Deliberately higher CYA to reduce maintenance frequency

[RouteVegetable]

Hey all, I couldn't find any answers to this question, though I feel like I have read about it before.

Background:

I use the Dichlor+bleach method to maintain my 450 gal hot tub, and so far things are all good. But a while back I overdid the Dichlor (I ran out of cal-hypo) and ended up with ~60ppm CYA. I've compensated by keeping my FC on the higher side - 5-7.5ppm, as I read that 60ppm of CYA will weaken chlorine 50% compared to with 30ppm of CYA. I maintain 50ppm borates, 60ppm TA and 350ppm CH.

My question is: is this high-CYA-so-double-the-FC-to-compensate a legitimate thing to do?

If so, that seems useful/convenient because it allows you to add more chlorine, but less frequently, and get effective sanitization for the same bather load, without overchlorinating your tub or bathers.

I want to know how far can one push this (eg: could I have 200ppm CYA with 30ppm of FC, jump right in the tub and be okay for a week?). What limits how far you can go?

 

Many thanks in advance, and thanks to all the helpful people on this forum who have got me this far in comprehending a surprisingly challenging craft!

 

Lee

[RDspaguy]

21 hours ago, RouteVegetable said:

so, that seems useful/convenient because it allows you to add more chlorine, but less frequently, and get effective sanitization for the same bather load, without overchlorinating your tub or bathers.

Missing the point. The cya binds up and renders useless a portion of your chlorine. It's like it's not even there, but you still read it on the test. So to get the same sanitation you need more chlorine, and once fc drops to a point you effectively have no chlorine, even though you still get a test reading. You gain nothing, and now have alot of guesswork regarding proper level to maintain.

Also, I do not recommend cal-hypo for spas. It's called the "dichlor/bleach method" because it uses dichlor and bleach, not cal-hypo.

[RouteVegetable]

I understand this; maybe I was unclear, I was talking about doing this deliberately. So to counter the non-useable chlorine, you would add more chlorine to get the same level of effective sanitization. This would ultimately let you "store" more chlorine in your tub, while not overchlorinating people or equipment, thanks to the chlorine blocking activity that you mention. Basically using the CYA as a chlorine buffer.

Indeed cal hypo isn't ideal, but I had the spare CH budget to spend on it, and for logistical reasons where I couldn't get hold of something better, I had been using it for a while. I only use bleach now.

[RDspaguy]

10 minutes ago, RouteVegetable said:

This would ultimately let you "store" more chlorine

It renders the chlorine useless. It is not "stored", as it cannot be later "unlocked" and used for sanitation. It is forever locked up and useless, while still affecting your test readings. You need a higher reading because part of your reading is false. And while there are methods for getting away with a high cya, there is no advantage to it. If there were, it would just be the way it's done. This "method" (chlorine and cya) has been in use well over a century, and if it could be improved upon, it would have been. 

Maybe @waterbearcan explain it better.

[RouteVegetable]

I'm no chemistry major, but I believe it's not correct, that the chlorine is locked up forever. I understood it was an fixed ratio/equilibrium. Cyanuric acid attaching to hypochlorous acid is a reversible reaction (see attached image).

 

I do understand that the Free Chlorine measurement includes that which is combined with the cyanuric acid, hence: With twice as much CYA, you need twice as much free chlorine, for the same level of sanitization. Or is that incorrect?

 

In any case, I am willing to accept that doing this is for some reason not useful. If so, I would like to understand why. My limited understanding of the chemistry suggests it would be useful. If it is not useful, my understanding is incorrect/incomplete, and I'd like to have the "why" explained to me.

 

[RDspaguy]

That's a question for our chemistry guru, @waterbear. I am a repair man, and no chemist either. I'm just telling it how I understand it and use it in the field. But I am certain, 100%, that if a high cya were somehow advantageous then the water parks and public pools that spend tens of thousands of dollars a year on chemicals would be doing it. I've worked at a few, and know they will do a half dump, and fire whoever caused it, if cya ever reaches 60ppm. That's alot of water in a million gallon wave pool and 600,000 gallon lazy river, and they have a good reason for doing it or they wouldn't do it.

[waterbear]

Running higher CYA necessitates the need for running higher FC levels. Period, end of story. It will not allow you to dose with chlorine less often. The CYA/Chlorine relationships is complex to say the least and the relationship is not linear. A simplified chart developed by Ben Powell of Pool Forum states that 30 to 50 ppm CYA requires a FC level of 3 to 6 ppm and a shock level of 15 PPM and a FC level of 60 to 90 PPM requires a FC level of 5 to 10 ppm and a shock level of 20 ppm.

This was later refined theoretically by Richard Falk aka Chemgeek, Chem Geek, chemgeek and a few other permutation (depending on which forum you find him on) where he broke it down by 10 ppm increments of CYA and can be found on the Troublefreepool forum. He theoretically determined that the desired (target) FC level is 11.5% of the CYA level (along with the percentages for minimum FC, shock, and shock levels for mustard algae).  He gives Ben Powell credit for originally developing the chart. Personally, I think Richard's chart is a bit of overkill in practical use and I tend to follow Ben's. FWIW, I was one of the original Moderators on the Troublefreepool forum when it first started and was/am a senior contributor on Pool Forum so  am very familiar with Ben's chart and its revisions by chemgeek. Much of the information about lower FC levels needed for a specific CYA level in salt pools came from me and observations that I had made in my  own and other pools (before I retired from the pool/spa industry).

The reason 20 to 30 ppm was initially picked for the dichlor/bleach method (developed by Nitro and Chemgeek) is becuse chlorine is aggressive when there is no CYA present and in the presence of a small amount of CYA it is essentially 'tamed" so it does not attach swimsuits and tub parts as readily.

As far as cal hypo goes, it is an unstabilized chlorine source so it can be used BUT for every 10 ppm of FC added by cal hypo you are adding 7 ppm calcium and this can lead to scale formation. I only really recommend the use of cal hypo in plaster pools or spas for this reason, along with maintaining a proper range on the calcium saturation index to help maintain the plaster surface. If you do not want to use bleach or liquid pool chlorine (both sodium hypochlorite) then the other good alternative is lithium hypochlorite, which is a fast dissolving powder). The main disadvantage is that lithium hypochlorite is the most expen$ive form of unstabilized chlorine.

In a nutshell, you want to maintain the proper FC for the level of CYA you currently have. You can successfully run a pool or spa with high CYA and a matching FC level but you still have to test and dose to make sure the FC level stays in proper range.

If you want to run a tub that requires less maintenance then I suggest 3 step bromine. The floater, once properly adjusted, will maintain the bromine level in range with less user intervention.