Chlorine disappearing too quickly

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63deejay

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Jun 30, 2012
11
Hi,
We have had our hot tub for about 2 months. I was using dichlor only until about 2 weeks ago I refilled and switched to using dichlor/bleach. The bleach I am using is 14% liquid chlorine. I used dichlor for a few days until I had added 22ppm CYA. My problem is if I take the FC level up to 12ppm at in the evening, by the following morning it has dropped down to around 5ppm and that is without using the hot tub. I didnt seem to have this problem with dichlor.

My thoughts are
a) this is normal - so I just live with it
b) I need to raise my CYA level
c) There is something in the water using up the chlorine so I just need to continue taking it above 10 ppm each night and eventually the usage will reduce (or I will run out of chlorine)

My other reading are

Ph : 7.4
TA : 60
CH: 200

Any help would be apprecicated

Thanks
 
If you are loosing FC overnight then there are organics in the water and you need to shock it. I am curious why your are using CYA in your hot tube. Normally a hot tub will have a cover to keep the heat in so as not to waste electricity. If you have a cover on it you do not need CYA since there is no sunlight to get to the water. Also, bleach is not the best way to treat a hot tub because bleach/chlorine is unstable at higher temperatures. The recommended method is bromine.
 
Carlos, you are forgetting that CYA is not just used to protect chlorine from breakdown from the UV in sunlight, but is also used to moderate chlorine's strength. If you do not use any CYA in the water, so just start out using bleach or chlorinating liquid, the chlorine will be too strong and will oxidize swimsuits, skin and hair too quickly, produce disinfection by-products faster, corrode equipment faster, outgas chlorine faster and degrade hot tub covers faster. It is also not true that bleach/chlorine are unstable at hot tub temps (it is concentrated chlorine that is less stable at higher temps, but diluted chlorine is not -- remember that chlorinating liquid is about 4 times less stable than bleach that is half the strength -- the concentration of bleach is around 61,700 ppm while a spa is on the order of 10 ppm or less). They do outgas some, but many people use it with no problem -- in fact, Dichlor is the most commonly used spa sanitation method.

63deejay, I believe you just need to raise your CYA some more to around 40 ppm (especially if you are raising the FC to 12 ppm). Also note that CYA slowly degrades in hot tubs at around 5 ppm per month so every month you'll need to use Dichlor for about a day or so once a month. I suspect that with Dichlor you would have found the same issues in the first days, but that after some time (even a week) the CYA had built up enough for the rate of chlorine loss to be slower. On the other hand, most people using Dichlor-then-bleach with 30-40 ppm CYA find they only lose around 25% of the FC after a fresh refill if there is no ozonator (do you have an ozonator? If so, then the loss can be 50% or more since ozone reacts with chlorine). You are losing more than that so Carlos may be right that you have something else consuming chlorine in your spa (unless you have an ozonator as I just described). You can shock again to see if the rate of loss is consistent or slowing down -- if slowing down, then you are getting rid of whatever is in the water consuming chlorine.
 
Hi
Thanks for the reply.
I will try adding dichlor to raise CYA level and also shock to see if chlorine demand reduces.

I don't have an ozonator so hopefully the combination of the above will solve my problem.
The hot tub gets used by 3 or 4 people every night for around 40-50 minutes, so previously my problem when using just dichlor was that the CYA level rose to around 100 in just over a couple of weeks.

Thanks again for your replies.
 
Sure sounds like your chlorine is being used up by something in the water, plumbing, or filters. You also have a pretty good bather load there. How many gallons? Are you regularly using enough oxidizer after soaking to deal with average bather waste? Here is the general rule-of-thumb from this forum:
For each person-hour of soaking, you'll need (conservatively): 3-1/2 teaspoons of Dichlor, or 5 fluid ounces of 6% bleach, or 7 teaspoons of MPS non-chlorine shock.

For comparison, my 215 gallon Hot Spring Jetsetter loses about 27% FC daily. I am keeping the CYA level at about 30 ppm. I'm also finding the above rule-of-thumb for oxidizing bather waste is a bit much in my tub, as I take a shower before soaking and keep the temperature at 102 instead of 104 (less sweating).

How are you measuring your chemistry? It is very handy to know Free Chlorine and Combined Chlorine with precision and accuracy. I'm using the TF100 test kit.
 
So 3-1/2 people for 45 minutes is 2.6 person-hours (a rather high bather-load) so the rough rule-of-thumb would be 9 teaspoons of Dichlor or 13 fluid ounces of 6% bleach or 18 teaspoons (6 tablespoons) of non-chlorine shock (43% MPS) though the latter is usually only for those using Nature2 with MPS. As BravoRomeo notes, this is a rough rule-of-thumb and your requirements may differ. The key is to add whatever you need to so that you have 1-2 ppm FC at the start of your next soak (I'm assuming you don't want chlorine smell during your soak; if instead you want more disinfection for having strangers over, then start with a higher FC level so that you end the soak with at least some FC).

Also, if you've never decontaminated the spa, then I suggest (perhaps at your next drain/refill) that you use Spa System Flush to get rid of any biofilms that may exist in the piping and follow the super-chlorination procedure. That should hopefully get you to a more normal chlorine loss rate if for some reason you aren't able to get there with raising the CYA and shocking that you are doing now.
 
I am using the LaMotte test kit (I think it is called Pro pool 7) which uses the titration method for measurement. I have tried mesuring FC and CC but I have used MPS and I thought that confused the CC measurement.

I think I have probably not been using enough chlorine after soaks because I was concerned that the amount of chlorine I had to add was consistently raising the chlorine level too high in the hot tub ie adding around 12ppm chlorine.

I have followed the suggestion and raised the CYA to around 30 and shocked again last night and I will see how that goes. I will also increase the amount of chlorine I add after each soak - I have been adding around 2/3rds of the amount you suggest.

Thanks again for your replies.
 
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