How To Get Fc Right With High Cya?

[simonc]

The Pool Calculator indicates that I need a MINIMUM of 9 ppm of FC in my hot tub given a CYA of 120ppm. It suggests a range of 9-15ppm. The detail on the Pool Calculator webpage says:

"The target FC level is a minimum. You always want the FC level to be at least as high as the target level."

Is the Pool Calculator correct on this point? (The information specifically references "pools", not hot tubs, but I assume for sanitation purposes, the numbers should be similar.)

My concern is two fold:

1. Generally, I don't want to use the tub if the FC is much higher than 5ppm.

2. Since my FC levels are normally between 2 and 7 ppm, I'm concerned that I'm not putting in enough chlorine (6% bleach) to keep the tub properly sanitized.

If I change the value of CYA to 30 ppm in the Pool Calculator, the minimum suggested is 2ppm with a range of 2-6ppm. I understand that CYA reduces the effect of chlorine. I use the diclor-then-bleach method. The reason my CYA is so high (about 120) is that I switched to bleach too late and when on vacation, I use a triclor floater in my tub.

My other numbers (pH, TA, etc.) are good. The water is clear, no foam, and smells and feels OK. My chlorine demand is about 50%. Yes, I know I can drain the tub and start over again, but I'm trying to conserve water (drought in California).

It seems to me that most hot tub owners must have a really high CYA, because most use only diclor (or triclor with a floater) ... that's what the dealers recommend ... or some expensive combination of chemicals/equipment. I never heard of CYA until I joined this forum. (Yes, I'm glad I joined!) Anyway ... do I really need to bring my FC level to 9ppm as a minimum. If so, thousands of people, including me, are using too little FC in their tubs.

Thanks,

- Simon

[Wannago]

I'm far from an expert, but the first thing that jumped out at me was that your CYA is waaaay too high.

You are lucky that you found this forum, 'cuz now you know what many others don't. You are right, that many others are probably in the same boat as you and don't know it. Thankfully you can avoid any potential issues given the information available here.

Hate to say it, but if I were you, I would drain and replace the water in your tub, then follow Nitro's approach. Drought be damned - I bet Arnie drains his tub /pool!

I wouldn't want to use a tub that is at 9 ppm but I guess that's up to you. 9 ppm would be too strong for my liking

[Dnepr Dave]

+1, Drain and replace!

Dave

[quantumchromodynamics]

Richard is the one who really understands this best. He is the only one that I think could explain this correctly. My information is somewhat of an estimate based on his posts.

I think that a chlorine level of 9 ppm with a cyanuric acid level of 120 should be about equal to a chlorine level of 0.315 ppm with no cyanuric acid in the water.

This is based on the fact that most of the chlorine forms a bond with cyanuric acid. Only about 3.5 % is free in the water at any time. (pH = 7.5, cyanuric acid = 120 and temp = 104 F)

If the water has a cyanuric acid level of 30 ppm, then the active, free chlorine would be about 14 % of the measured chlorine. Therefore, a chlorine level of 2.25 ppm with a cyanuric acid of 30 ppm would be equal to 0.315 ppm of chlorine with no cyanuric acid.

The chlorine that is bound to the cyanuric acid is continuously released as the active, free chlorine is used up.

9 ppm chlorine at 120 ppm cyanuric acid = 2.25 ppm chlorine at 30 ppm cyanuric acid = 0.315 ppm chlorine with 0 cyanuric acid.

It would be best for you to drain and refill to lower the cyanuric acid level.

[chem geek]

QCD is basically correct. The actual numbers are (at 104F) that 9 ppm FC with 120 ppm CYA has the same active chlorine (hypochlorous acid) concentration at pH 7.5 as 2.6 ppm FC with 30 ppm CYA or 0.4 ppm FC with no CYA. So you have several choices. You can operate your spa at a higher FC level, but that would probably mean soaking with more chlorine and you may not want to do that. Another option is to ignore the higher CYA and do what most Dichlor-only users do and just take the risk of having around 1/5th of the sanitation level recommended with the Dichlor-then-bleach method. The option for most consistent sanitation while minimizing chlorine exposure during soaking is to drain/refill, start with Dichlor, then switch to bleach when the CYA is around 30 ppm.

Basically, your spa is roughly where Dichlor-only users are at after a little over a month of use if they add around 4 ppm FC per day.

[hrustar]

I'm in a very similar boat.

Here's what I've done/doing.

I cant switch water due to weather. So I've switched to 6% bleach so i don;t keep adding CYA. I've also just got a Taylor drop kit and found my TA is very high. Therefore I'm working on lowering TA done to 60 or so. Then I'll add borates (using poolcalculator specs) to stabalize my ph around 7.2.

Then I'll ride it out until I get a day of above freezing weather.

[NewB@Spa]

I'm in a very similar boat.

Here's what I've done/doing.

I cant switch water due to weather. So I've switched to 6% bleach so i don;t keep adding CYA. I've also just got a Taylor drop kit and found my TA is very high. Therefore I'm working on lowering TA done to 60 or so. Then I'll add borates (using poolcalculator specs) to stabalize my ph around 7.2.

Then I'll ride it out until I get a day of above freezing weather.

I am in a similar "boat" - My tub (brand new) and started with Dichlor (at dealers recommendation), now (they say because of my MSP once a week) that my CYA (of 60) isn't accurate so I am not sure where I am. I know draining (now in the 20s/30s) is a remote option. I keep reading (damned strips) a 0 (or very near) FC. Anything I can do?

[hrustar]

Assuming you've been using Dichlor for more than two weeks, you can probably switch over to Clorox Regular 6% bleach.

Like you, I had troubles with my FC count. Over the last few days, I've switched to bleach with great results. My tub is 425 gallons. A few days ago, I added 6 oz. to get my FC from about 1.5 to 6. and since I've found adding about 3 oz. a day is good maintenance, plus adding 2 oz. or so after a soak.

But I'm still early on so this mostly like will be tweaked some more.

[NewB@Spa]

I added ~ 6oz this morning and will test this afternoon. Going to stop by the pool store and pickup a drop kit (for now). I will test and then get in the habit of throwing in 2 oz after soak.

[mcw53]

Chem geek's rule-of-thumb (add 5 ounces 6% bleach per 1 person-hour soak) works pretty good for me. The wife and I typically soak 20 minutes every night. That's a total of 40 minutes, so I add 3 or 4 ounces of bleach. On the rare occasion that we miss a night, I'll add 1 or 2 ounces. Basically, my goal is to soak in water with 1 ppm FC.

[simonc]

QCD is basically correct. The actual numbers are (at 104F) that 9 ppm FC with 120 ppm CYA has the same active chlorine (hypochlorous acid) concentration at pH 7.5 as 2.6 ppm FC with 30 ppm CYA or 0.4 ppm FC with no CYA. So you have several choices. You can operate your spa at a higher FC level, but that would probably mean soaking with more chlorine and you may not want to do that. Another option is to ignore the higher CYA and do what most Dichlor-only users do and just take the risk of having around 1/5th of the sanitation level recommended with the Dichlor-then-bleach method. The option for most consistent sanitation while minimizing chlorine exposure during soaking is to drain/refill, start with Dichlor, then switch to bleach when the CYA is around 30 ppm.

Basically, your spa is roughly where Dichlor-only users are at after a little over a month of use if they add around 4 ppm FC per day.

Chem Geek & QCD -

Sorry, it's taken me a couple of days to thank you (and others) who responded to my question.

This is what I've taken away from the information you provided. Please correct me if I'm wrong.

1. FC is not the same as available chlorine. Just because my test kit registers FC 5ppm, doesn't mean that I have 5ppm available for sanitation. The CYA (in combination with other factors like, temp, pH, etc.) limits the availability of chlorine for sanitation purposes. (Just some food for thought ... Why don't test kits measure the available chlorine? Is it too hard ... impossible ... to do? It seems that the FC number can be misleading. I think I have plenty of chlorine for sanitation, but I really don't. Hmmm ...)

2. I got the impression that it is "safe" to bathe in water that has an FC of 9ppm, if the the CYA level is high >120. I'm not so sure about this safety issue. Even though FC is 9, the available chlorine (that effects my body) is really much lower depending on the CYA (and other factors), therefore the chlorine is not too strong for bathing.

3. If I keep my FC level at about 5ppm (with CYA >120), I am taking the risk of under sanitizing my tub. This one is a little scary, but this has been the case for 4 years for me, before I joined this forum. Before switching to bleach, I used diclor and/or a triclor floater exclusively. My CYA must have been sky high! I used test strips and tried to keep the FC in the 2-5 range. Looking at some of the other posts on this thread with people saying that they are "in the same boat" with me, under sanitation must be very very common among hot tub owners.

Given all this information, I will probably refill my tub sooner than later. I don't like the idea of bathing in 9ppm FC (as a MINIMUM). Maybe it's "safe", but it still makes me uncomfortable. Also, I don't like the idea of under sanitizing with "only" 5ppm FC, but I've been doing this for years with no bad results.

- Simon

[quantumchromodynamics]

If the tub is not heavily used, then the chlorine demand should be small. If you use 5 ppm with 120 ppm of cyanuric acid, you will probably be OK, especially if you are very consistent with maintaining the level at 5 and not allowing it to drop below 4.

Adding an oxidizer based on the usage of the tub will also help insure good sanitation.

The rough rule of thumb (without an ozonator) is that every person-hour of soaking in a hot (100-104F) spa needs around 5 fluid ounces of 6% bleach or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (MPS) to oxidize the bather waste. - Richard (chem geek)

Periodic shocking to about 9 or 10 ppm will help make sure that you maintain good water quality. You can shock when you know that you won't be using the tub for at least 12 hours to allow the chlorine to drop back down to 5 before you use it.

I think that if your chlorine demand is low, you don't get any combined chlorine and your water looks and smells good, then you're probably maintaining adequate sanitizer. I think that you are taking a small risk by not maintaining a chlorine level of at least 7.5 % of the cyanuric acid level, but how much is unclear.

The equations that describe the relative concentrations of the different chemical species are fairly complex and difficult to understand. Richard is the one who understands the chemistry best.

While I have a lot of confidence in his analysis, I do not understand the chemistry well enough to be 100 % sure about all of the possible effects of high chlorine and high cyanuric acid.

Part of the problem is that many people in the pool and spa industry and in the government agencies responsible for determining rules and guidelines have not given this the proper attention that it deserves.

I think that the recommendation to limit the cyanuric acid to about 30 ppm indicates a conservative approach by many board advisors. The best thing to do is lower the cyanuric acid level.

CHAPTER 14

EQUILIBRIA IN AQUEOUS SOLUTIONS OF CHLORINATED ISOCYANURATE

J. E. O'Brien, J. C. Morris, and J. N. Butler*

Although chlorinated cyanurates serve as a reservoir of free chlorine, bactericidal efficacy is more closely related to the relatively small fraction of free chlorine present at equilibrium.

Copyright ~ 1974 by Ann Arbor Science Publishers, Inc.

[chem geek]

Most Dichlor-only users don't get hot tub infections, but they do tend to change the water after around 3 months because the water becomes dull or cloudy or harder to maintain (as opposed to Dichlor-then-bleach users who mostly are able to go twice as long before changing the water and even then the water tends to be cleaner at that time). Some Dichlor-only (and other sanitation system) users do have problems as reported mostly on this forum and that I compiled a while ago in this this post. The main reason for problems was too low or zero sanitizer level, but there also seemed to be a small trend towards Dichlor-only users after 1-2 months having more likelihood of a problem.

This in no way proves a correlation, but the science behind chlorine and CYA as well as the numerous studies (many listed in this post) show a reduction in sanitation as well as oxidation that is significant such that it is mostly the small remaining hypochlorous acid that does the bulk of sanitation and oxidation. As for soaking in high FC levels with high CYA levels, the bulk of the chlorine is attached to CYA and as shown in a link in the post I referred to, there is minimal to no skin absorption of the chlorinated cyanurate compounds. They are not volatile either -- only the hypochlorous acid (and even smaller amount of chlorine gas) are volatile, along with disinfection by-products. So the main issue would be drinking the water and that's not something you are apt to do.

As QCD said, just having the FC at 5 ppm should be fine and is far better than having just 1 ppm given your CYA level.

[simonc]

Thanks to all who responded to my questions.

For now, I'll try to keep FC in the 3-6 ppm range. Also, I'll probably change my water sooner than later to reduce my CYA levels and to "start over" again. This will allow me to keep FCs at a lower level in the future.

- Simon