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Thread: Thinking about dumping a gallon of muriatic acid into my spa

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    Thinking about dumping a gallon of muriatic acid into my spa

    I have a 450 gal. Caldera spa that is 2 years old. I have maintained ph and alkalinity very well, but we do have hard water here (250ppm). Recently my ozone generator quit producing any bubbles and I discovered the Venturi was clogged with hard water scale, I assume had flaked off the heater element.
    I'm thinking of dumping a gallon of muriatic acid into the full tub and circulating it overnight before draining, flushing and refilling. This should descale the entire system, right? Thoughts? I can't imagine a 1:450 ratio is strong enough to hurt anything in 8 hours?
    25K Gunite, rectangle with spa, sand filter, Hayward natural gas heater, Polaris pool sweeper

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    Richard320's Avatar
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    Re: Thinking about dumping a gallon of muriatic acid into my

    I wouldn't do that. That will be super low pH, and will probably do bad things to the plaster and the metal inside your heater. Why not check the pool calculator and see what you need to remove all the alkalinity? To do a no-drain acid wash on a large pool only uses 4 gallons or so - and they do it without circulating it!
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
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    linen's Avatar
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    Re: Thinking about dumping a gallon of muriatic acid into my

    I agree with Richard, I wouldn't...that is a huge amount of acid. Only 1.1 ounce of 31.45% Muriatic Acid will drop your tub from 7.5 to 7.0 ph (assuming 75 ppm TA and no borates). The calculator can not handle 128 ounces...it will drop your ph quite far and can damage equipment including heaters, etc.
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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    Re: Thinking about dumping a gallon of muriatic acid into my

    This link is where I got the idea from:
    http://www.poolsplusinc.com/eSTORE/acatalog/FAQ_s.html
    25K Gunite, rectangle with spa, sand filter, Hayward natural gas heater, Polaris pool sweeper

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    Re: Thinking about dumping a gallon of muriatic acid into my

    I ran the numbers on the pool calculator. Starting with 450 gals of water at a pH of 7.8, and a TA of 180, if I add a gallon of muriatic acid, it will drop the pH to 3.8.
    Lemon juice has a pH of 2 for comparison.
    25K Gunite, rectangle with spa, sand filter, Hayward natural gas heater, Polaris pool sweeper

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    Re: Thinking about dumping a gallon of muriatic acid into my

    The Pool Calculator does not calculate pH correctly for large swings beyond normal pool pH levels. My Pool Equations spreadsheet will calculate it accurately though it too has some trouble after the TA is exhausted (though there are other ways to estimate when that happens).

    If you start with 450 gallons at a TA of 180 ppm and pH of 7.8, then one gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) will lower the pH to around 1.8 which is very low. Basically what happens is that every 10 ppm of TA takes 1.15 fluid ounces so to exhaust the 180 ppm of TA it takes around 20.7 fluid ounces where the pH gets to around 4.5 and then crashes after that since the pH buffer is exhausted.

    pH is logarithmic where a 10:1 dilution is a pH difference of 1.0 and a 100:1 dilution is a pH difference of 2.0. So most of the acid you add is towards dilution since the 20.7 fluid ounces to use up the TA is only a portion of the 128 fluid ounces in a gallon so you have 128-20.7 = 107.3 or 0.838 gallons left. A 450:0.838 dilution is a pH difference of log10(450/0.838) = 2.73 and the acid starts out with a pH of -1.0 so the result is around -1.0+2.73 = 1.73 which is close to what the spreadsheet calculated (the spreadsheet accounts for ionic strength and other factors).

    Just because lemon juice is very acidic does not mean that pH is OK for a spa. It will start to corrode/dissolve metal including the copper in your heat exchanger, but how much depends on how long you expose it to the low pH. You usually don't need such a low pH to dissolve scale. It will dissolve with a pH of 6 or 5 or even 7 if you wait longer and will be less harsh on your spa components.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: Thinking about dumping a gallon of muriatic acid into my

    Great info. from all. That is why I asked first. I knew the knowledge base here is awesome.
    So my question becomes:
    Given a buildup of hard water deposits on the heater element and probably elsewhere in my spa, what level of pH should I strive for to initiate an overnight descaling of the system without doing harm to components? What ratio of muriatic acid to water?
    Whereas this calcium scale is flaking off the heater element, traveling downstream to the ozone mixing venturi, and totally clogging it off with large flakes of scale, I need to deal with this somehow.

    Next thought I have is, should I mix about half my 450 gallons with soft water in order to drop my calcium hardness from it's current 250 ppm in order to try and prevent this scale buildup in the future?

    Thanks again for the input.
    25K Gunite, rectangle with spa, sand filter, Hayward natural gas heater, Polaris pool sweeper

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