Combined Chlorine Issues

[eagle12]

Here's what our spa dealer prescribed for our Hotsprings Envoy spa with Ozone;

Nature2

MPS 2Tbs once every day

dichlor 2Tbs once per week

We average two people every other day for about an hour in the spa

Our water is balanced, clear, but has an odor

Using a test strip mps is always high

I bought a Taylor K2006 kit and a K-2042 deox reagent kit

Thinking that we were putting to much mps in the spa, I suspended adding anything for ~4 days...

Test strips now indicate low mps

The Taylor tests using the deox reagent indicate the following;

FC = .4ppm

CC = 7.4ppm

MPS= -2.4ppm

I think I understand the readings...FC and MPS are low (because I stopped adding to the water)

CC is where the odor is coming from...so, I've read enough to be dangerous...the breakpoint chlorination need is (7.4ppmCC * 10) 74ppm needed

The dichlor we use has available chlorine of 28%, so using a formula(see below) I took from my Taylor booklet I need to add 13.875oz of dichlor to get rid of the CC...?

Treatment Table Strength / Label Strength x Table Treatment Value = Adjusted Treatment Value

Treatment value is based on the table in the Taylor booklet, @ 35% available chlorine it takes .15oz to raise 1ppm chlorine

74 x .15 is 11.1oz @ the 35% strength

35 / 28 * 11.1 = 13.875oz that's 1.7 cups of 28% dichlor?

Did I do this right, sure seems like a lot to add?

Thank you

[gman]

eagle 12,

The smell is from the high CC as you already know. Part of your high CC problem may be that your cyanuric acid level (CYA) is getting too high. Another could be your filters need cleaning. If you've just been using dichlor for sanitation, then you should be checking your CYA levels, since dichlor also contains CYA along with chlorine. As you may know, CYA acts as a buffer to chlorine, which helps to keep the chlorine from dissipating too quickly. However, CYA does not dissipate like chlorine, so as the CYA gets higher, the buffering of CYA is greater. Unless you stop adding CYA, you will need to keep increasing the amount of chlorine to maintain effective sanitation. That is why many happy hot tub users follow the Dichlor/Bleach method recommended on this forum. Essentially, once your CYA level reaches about 20 - 30 ppm, dichlor is replaced by chlorine without CYA (ordinary regular chlorine bleach, ex: regular Chlorox). I would also recommend checking your filters and clean them if you haven't cleaned them in a while.

The way to get rid of the high CC is to add chlorine and leave your water exposed to the open air and sunlight. I would recommend adding regular bleach to increase the FC level (rather than dichlor just in case your CYA level is too high). Exposing your water to sunlight and air for a while will allow the chloromines formed from the CC to dissipate. If your tub is exposed to sunlight, though, you will need to add a little extra chlorine to compensate for the chlorine loss caused by chlorines extended exposure to sunlight. Let your hot tub run with the cover off. Once your tub runs through its standard cycle, leave the cover off and the water exposed to the sun for a while, so the chloromines can dissipate into the air. How long to leave it exposed (i.e., half hour or hour) will depend on how strong the odor and/or how large the tub. Re-check your FC and CC levels. Your nose will be a good indicator of any chloromines in the water.

gman

[Mikey_in_NY]

Here's what our spa dealer prescribed for our Hotsprings Envoy spa with Ozone;

Nature2

MPS 2Tbs once every day

dichlor 2Tbs once per week

I'm no expert but 2 TBSP of MPS every day seems very high to me - I use that dosage once per week as a non-chlorine shock. 2 TBSP of dichlor once a week is OK until you build up a sufficient CYA reserve (probably 3 to 4 weeks), then - like gman says - you should stop using dichlor and switch to a dose of Chlorox regular every day or two. Once you get the dosages correct your CC should be no more than 0.5 PPM more than your FC. After adding MPS wait a day or two before taking chlorine mneasurements as the MPS will give a false high chlorine reading.

[quantumchromodynamics]

I suspect test error. I recommend that you redo the test according to the instructions posted here.

The test can be somewhat confusing.

Either way, the 10X breakpoint formula is wrong and should not be used.

You should not be using dichlor once your cyanuric acid reaches 20 to 30 ppm.

What is your cyanuric acid level?

Right now, add enough regular, unscented 6 % bleach or liquid chlorine to reach 20 ppm.

You can use the Pool Calculator to calculate the correct amount of bleach or liquid chlorine to add.

I recommend that you read the following posts:

Nitro's Approach to Water Maintenance

Dichlor/bleach Method In A Nutshell

Here is the rule of thumb for adding oxidizer based on tub usage.

For every person-hour of soak time you need to add 3.5 teaspoons of dichlor or 5 fluid ounces of 6% unscented bleach or 7 teaspoons of 43% MPS. - chem geek

[chem geek]

MPS 2Tbs once every day

dichlor 2Tbs once per week

We average two people every other day for about an hour in the spa

So that's around one person-hour per day. Your MPS dosage is around 1 tablespoon (3 teaspoons) per day and Dichlor is 2/7 tablespoons or 6/7 teaspoons per day. So you are using enough oxidizer for 3/7 + (28%/55%)*(6/7)/3.5 = 0.55 person-hours per day. If you didn't have an ozonator, then this would not be enough oxidizer being added (either MPS or chlorine). If you were using more chlorine, then I could understand the CC being a buildup of chlorine combined with ammonia (known as monochloramine) or urea, but that is odd when you are using MPS. MPS will register as CC so perhaps you aren't using enough deox reagent. Note that your Dichlor is about half-strength.

How long have you had the water in this spa and how many gallons of water are in the spa? We can then calculate how much CYA has built up, but if your spa were 350 gallons, then with your half-strength Dichlor you've been adding around 6 ppm FC and 5.5 ppm CYA per week so probably not too much buildup.

However, you have an ozonator and that usually cuts down the oxidizer demand roughly in half (assuming the ozonator is actually working). I suspect that the ozonator is working properly and that you have a buildup of MPS and are simply not using enough de-ox reagent to properly measure it. On the other hand, you notice a smell so it's possible there is a buildup of contaminants, but that is very strange given the ozonator and amount of oxidizer you are using.

As others have noted, you could use the Dichlor-then-bleach method which tends to not have these problems -- you won't get false CC readings and you generally keep the water in good shape without smell. If you want to use the Nature2 with MPS approach as you have been doing, then if you redo the test and find that the CC is real, then I suspect that your ozonator is not working properly and you either need to get that fixed or need to use more oxidizer than you have been using.