Very High Acid Demand with SWCG

PoolFlunkie · Oct 13, 2015

I have been adding about 1-1/2 gallons of muriatic acid each week to maintain my pool pH. When I check my pH it is normally around 8.0. I add about 60-80 oz. and decrease the pH down to about 7.2 to 7.4 and it increases back to 8 by the next week. I have an intellipH system that adds acid while the pool is circulating but its maximum setting is 2 oz per hour (about 12 oz. per day or 84 oz per week). After a week the pH is back to 8 and I manually add 60-80 oz. more to reduce the pH (total of about 1-1/2 gallons between the manual addition and automatic). Pool is 10 years old with original plaster. I installed the SWCG and pH control about a year ago (been fighting the issue since then). I use the intellipH system to add the large amounts of acid by connecting the acid pump to a 24v supply and running it for 15-20 minutes. I have the intellichlor SWCG set at ~20%. Any help would be appreciated. thanks.

FC=3.6
CC=<0.2
TA=60
CYA=60
Salt=3600
CH=300

pabeader · Oct 13, 2015

I see that your TA is 60. How are you maintaining this level?

Texas Splash · Oct 13, 2015

That's a lot of pH increase just for the SWG. Do you have any other forms of aeration (i.e. spa, waterfall, bubblers, etc)?

PAGirl · Oct 13, 2015

Are you using the full strength MA?

JoyfulNoise · Oct 13, 2015

I'd also ask - why are you trying to drop your pH so low??

At a pH of 7.2 your CSI drops to -0.79 :shock:

That's way too low. If you operate your pool so the pH stays between 7.6-7.8, your pH will not rise so quickly. pH rise is almost exclusively caused by CO2 out-gassing from your water. When you drop your pH down to 7.2, you create twice as much dissolved CO2 in your pool water. As that CO2 out-gasses, the pH rises.

So you would probably use less acid less frequently if you kept your pH at a higher level. You certainly have no cause to worry about scaling from high pH given your other water parameters.

Jaimslaw · Oct 13, 2015

I too have had some fairly serious SWG ph creep over the years, which I attribute to my dual spill over spa. Really does cause a lot of aeration and thus, a fairly sizeable ph bump.

Long story short, just redid my plaster pool (to mini pebble ) and am still in the 30 day, no salt/SWG break in period. PH sure easy to maintain without the SWG; but next week, I add the salt and fire up my SWG. In the past, and because I could not be around to add acid more than just the weekends, I have just set up a nice Stenner pump acid injection system. Will now be able to maintain constant ph levels vs before, where ph levels probably were kept too high for much of the week I was not monitoring it.

But am glad to read that mantaining slightly higher ph levels will not cause the scale buildup that led to me doing a replaster job, and hopefully will cut down on the acid demand. Will also try a lower TA level (60ppm?) to see if I can strike a good balance and lower the rate of that ph rise.

But yeah, was in that same boat where it seemed ph levels would rise way up just a day or two after getting it to the 7.4 area. Was going thru quite a lot of acid. Looking forward to being able to cut down on acid consumption even with the convenience of the automated process of adding my acid.

JoyfulNoise · Oct 13, 2015

For pool owners with plaster pools, SWGs and aerating water features, there is some amount of experimenting you must do to get the pH stable. What you use as your guide during the experimenting is the CSI calculation. You vary your TA and pH parameters, as well as aeration feature on-times, to see if you can find a sweet-spot where the pH is stable. Often times, plaster pool owners find that they can go quite a bit lower then the recommended values of TA (down as low as 50ppm) and find a region of stable pH. The trick is to make sure your CSI stays between -0.3 and 0 (a little positive is ok too).

Adding borates to your pool water helps as well since borates act as a secondary buffer system that resists rising pH. When I added borates to my pool water, I went from adding acid every other day to longer than once per week. Now I have to add more acid to drop the pH so the total amount of acid remains the same, but my pH is more stable. I also keep my TA at 60ppm or lower and I don't try to drop my pH lower than 7.6 (unless I'm trying to actively lower my TA).

So you need to find the combination of TA and pH that works for your pool. Also, if you can, ease off on the water features. Many people with spillways, scuppers, bubblers, etc, run them way too much and too often. I try to only run my waterfall for a few times each day and just enough to get a good turn over of water in the little pools that form in it. That's it. Running water features all the time is simply unnecessary and drives up the CO2 outgassing.

PoolFlunkie · Oct 14, 2015

Thanks very much for all the feedback. I'm using full strength muriatic acid (31.45%) and I do have a spillway between my spa and pool that flows whenever I circulate the pool (6-8 hours/day, probably at 50+gpm over the spill way). As far as how I maintain the TA at 60, I guess it's due to checking the TA when the pH is high. After I manually add acid I only check the pH and not the other items. I do have borates added at about 40 ppm although I have a tough time telling the amount since I use the test strips which are not very precise (I usually add borax when I add salt).

The reason for shooting for a lower pH was to try to remove some calcium stains on the plaster. It was suggested in another thread that maintaining the water on the low side of the CSI range could help with the stains over time. I also have been analyzing and adjusting the water once a week so I've been overshooting the pH on the low side so that I'm hopefully less than 8 when I go to check it the next week.

I think I will live with the stains for now (they're not that bad and is running at the lower CSI really an effective way to remove stains?? - there was quite a bit of debate on that item), reduce the spillway water flow (will need to add a valve), and try to find the sweet spot for the TA, pH (probably around 7.8 and 60). Does this make sense? Thanks again for the extremely useful feedback!

As the water cools (it was almost 90 and now it's about 75F) should the CO2 degassing noticeably decrease and the pH not increase as quickly? Or is the degassing less about the temperature and more due to the high amount of splashing and air to liquid contact from the spillway? Thx

JoyfulNoise · Oct 14, 2015

I don't think lowering the CSI is going to do much to calcium scale stains. When plaster companies do a "no-drain" acid wash, they add lots of acid to effectively drop the TA to 0ppm and the pH to 4.5-5.0 range. But the thing to remember with low CSI and no-drain acid washes is that they are not selective. You are etching everything in your pool at about the same rate. A patch of scale is going to etch at roughly the same rate as the rest of the plaster and, in fact, could etch slower if it's a very old stain. My personal preference is that you can more effectively remove calcium scale spots by directly applying acid to the affected area using one of many pole-end tools sold on the internet for scale removal. Jandy even sells one that you can attach a siphon to a bottle of acid and pump acid directly onto a spot while brushing it.

As for temperature, aeration, TA and pH, carbonates typically have retrograde solubility, which means they are more soluble at lower temperatures than at higher temperatures. So yes, when your water is hot, it will outgas CO2 more rapidly than when it's cooler (think about a warm beer versus a cold beer). So, during cooler winter months, pH fluctuations will be a lot slower as CO2 outgassing rates drop assuming your aeration stays roughly the same.