Making the switch to dichlor-then-bleach

[gourmatt]

I've been reading up on the dichlor-then-bleach method and would like to make the switch. Ever since buying the spa this past March I've been using dichlor as the chlorine source along with non-chlorine shock (weekly). My first water lasted two months before it needed to be changed. I'm now on month three after the water change, and it still looks good but I doubt I can get another month out of it if I keep using dichlor alone.

The current readings (using Taylor K-2006) are:
TA 80, CH 190, pH 7.2, and CYA 100

The spa is 310 gallons and it DOES have an ozonator.

I have not added any borates knowingly, nor do I have the means to test for them currently, but at the water change I added a product branded by the local spa store that they call "Silky Smooth". It's a very thick white liquid, almost like latex paint. There are no ingredients on the bottle but I'm wondering if it could be a borate-type product like Pro Team Gentle Spa since the product descriptions are similar. I will say that before adding the Silky Smooth the rotating jets were kind of "jerky" whereas after adding the product they spin freely. So it had some effect on the water.

My question is whether you think I can switch over to bleach with the water in it's current state, or whether I should do a water change first. I'd prefer to keep going on the existing water just so that I can get further into the fall before doing a water change which would then hopefully enable me to get through the winter without having to do one at the coldest time of the year. I live in Rhode Island.

Are there any adjustments to the chemistry recommended? My saturation index is a little low, but I'm thinking that the pH might come up simply by making the switch away from dichlor over to bleach. The CYA is obviously high, but not much that I can do about that without a water change.

Thanks in advance for any advice!

 

[scott.MI]

Yes, you can switch. Bleach and dichlor are both chlorine sources so your not really "switching" anything. Your just using an alternate source of chlorine, one that doesn't further increase your CYA levels. Chem Geek has regularly said most problems are with CYA level higher, like around 200.

 

[chem geek]

100 ppm CYA would come from 110 ppm FC so over 2 months that would be about 1.8 ppm FC per day. Since you have an ozonator, it will react with chlorine so I'm guessing that you don't have a high bather-load with this spa. How often do you use it (days per week) for how long (hours) with how many people? Do you find that you tend to add chlorine to get to 4 ppm FC but it drops to 2 ppm FC if you are not using the spa so does that mean you are adding chlorine every day?

The main issue with switching to bleach is that you will probably find the pH rising too quickly unless you get the TA lower to 50 ppm but you would then need an additional pH buffer so need the 50 ppm Borates which you can add from boric acid or from a combination of 20 Mule Team Borax and acid, separately added.

 

[gourmatt]

Our usage varies week to week, but you're right that it is not a terribly high bather load. Typically it is just my wife and me, maybe 2-3 times per week for an hour or so each time. Over the whole summer there have been maybe four times that we've had more than just the two of us in the spa.

I've probably erred on the side of having too little chlorine since our last fill, overall. When I first got the tub the spa store gave basic maintenance instructions but of course they don't tell you about the effects of CYA. So I was certainly overdosing which explains why my water lasted only 2 months. In fact, it was about that time that I started researching spa maintenance on the internet, bought the Taylor kit, measured a CYA of 200 and decided a water change was in order. So this time around I've been much more "measured" with my dichlor additions, probably averaging 1/4 to 1/2 oz after each soak, but typically not between soaks unless I took a test strip out and saw no FC. Somewhat to my surprise, even with these modest additions the FC would spike to over 10 then drop off very low within a day or two.

As you can tell, I haven't been as diligent as I should have been with measuring and logging FC. I want to be more diligent this time around, but work schedules and occasional work travel interrupt the best plans. Any advice?

I'm going to give the bleach a try over the weekend, carefully monitoring pH. I'll pick up some borax and acid as well, and go on the premise that the previously-added "Silky Smooth" was not a borate-adding chemical. I wish I knew what was in that stuff....

 

[chem geek]

Chlorine in a spa, especially with an ozonator, is more maintenance requiring daily attention or certainly at least every other day. That's the main reason some people use bromine since tabs can be used for more continual dosing and ozone can make bromine from a bromide bank. Bromine smells different and still requires some chlorine use every once and a while to keep the water clear.

I think you will be able to get a handle on your chlorine demand both from bather load and in between soaks so you'll be able to figure out how much to dose, but I don't know what you could do if you think you won't be able to dose when needed, especially in between soaks. If you knew there were periods when you weren't going to be using the spa, it would be better to turn off the ozonator since the chlorine would then last longer, but unfortunately spas don't have an "ozonator disable" switch. For more extended periods, lowering the temperature of the water would have the chlorine last even longer.

You can try the manual dosing and if you find it to bothersome you could consider a saltwater chlorine generator for background dosing, such as the ControlOMatic TechniChlor.

 

[gourmatt]

Thanks for all the advice, chem geek. The saltwater chlorine generator sounds interesting. Does anyone here have experience with it?

I made the switch to bleach yesterday and gave it a pretty good dose in order to shock it. Based on the chlorine / CYA table that I found posted (put together by you, I think) I was targeting a Shock FC of 39. I then referenced the pool calculator to obtain a dose of 17 oz of 8.25% bleach based on my tub size of 310 gallons, along with adding 1.4 oz of washing soda to raise my pH to 7.6. After dosing the pH was at the 8.0 indicator on the Taylor kit (so perhaps higher) and the FC was 50! I played around with the pool calculator a bit and suspect that the culprit for the discrepancy could be that I don't have 310 gallons in the tub, but maybe more like 280 or 290. Lesson learned .... undershoot the dosing and retest.

I then added a cumulative of about 3.5 tsp of dry acid in three steps to bring the pH back down.

Readings as of 7:30 this morning (22 hours later) are pH 7.4, TA 110, and FC 26.

By the way, I did not add any borax since I don't have muriatic acid on hand, and only a little dry acid. I didn't feel prepared to try to battle with the pH any more than I already was. I also read about the sulfate problem associated with dry acid in another post, so that was in the back of my mind as well. I think I'll go with boric acid so that I don't have to worry about the pH swing or sulfates. Any opinions on the best borate test strip? Lamotte? And I'll pick up some muriatic acid since I may need that when adding the bleach.

What caused the TA increase? The washing soda? Is there any way to prevent it from rising?

So the FC dropped by 50% in 22 hours. Pretty heavy demand, no?

Thanks!

 

[chem geek]

Any opinions on the best borate test strip? Lamotte? And I'll pick up some muriatic acid since I may need that when adding the bleach.

What caused the TA increase? The washing soda? Is there any way to prevent it from rising?

So the FC dropped by 50% in 22 hours. Pretty heavy demand, no?

The Lamotte Borate test strips are the ones to get. Washing soda (same as pH Up) raises the TA. You can have half the TA rise when increasing pH if you use 20 Mule Team Borax instead. To have the pH rise with no change in TA, aerate the water.

As for the FC drop of 50% in 22 hours, that is pretty typical when you have an ozonator. Ozone reacts with chlorine. If you had no ozonator, but had the spa still be hot, you'd lose closer to 25%. If the spa was cooler, you'd lose even less (perhaps 15%) assuming there is no exposure to sunlight.