see how it is doing.
 

mguzzy

Gold Supporter
I shoot for an FC of 9.. the extra elbow room is nice in case there is a sudden demand on my FC.. like my son(s) brings all his stinky teenage friends over for a swim.. or its all of a sudden over 100 deg for three days.. or or or... you get the point. It really doesn't take any extra Cl to maintain a higher FC only to get there. Your FC usage is the same either way whether your at 5 or 9.. in the heat of the summer most people lose 5-6 ppm per day. so maintaining a higher threshold will help.