CYA/FC Offsets?

[H2O_Keeper]

This is my first season doing BBB and I am going through waaay more clorox than I thought. I am averaging at least 2-3 points of FC a day trying to hold a 5FC based off my CYA level (30 with a 3-5FC range).

I realize the CYA/FC numbers are based off experience, theory, and probably the worst case scenario should you be trying to maintain your pool in a texas or tampa area.

I live in the midwest and my pool temp rarely reaches 90 neither does the outside temps. I also have a peristaltic setup so I get nightly doses of chlorine to keep things level. Is there any guesstimates with someone that can hold a low pretty precisely and not have intense sun what the potential CYA/FC multiplier could be?

I was wondering for my 30 CYA if I couldnt get away with a night time (before adding chlorine) level of 1. I tried this for a one week period and no algae nor did the CC go above 0.5. Am I gambling asking for algae in this range with my temp and daily low levels?

 

[JasonLion]

Am I gambling asking for algae in this range with my temp and daily low levels?

Yes. You won't get algae right away, but over the course of an entire season you will get algae eventually if you let FC fall to 1 every day.

If you are concerned about reducing your chlorine usage you should try raising CYA up to around 50.

 

[CurtisT]

I guess your other option (based on what I've read here) if you want to maintain slightly lower chlorine levels at times is to back up the chlorine with regular doses of a Ployquat algaecide.

 

[chem geek]

It also depends on your pool's natural nutrient (phosphate, nitrate) level, the amount of sun exposure, the water temperature and other factors. The Best Guess CYA chart was originally created by Ben Powell with levels from his experience servicing hundreds of commercial pools and then having feedback on The PoolForum from thousands of residential pool owners. It is a set of levels for manually dosed pools that works to prevent algae in nearly every case so is somewhat conservative. In my own pool with 2000-3000 ppb phosphates and 86-90F water temperature, it's not that conservative -- if my pool got to 2 ppm FC with 30 ppm CYA for too long, chlorine usage would shoot up.

You are, of course, free to experiment with your own pool. You may be able to get by with a lower FC, but keep in mind that the first sign of algae growth will likely not be visible but will instead be a higher-than-normal chlorine demand (even overnight). Eventually, the water will turn dull, then cloudy, then green (depending on the type of algae -- sometimes it goes straight to green on pool surfaces, often slimy first). As Jason noted, a higher CYA level will result in lower chlorine usage in spite of a higher required FC level -- it's another irony we discovered, possibly due to CYA's non-linear "shielding" effect protecting lower depths from sunlight. In very sun-intense areas, some people run their pools with 60-80 ppm CYA and find that it uses a lot less chlorine in spite of minimum FC levels in the 4.5 - 6.0 ppm range. This post from Mark describes an experiment he did that showed this effect. At 45 ppm CYA, the chlorine dropped by 50% from 5 to 2.5 ppm for a 2.5 ppm drop while at 80 ppm CYA, the chlorine dropped by only around 15% so even doubling the FC would still result in a lower absolute FC loss per day of 1.5 vs. 2.5.

As Curtis noted, if you want to maintain a lower FC level at any CYA level, then there are supplemental algaecide products at extra cost, such as PolyQuat 60. It is possible that the use of 50 ppm Borates might let one have a lower FC/CYA ratio, but it's too early to tell. It certainly seems to be good insurance and slows down algae growth even when the FC level gets to zero -- we just don't know how much it helps in terms of the maintenance FC/CYA level.

Another option is to use a pool cover. That will significantly cut down chlorine usage and also saves on water, reduces the rate of TA and CH rise from fill water and stabilizes pH from carbon dioxide outgassing since it virtually eliminates evaporation and outgassing.

Personally, I'd try the higher CYA level first. You can test this in a basin of water if you don't want to try it in the pool, though the effect in a bucket or basin of water may be less noticeable due to the shallower depth (Mark still noted the effect in a later bucket test, however). It's also hard to get good stable readings in such a small volume of water.

Richard

 

[astrolite]

I find this thread very interesting! Chem Geek, I hope you'll respond to this. Last year was my first year with the BBB method my CYA was around 60 ppm, I was maintaining my FC between 5 & 9 ppm and had fairly low chlorine loss in a 24 hr period, maybe 2 to 3 ppm. Thinking lower CYA was better concerning the FC level I would have to maintain I did a partial drain in the fall and refill in the spring to lower the CYA.

This season my CYA is down to around 45 ppm and the recommended level for FC is 3 to 8 ppm. Great I thought, I've accomplished what I wanted! However, now I find that my 24 hr chlorine usage has skyrocketed to 5.5 ppm! So I'm using way more chlorine this year than last even though I don't need as high a level!

I've done an overnight chlorine loss test to be sure I don't have organics eating the chlorine and only lost 0.5 ppm. I guess I just need a slightly higher CYA level than the average person to cut my FC demand.

I'm in Central PA, my pool gets moderate sun and my water temperature is only 80 deg. Should I be looking into my pool's "natural nutrient levels" and be making some adjustments there? BTW I should add, my pool has been great ever since starting the BBB method and is crystal clear right now.

Thanks

 

[JasonLion]

astrolite, Higher CYA levels always result in lower total chlorine used.

Your results with a CYA of 60 are about what I would expect. With a CYA of 45 I would expect you to use more chlorine, but not quite that much more.

 

[H2O_Keeper]

In my own pool with 2000-3000 ppb phosphates and 86-90F water temperature, it's not that conservative -- if my pool got to 2 ppm FC with 30 ppm CYA for too long, chlorine usage would shoot up.


Richard

I am still kinda new on good data I was using test strips last summer. So you are saying it may be cheaper/better trying to hold a slightly higer level everyday. So hypothetically say I move it from 1 to 4 every night now. But if I was try to maintain a 3 everynight I might be able to just move it to 5 with the daily dose. The net usage would be less (2FC per day instead of 3FC???). Haven't tried to hold it higher yet to see if this is true, looking for some insight. I will definitely try to bump the stabilizer a little but was just wondering about my interpretation of the comment.

Richard, awesome link. I never thought of opening that one because of the SWG being in the title (I dont have one). Good stuff. Thanks!

 

[chem geek]

As to whether your chlorine demand will go down if you somewhat raise the FC level, the answer depends on the source of the chlorine demand. If you've got some nascent algae growth "on the edge" of being beaten back by chlorine, then raising the FC level a little higher can result in a lower chlorine demand. Normally the way an unusual chlorine demand (which shows up overnight as well) is handled is to shock the pool to thoroughly kill the algae and then to maintain the recommended FC/CYA levels.