Can't Maintain Chlorine Levels On New Tub With Fresh Balanced Water

[cin]

Hi all,

I just got a small (216 gallons, 818L) new hot tub, and after letting the chlorine go to 0 for a few days on the original fill (by accident), I ended up here reading all about proper water chemistry. So I drained the tub after shocking it to 10ppm a bunch of times (Chlorine was disappearing fast, pH was drifting up all the time, I had added too much Calcium Chloride and TA) and started over, following the instructions of Nitro's posts:

- Filled the tub with hot tap water (Vancouver BC, best tap water in the world, perfect ph)

- Tested it, used the Pool Calculator to estimate quantities

- Added 53ml of Calcium Chloride (water was at 50-80 ppm)

- Added 26ml of baking soda to up the TA (was at 60 ppm or so)

- Added 361ml of Borax

- Balanced the pH (it was WAY up, like off the chart at 10) with pH- dry acid and also increased the CH as it had gone back down to 50-80 ppm (I blame the Borax)

- Aerated the water for 15min

- Balanced the water again, to 7.4

- Added 7ml of Cal-Hypo chlorine at 47%

- Tested the water (using strips and a pH/Cl rainbow drop test)

At this point the chlorine was at about 9ppm and everything else was perfect.

Waited a bit then 3 clean bathers with swim suits got in for an hour and a half

Tested the water again and the chlorine had gone down to about 3ppm. So I added a few more ml, to bring it back to about 9ppm.

The morning after the chlorine had completely disappeared. CH and TA were also down again, so I topped that up a little. Perfect pH. So I added 7ml of chlorine granules and went to work.

When I got back the chlorine had disappeared. So I added some more.

We've been using the tub, the water is fantastically clear, but I am constantly adding chlorine as it gets eaten really quick. I'm adding chlorine twice a day at this point. The CH also seems to keep drifting down, which I wasn't expecting.

I read about Chlorine Demand and all but I'm not sure I understand why our tub is eating so much chlorine. The tub is about a week old, the filter is clean, the water is 2 days old.

Could it be because of the small volume of water combined with 3 avid bathers, and possibly the rain?

Or should I conclude there's something in there eating the chlorine, and I need to decontaminate the tub even though it's so new and clean? (something that may have settled in when I let the chlorine fall to 0 in the first few days?)

Any idea why the CH is drifting down?

Thanks!

(curses, my carriage returns are not showing proper)

[chem geek]

You can use "[ left ]" and "[ /left ]" without the spaces and quotes to have left-justification return though if you edit the post you have to add these back again.

The amount of chlorine that needs to be added to oxidize bather waste is independent of the spa size. If you do not have an ozonator, then 3 people for an hour and a half is 4.5 person-hours of soaking. The rough rule-of-thumb is that every person-hour soaking in a hot (104ºF) spa requires around 3-1/2 teaspoons of Dichlor or 3-1/2 fluid ounces of 8.25% bleach or 7 teaspoons of non-chlorine shock (43% MPS). That would be 16 teaspoons of Dichlor or 16 fluid ounces of 8.25% bleach or 32 teaspoons of MPS in your situation, BUT there's no way your spa was 104ºF for that long since you can't soak that hot for that long (it's not healthy) so your chlorine demand would be less.

You only add enough chlorine for 6 ppm FC in 216 gallons which would be about 2 teaspoons of Dichlor or 2 fluid ounces of 8.25% bleach. So you only added enough chlorine to handle 30-35 person-minutes of bather load. Even when you doubled this by adding more, that was still only 1 person-hour and you had 4.5 though equivalently to a hot spa perhaps 3 person-hours. Basically you are adding woefully too low amounts of chlorine for your bather load

[cin]

Hi Chem Geek,

Thanks for the help!

What you say makes a lot of sense, but if I put much more chlorine in at once the levels will go above 10ppm. Too corrosive for bathing.

As it is now, I test the water 3 times a day, and add chlorine as needed (twice a day at least). I add enough to bring it up to 9ppm or so in the morning and before bed, so that it's usable during the day, and that it doesn't go down to 0 at night. I was under the impression that I'd be able to stabilize the chem situation so that I only need to add chlorine once every 2 days or so. But what you are suggesting is that my bather load is just too heavy for that, and that with that load I will have to constantly add chlorine. Which is fine, as long as it "normal".

So my plan now, given your clarifications, is to continue tweaking the chlorine level to 9ppm in the morning (to make it usable during the day) but increase the amount I put in at night. It'll be above 10ppm at night, but then there should be some left in the morning, after it has oxidized the nightly bather load.

Does that sound right?

Thanks a bunch!!

[chem geek]

If you put in the needed amount of chlorine, then yes the FC level will be high but it will not last very long since it will oxidize bather waste. The idea is to add enough so that there will still be at least 1-2 ppm FC for the start of your next soak. So no, the chlorine will not be too much for bathing. You aren't bathing in the water after your soak.

How often are you using the spa? If you are using it once per day then just add a lot of chlorine after the soak and it should be low for the start of your next soak. In a hot spa, the chlorine usually oxidizes nearly all the bather waste within 24 hours. If you are soaking twice a day then that can be more difficult since the chlorine will have oxidized much but not all of the bather load in that time.

The Borax would not lower the CH level so not sure why that happened -- more likely to be measurement error as that test can be a bit tricky to read. If you have a fading endpoint, then add some titrant drops initially before the calcium buffer and indicator dye, then be sure to include those drops in the total count. And yes, Borax raises the pH a lot so for adding borates you add acid and borates separately and usually split the doses into one-fourth. It's easier to just use Boric Acid if you can get it (say at DudaDiesel or The Chemistry Store).

Because of the high bather load of 4.5 person-hours and the small water volume of 216 gallons, you won't go as long between needing a water change. If you were to use Dichlor-only then the standard Water Replacement Interval (WRI) would apply:

WRI = (1/9) x (Spa Size in Gallons) / (Person-Hours per Day) = (1/9) x 216 / 4.5 = 5.3 days

If using the Dichlor-then-bleach method so that you switch to using bleach after the CYA has risen to 30-40 ppm (so after you've cumulatively used 33-44 ppm FC from Dichlor) then you can go at least twice as long so 10.6 days, so perhaps two weeks, at least if you intend to have 3 people using the spa for 1-1/2 hours every day.