Combined Chlorine in Hot Tub

ChrisL

Well-known member
Apr 3, 2007
95
0
#1
I recently fired up a new hot tub. As with the pool, I am using bleach and muriatic acid to keep numbers in line. (No need to raise ph yet.) I am having trouble going to 0 with combined chlorie. It stays around 0.4 ppm after several shock doses of bleach. Is this normal or do I just have to keep the chlorine levels up until the cc goes down? As always, thanks for any input,
Chris
 

chem geek

LifeTime Supporter
TFP Expert
Mar 28, 2007
12,082
2
San Rafael, CA USA
#2
It's much harder to get rid of Combined Chlorine in a hot tub or an indoor pool since they are not usually exposed to sunlight which helps break them down. Also, a hot tub has a much higher bather load than a pool due to the small volume of water. That means that organics tend to overwhelm chlorine faster and more combined chlorine forms.

First of all, you should have at least some Cyanuric Acid (CYA) in the water or else you will be overdosing with chlorine. I suggest using Dichlor for about a week to get to a CYA level of around 20 ppm. After that you can use bleach by dosing to around 4 ppm Free Chlorine (FC). You may need to add some chlorine every day after you get out of the tub if you are using it regularly.

To keep the CCs lower and to lessen your chlorine usage, you can use non-chlorine shock, potassium monopersulfate (MPS), and can use a little each day after you get out of the tub -- probably around 1 teaspoon. Note, however, that MPS will register as Combined Chlorine in the chlorine test, but you won't create any true combined chlorine if you are using MPS since MPS will oxidize organics (and ammonia) before chlorine gets a chance to.

0.4 ppm CC is not bad so if you want to just use a regimen of shocking with chlorine (bleach) weekly instead of using MPS, then that's OK. It's really up to you. I recently used some MPS in my pool to get rid of an oil film that didn't go away with scum balls nor enzymes. I could have shocked with chlorine, but since my wife was using the pool every day I didn't want to have to wait for the chlorine level to drop so I used MPS. It worked great -- it's very effective, but not cheap, but for a hot tub you use less than you would for a pool (though it's not proportional).

On a separate note, if you find the pH to rise in your hot tub, then you can lower the TA level to reduce that problem. Lowering the TA to around 70 is usually enough though you could lower it even to 50 if necessary (assuming you have a fiberglass tub and not plaster nor any tile with exposed grout). Using bleach is pH neutral when accounting for chlorine usage, but the aeration in a hot tub drives a lot of carbon dioxide out of the water and that makes the pH rise. Keeping the TA lower reduces this effect since TA mostly measures the amount of carbonation in the water (pools and spas are intentionally over-carbonated to provide a pH buffer and, for plaster pools along with calcium hardness, for calcium carbonate saturation).

Richard
 

ChrisL

Well-known member
Apr 3, 2007
95
0
#3
Thanks for the help. I didn't think of the cya as helping keep the working level of chlorine down. One question, why is the use of mps shock for a hot tub not proportional to the use in a pool? How would I figure the amount, as I have a couple of lbs of mps pool shock around.
Chris
 

Sabot

LifeTime Supporter
Aug 3, 2007
343
0
Austin, TX
#4
Chris,

I have a small hot tub which we have been using now for around a month or so. Even tho it came with a Nature2 stick and a Ozonator I am using the BBB this year until I get into trouble. (I will not replace the Nature2) Since the new toy is becoming a little old, we are not using it daily. This means my chems stay a bit more calm. When we use it for a few days in a row, the numbers jump around a bit. The PH moves a bit more when we use all the jets as well. I find it really hard to get my CC below .5 for I keep it covered all the time unless it's in use.

I found that I can lower my CC when I turn off the heater and open the cover for the day. I also do this when I shock the spa so the Chlorine burns off, otherwise to get the chlorine number down would take a week or so.

I found that taking my measurements after the spa has been at rest (the next day) results in the best numbers. Now if I am adjusting some of the chems, I will test an hour or so after I made the adjustment.
 

chem geek

LifeTime Supporter
TFP Expert
Mar 28, 2007
12,082
2
San Rafael, CA USA
#5
ChrisL said:
One question, why is the use of mps shock for a hot tub not proportional to the use in a pool? How would I figure the amount, as I have a couple of lbs of mps pool shock around.
Chris
It's not proportional because MPS isn't killing bugs, so it's not a rate-based concentration, but rather is oxidizing organics. So the amount of MPS needed is more a function of the total amount of organics that needs to be oxidized and this isn't directly related to pool volume. Yes, larger pools tend to accumulate more junk from leaves, pollen, etc. as well as bathers, but one person swimming in a pool is still the same one person in a hot tub and both release sweat with organics (and ammonia). I don't know who sweats more -- a person in the hotter water of a hot tub or a swimmer in a pool (clearly an idle bather in a pool sweats less).

Dupont recommends dosing with MPS at a rate of one pound per 10,000 gallons for pools one to two times per week which is roughly 12 ppm per dose (that's ppm Oxone -- the CC equivalent is about 2.5 ppm). For spas and hot tubs, the recommended rate is 1-2 ounces weight per 250 gallons after each use or at least once per week if not used which is roughly 30-60 ppm per dose (the CC equivalent is about 6 to 13 ppm).

I think their recommendation for the spas and hot tubs is on the high side of what is needed. Technically, you could just add MPS after each use and then measure the CC level the next time you get into the tub. If it keeps climbing over time, then you are using too much; if it drops close to zero, then you aren't using enough. Having around 0.5 to 1.0 ppm CC would be a residual of MPS that should handle immediate needs. Some people have a skin sensitivity to MPS so would have to dose more carefully or find other alternatives.

Again, MPS may be unnecessary or may only be needed on occasion. If you used it at the dose Dupont recommends, you'd be building up a lot of sulfates in the water. I don't know if that matters for a typical fiberglass spa.

Richard
 

ChrisL

Well-known member
Apr 3, 2007
95
0
#6
Thanks to both of you for the tips and information. Its very helpful on this new adventure in water care.