Keeping FC at 20% of CYA: Yes or No?

Mcmitzi

New member
May 31, 2021
4
El Dorado County, CA
Pool Size
6600
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Vinyl
Chlorine
Liquid Chlorine
First and foremost thank you to every contributor on this site. I’ve mined the old threads here and my pool is transformed!

A piece of advice I see sometimes is “keep the chlorine at 20% of CYA” and I want to be sure that I am 1. Calculating that correctly and 2. That my pool qualifies for this advice.

If my CYA is 40 then according to this advice, my FC should be 8, correct? That seems reasonable and doable but is also just outside the value on the chart.

Which leads me to my next question, who should employ this advice? My pool has 3 people in it daily: 2 little kids and a construction worker husband who takes a dip in the pool before he even comes in from the truck after work and makes a second trip in with the kids later. Does that count as a high bather load? I feel like it’s not many people but the grime factor is high and my pool is smallish😬. Additionally, my pool is under a canopy of oaks which drop leaves, sticks, and acorns relentlessly. I’ve included a picture of the trees as well as a shot of half of what was in the skimmer this morning 🤦🏼‍♀. Is 20% of CYA for me?

Thanks in advance!!
 

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HermanTX

Gold Supporter
May 20, 2020
2,536
Katy TX
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Liquid Chlorine
In the summer, I have found that staying at the high end of the range is good. You can swim all the way up to SLAM level so you are well within that range. Have you tested your pool in the morning then in the evening to determine how much FC loss is occurring throughout the day. It could be 2-3ppm due to bather load and debris. So if you are at 8ppm then it is dropping to 5 sometime during the day. Not sure when you add your LC. I add in the morning then again in late afternoon. This way I try to have a consistent level of FC.
My only concern would be bleaching out the color of your AG pool. If you can spread out your addition of LC, you should be fine.
 
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Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
10,798
NY
I am a well known fan of running hot. My SWG constantly overproduced and by the time I got around to adjusting it, I was nearing 20%. I targeted 15% and almost always went over.

In your case, it’s less FC with a CYA about half of mine. The daily 2-4 FC loss will affect you more %wise than it did me. I would target your normal loss (at any point in the season) to still fall into range. That would buy you a second day if you started on the high side of in range.

So if your 40 CYA target range is 5-7 and you are currently losing 2ppm a day, I would add that to your 6 average target and shoot for 8. After one days loss you would be perfectly in range with up to 2 more to go before minimum.

*assuming you don’t mind ‘wasting’ the slightly higher FC burn off in exchange for the safety net.
 

Saturn94

Bronze Supporter
Mar 11, 2015
1,273
SE Virginia
Pool Size
20000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
First and foremost thank you to every contributor on this site. I’ve mined the old threads here and my pool is transformed!

A piece of advice I see sometimes is “keep the chlorine at 20% of CYA” and I want to be sure that I am 1. Calculating that correctly and 2. That my pool qualifies for this advice.

If my CYA is 40 then according to this advice, my FC should be 8, correct? That seems reasonable and doable but is also just outside the value on the chart.

Which leads me to my next question, who should employ this advice? My pool has 3 people in it daily: 2 little kids and a construction worker husband who takes a dip in the pool before he even comes in from the truck after work and makes a second trip in with the kids later. Does that count as a high bather load? I feel like it’s not many people but the grime factor is high and my pool is smallish😬. Additionally, my pool is under a canopy of oaks which drop leaves, sticks, and acorns relentlessly. I’ve included a picture of the trees as well as a shot of half of what was in the skimmer this morning 🤦🏼‍♀. Is 20% of CYA for me?

Thanks in advance!!

Given your situation that sounds like a reasonable plan. 👍
 
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Mcmitzi

New member
May 31, 2021
4
El Dorado County, CA
Pool Size
6600
Surface
Vinyl
Chlorine
Liquid Chlorine
In the summer, I have found that staying at the high end of the range is good. You can swim all the way up to SLAM level so you are well within that range. Have you tested your pool in the morning then in the evening to determine how much FC loss is occurring throughout the day. It could be 2-3ppm due to bather load and debris. So if you are at 8ppm then it is dropping to 5 sometime during the day. Not sure when you add your LC. I add in the morning then again in late afternoon. This way I try to have a consistent level of FC.
My only concern would be bleaching out the color of your AG pool. If you can spread out your addition of LC, you should be fine.
I test and add in the morning and few times a week also test at night. I need to look back over my logs to get an average loss rate.
Thanks for replying!
 

Mcmitzi

New member
May 31, 2021
4
El Dorado County, CA
Pool Size
6600
Surface
Vinyl
Chlorine
Liquid Chlorine
I am a well known fan of running hot. My SWG constantly overproduced and by the time I got around to adjusting it, I was nearing 20%. I targeted 15% and almost always went over.

In your case, it’s less FC with a CYA about half of mine. The daily 2-4 FC loss will affect you more %wise than it did me. I would target your normal loss (at any point in the season) to still fall into range. That would buy you a second day if you started on the high side of in range.

So if your 40 CYA target range is 5-7 and you are currently losing 2ppm a day, I would add that to your 6 average target and shoot for 8. After one days loss you would be perfectly in range with up to 2 more to go before minimum.

*assuming you don’t mind ‘wasting’ the slightly higher FC burn off in exchange for the safety net.
I love the idea of a buffer. It’s not a waste if it saves me from dipping too low. Thanks so much for the reply and calculations!
 
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