Recommended to disable uv system?

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dan1333

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Nov 1, 2020
60
Orange, CA
Hi again everyone, my next question... I recently purchased a sundance hot tub (Edison) and it comes with the clearray UV system. From what I understand from reading these forums and this website, the UV is of little benefit and will essentially eat up a good bit of FC. Do you recommend just disabling the UV system? I was thinking of just taking out the UV bulb.

Thanks again
 
That depends upon whether you like soaking in bleach or not. ;)
As a sanitizer it is basically useless, since it only sanitizes a small amount of water at a time then returns it to a dirty tub. But as a chlorine neutralizer it does a fantastic job. So you can add high levels of chlorine after use to destroy pathogens, and have that high fc and cc burned off by the time you open your cover the next day. This is how it provides you with a "low chlorine" spa. It does not mean you need less chlorine, but that there is low chlorine in the water when you get in.
 
Just a note that adding 20-30 ppm of CYA buffers most of the chlorine and you are no longer soaking in bleach.
 
ajw22 said:
Just a note that adding 20-30 ppm of CYA buffers most of the chlorine and you are no longer soaking in bleach.
Click to expand...
I hear you chemical gurus say stuff like that, but I don't understand what you mean. If the chlorine is able to react with organic contaminants, which it must to sanitize the water, then it will react with organic contaminants. You are an organic contaminant, and the chlorine will react. I am no chemist by any means, but it seems pretty straightforward to me.
CYA, being a chemical added to mitigate chlorine burn off from sunlight (UV), would play a role in how effective your UV is at eliminating it. How much of a role I really couldn't say. But if asked, I would recommend not using it in a UV system, as it could be somewhat defeating the purpose (in my opinion) of the UV.
What does the manufacturer say?
 
RDspaguy said:
I hear you chemical gurus say stuff like that, but I don't understand what you mean. If the chlorine is able to react with organic contaminants, which it must to sanitize the water, then it will react with organic contaminants. You are an organic contaminant, and the chlorine will react. I am no chemist by any means, but it seems pretty straightforward to me.
Click to expand...

The point is that the chlorine that is bound to CYA doesn't react with organics. That's why we have to increase the FC level according to the FC/CYA Levels, keeping the FC-CYA ratio constant, so that the active chlorine level (HOCl) is high enough to kill organics. As soon as the HOCl starts doing its job and its concentration in the water gets down, more chlorine gets released from CYA (following the equilibrium equations) to form fresh HOCl that keeps the sanitation process going.

The chlorine that is bound to CYA (and that is most of it) doesn't react with organics - that's why a pool with sky high CYA and standard FC levels (not following the FC-CYA chart) turns green. But in the same way, this chlorine that's bound to CYA is also not aggressive to swimmers or pool equipment - you can safely add more chlorine following the FC-CYA chart until you have enough HOCl in the water to keep it clean and safe.

The extra chlorine that's bound to CYA is a kind of reservoir chlorine that is inactive while bound, but once HOCl gets used up, it gets released fairly quickly. Like that you can have enough chlorine in the water to eventually react with all organics or survive a day of high UV load, without all of the chlorine being "active" at the same time, i.e. the chlorine won't be harsh.

In fact, the chlorine bound to CYA gets released fast enough that it shows up as Free Chlorine in the FAS-DPD test (and other chlorine tests). As the titrating reagent "deactivates" the HOCl (and OCl-), more chlorine gets released from CYA, until all of it is released and showing as Free Chlorine - in that way the titrating reagent is a kind of "super contaminant".

That's the big misunderstanding - the "Reservoir Chlorine" shows as "Free Chlorine", but it's actually not "free". Unfortunately, there is no easy test to directly measure HOCl. The best workaround is to measure FC and CYA, and calculate HOCl based on the chemical equilibrium equations - that's basically the theoretical background of the FC-CYA chart.

Sorry for that big detour around the original question, but I thought it was important to make that point after RDspaguy brought it up.
 
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RDspaguy said:
I hear you chemical gurus say stuff like that, but I don't understand what you mean. If the chlorine is able to react with organic contaminants, which it must to sanitize the water, then it will react with organic contaminants. You are an organic contaminant, and the chlorine will react. I am no chemist by any means, but it seems pretty straightforward to me.
CYA, being a chemical added to mitigate chlorine burn off from sunlight (UV), would play a role in how effective your UV is at eliminating it. How much of a role I really couldn't say. But if asked, I would recommend not using it in a UV system, as it could be somewhat defeating the purpose (in my opinion) of the UV.
What does the manufacturer say?
Click to expand...

Let me grab Matt, @JoyfulNoise he has explained this before, and I’m not about to butcher his words so I’ll let him do it.
 
Bottom line - leave the UV bulb in place as long as it doesn’t run 24/7. I’m assuming it only comes on intermittently or has some on/off feature on the controls.

UV in a hot tub is useful as long as the bulb has good wattage and the quartz sleeve it is sit in is clean and clear. It is true that some UV will neutralize the chlorine but it also helps to oxidize organics much like ozone does. In fact, if I had my choice, I’d prefer to have UV over ozone. But just like ozone, power output matters and you don’t want it running all the time.
 
mgtfp said:
The point is that the chlorine that is bound to CYA doesn't react with organics. That's why we have to increase the FC level according to the FC/CYA Levels, keeping the FC-CYA ratio constant, so that the active chlorine level (HOCl) is high enough to kill organics. As soon as the HOCl starts doing its job and its concentration in the water gets down, more chlorine gets released from CYA (following the equilibrium equations) to form fresh HOCl that keeps the sanitation process going.

The chlorine that is bound to CYA (and that is most of it) doesn't react with organics - that's why a pool with sky high CYA and standard FC levels (not following the FC-CYA chart) turns green. But in the same way, this chlorine that's bound to CYA is also not aggressive to swimmers or pool equipment - you can safely add more chlorine following the FC-CYA chart until you have enough HOCl in the water to keep it clean and safe.

The extra chlorine that's bound to CYA is a kind of reservoir chlorine that is inactive while bound, but once HOCl gets used up, it gets released fairly quickly. Like that you can have enough chlorine in the water to eventually react with all organics or survive a day of high UV load, without all of the chlorine being "active" at the same time, i.e. the chlorine won't be harsh.

In fact, the chlorine bound to CYA gets released fast enough that it shows up as Free Chlorine in the FAS-DPD test (and other chlorine tests). As the titrating reagent "deactivates" the HOCl (and OCl-), more chlorine gets released from CYA, until all of it is released and showing as Free Chlorine - in that way the titrating reagent is a kind of "super contaminant".

That's the big misunderstanding - the "Reservoir Chlorine" shows as "Free Chlorine", but it's actually not "free". Unfortunately, there is no easy test to directly measure HOCl. The best workaround is to measure FC and CYA, and calculate HOCl based on the chemical equilibrium equations - that's basically the theoretical background of the FC-CYA chart.

Sorry for that big detour around the original question, but I thought it was important to make that point after RDspaguy brought it up.
Click to expand...
Thanks. That helps with (some of) my confusion. Nothing can help with all of it, I'm afraid. ;)
 
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