New hot tub questions - rash

Sep 1, 2021
13
Lincoln,CA
Pool Size
515
Surface
Vinyl
Chlorine
Liquid Chlorine
Hi Everyone,

Quick summary of my setup before my questions - 500gal Masterspa hot tub with Ozone (ozone runs with jets only, 2 hours 2x/day, plus whenever jets are run during a soak). Typical bather use is 2 adults, 3-4x per week for 30-60 minutes each time.

Initial guidance from the hot tub store was to use Dichlor to maintain 3-5ppm FC, and to shock with MPS once a week or after heavy usage. I didn't trust the accuracy/precision of the test strips that the store supplied, so I got a DPD test kit from Leslie's (I've since ordered the K-2006 kit, but it hasn't arrived yet). In general, I needed to use about 1T of Dichlor every other use. Hot tub store said the shock wouldn't hurt anything, so used about 1-2T after each use (more on this later).

I got a rash after about 2 weeks of use, but my wife didn't. We would both use the hot tub the same amount and shower right after. Hard to say how soon after using the hot tub I had a reaction since we were using it most nights since it's new. I took a week or so off the hot tub and it cleared up, then used it again and got another rash within a few hours of getting out (I had again showered right after). Wife had no problem again. Went to the doctor and they're not convinced it's due to bacteria (wife had no reaction 2x, and how fast it came on the second time), but gave me a course of antibiotics in case it is.

Spent some time reading up on hot tub chemistry and found my way here. I'm fully onboard with the TFP methods after reading the rest of the website. Now, onto my questions:
1) The DPD test kit I had been using measured TC, not FC. There wasn't any chlorine smell that I've heard you get with high CC, so I can't confirm that my FC was in the 3-5ppm range suggested. I've also read that the MPS can show up as chlorine on test kits. Does the DPD test register MPS as total chlorine (meaning I may have had very low FC)? I've seen that there's a separate chemical to test FC/TC with MPS in the water, but also read here that it's not so reliable (also doesn't seem to be in stock anywhere right now).
2) The Ozone - How effective is it really only running 4 hours per day? I've also read that they have fairly short lifetimes, so it could fail and I may not know. Should I just consider the ozone as a bonus and maintain chlorine level recommended without ozone?
3) I've also read a bit about allergies to MPS (here and elsewhere). Assuming that's the case, does the MPS that I've been adding go away, or do I need to drain it if I want to get it out of the water?
4) If I go with the TFP methods (dichlor and then bleach), should I ever use the MPS? Or just shock with chlorine as needed?
5) When is a shock truly needed, should I do it on a schedule, or when a test kit shows CC over some limit?

I'm sure I'll have more questions once my test kit arrives.

Thanks in advance!
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
33,620
Texas, San Antonio/Marion, South-Central Area
Pool Size
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Surface
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Chlorine
Salt Water Generator
SWG Type
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Welcome to TFP! :wave: I would first say that the MPS may not be helping you. It will certainly skew total chlorine test results since you currently cannot test the combined chlorine levels separately. In fact, since the DPD kit only tests for total chlorine, you may "think" you have adequate sanitation when in reality it the spikes CC level throwing you off (FC + CC = TC). Once you get your K-2006 test kit you'll be able to tell for sure because that kit has the FAS-DPD (powder & drops) which tests FC and CC separately. But I would use strictly dichlor for the initial fill increase followed by liquid chlorine thereafter.

In addition, did you ever do an Ahh-Some purge of that new tub? That's another very import task that clean out the lines of new systems.

The link below is a great resource if you haven't seen it already.

 
Last edited:
Sep 1, 2021
13
Lincoln,CA
Pool Size
515
Surface
Vinyl
Chlorine
Liquid Chlorine
Thanks for the help. Still waiting on my test kit, but a few more questions:

1) Given that MPS can throw off the chlorine reading on a test kit, is there really any benefit to using MPS? The guy at the hot tub store (I know...) said it's good to use the MPS since it lets you use less chlorine. Let the chlorine sanitize bacteria, and let the MPS oxidize bather wastes/oils/etc. But if that just results in me thinking I have chlorine when I don't it causes other issues.

2) When is a shock really needed? I've seen recommendations for shocking using MPS after every use, or once a week, or when combined chlorine gets above free chlorine.

3) If I'm following the TFP methods, would I ever use MPS, or just chlorine (dichlor then bleach).

4) When you shock to reduce CC, where does the CC go? Does it offgass into the air? Is it different if you shock with chlorine vs. MPS?
 
Sep 1, 2021
13
Lincoln,CA
Pool Size
515
Surface
Vinyl
Chlorine
Liquid Chlorine
Another question, but not really related to my specific problem.
If I decide to use MPS along with dichlor/bleach, how do I deal with the ‘false’ CC reading? I’ve read that the kit for testing with MPS isn’t the most reliable.
Let’s say a test shows some combined chlorine, but water doesn’t have any of the odors that CC cause, can I assume it’s just the MPS showing as CC?
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
33,620
Texas, San Antonio/Marion, South-Central Area
Pool Size
17888
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
If I decide to use MPS along with dichlor/bleach, how do I deal with the ‘false’ CC reading?
For those who mistakenly add MPS to their pool, it can take weeks for the excessive CC readings to resolve. But I'll tag @Donldson for confirmation on this or your last couple questions.
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
5,150
NW Ohio
If you're going to be using MPS on a more-than-occasional basis you'll want to get the Taylor k-2041 or k-2042 MPS Interference Remover kit. This is the R-0867 DeOx Reagent which is used with the DPD or FAS-DPD test (which if you only have a DPD test you should upgrade to FAS-DPD for more precise readings) to cancel out the MPS in the sample so you get a correct reading.

Don't assume a CC reading is just interference. A high CC level is an early indicator of a problem that you don't want to ignore.

And I'll chime in on the ozone question: consider it a bonus. It can help break things down but it doesn't provide sanitation so you still need to maintain an adequate chlorine level, which makes it redundant for the most part. That's on top of it potentially failing without any indication, as you mentioned. They aren't bad things to have in a hot tub, but can be more trouble than they're worth.

And welcome to TFP!
 
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Mdragger88

Bronze Supporter
Jun 1, 2018
3,140
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
Thanks for the help. Still waiting on my test kit, but a few more questions:

1) Given that MPS can throw off the chlorine reading on a test kit, is there really any benefit to using MPS? The guy at the hot tub store (I know...) said it's good to use the MPS since it lets you use less chlorine. Let the chlorine sanitize bacteria, and let the MPS oxidize bather wastes/oils/etc. But if that just results in me thinking I have chlorine when I don't it causes other issues.

2) When is a shock really needed? I've seen recommendations for shocking using MPS after every use, or once a week, or when combined chlorine gets above free chlorine.

3) If I'm following the TFP methods, would I ever use MPS, or just chlorine (dichlor then bleach).

4) When you shock to reduce CC, where does the CC go? Does it offgass into the air? Is it different if you shock with chlorine vs. MPS?
Welcome to tfp👋
To help answer some of your questions:
1. No real benefit in my opinion as it causes as many issues as it solves.
Mps is only an oxidizer, chlorine is both an oxidizer & sanitizer so no real need for the mps in my opinion.
2. Go to shock level for your cya FC/CYA Chart
with liquid chlorine if you have cc’s over 1ppm.
Do not wait until cc is higher than fc .
After heavy bather loads or maybe a bunch of kids in & out I tend to go to shock level instead of just replenishing what was lost. In any case you never want your tub to go to zero so dose accordingly before you get in so that doesn’t occur.
3. You can use mps but as I mentioned above why use 2 separate products when chlorine is capable of doing it all. I have never used mps for that reason along with the fact there’s really no way to test the level of it - I don’t put things in my tub or pool I can’t test for. There’s also no listing for it in PoolMath which tells me it’s superfluous & I don’t like flying blind.
4. someone with a more technical chemistry ⚛️ type answer may chime in on this but to keep it simple- it off gasses. This is why you should leave the cover off after you soak/dose for a bit so it has somewhere to go (this is also better for your cover).
This is likely the same “action” for any oxidizer
 
Sep 1, 2021
13
Lincoln,CA
Pool Size
515
Surface
Vinyl
Chlorine
Liquid Chlorine
Thanks everyone.

I have a K-2006 kit coming later today, and I ordered the MPS interference remover as well. Testing for MPS sounds like a bit of a PITA, so I'll probably just use it sparingly.
 
Sep 1, 2021
13
Lincoln,CA
Pool Size
515
Surface
Vinyl
Chlorine
Liquid Chlorine
Got my test kit. CYA was up over 100 so it’s time to drain it. I’ll be starting over with the TFP recommended methods. Going to get some bleach while it drains.
 
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Mdragger88

Bronze Supporter
Jun 1, 2018
3,140
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
Yup- dichlor can raise your cya in a hurry.
just follow the guide linked above & you’ll be golden.
Always follow the FC/CYA Chart
Before draining an Ahhsome purge is recommended as biofilms can harbor some pretty nasty stuff 🤢 especially if you’ve never done it before.
 

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