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Thread: Can a week's worth of constant high FC burn off the CYA?

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    DonW's Avatar
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    Jun 2008
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    Can a week's worth of constant high FC burn off the CYA?

    I have a curiosity happening with my CYA - it's gone! I've just finished a bout with Mustard Algae and 8 days of shocking at MA+ levels. Before the process I checked and my CYA was 55-60. Prior to that in late July I took a reading of 45-ish. After my bout with MA I checked and it was zero. O.K. My CYA reagent must be bad. Ordered a new 8 oz bottle and a dedicated graduated cylinder and just got done checking. I can fill the test tube to the top and still easily see the dot. What's going on?
    Pool has very light use, constant sun all day, water replacement has been < 5 inches since late July. I have the LQ going and additionaly am putting in 96 oz of 6% every other day. Topping off the LQ at about 1 jug per day = daily FC consumption at about 1.5 -2 jugs per day in high 90 degree heat. Keeping my FC at around 13, I want to get to closing without MA rearing its ugly head.
    So, can a week's worth of constant high FC (40ish), burn off the CYA level in a pool?
    18' x 33' AGP
    16,715 gal.
    200 lb. Sand Filter w/Hayward PowerFlo MATRIX 1.5 H.P. 2 Spd. pump
    Stenner Injection Pump System

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    Mod Squad JohnT's Avatar
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    Re: Can a week's worth of constant high FC burn off the CYA?

    Not likely. Do you add much water to the pool?
    TFP Moderator
    20K Gallon 20X36 Vinyl Inground
    Hayward S244T Sand Filter with 1HP Whisperflo Pump. Liquidator C-201 and Solar Heat

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    DonW's Avatar
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    Jun 2008
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    Re: Can a week's worth of constant high FC burn off the CYA?

    Less than 5 inches since July. I didn't think so, but my pool has always been funny about loosing CYA, just not this much over this short a span.
    18' x 33' AGP
    16,715 gal.
    200 lb. Sand Filter w/Hayward PowerFlo MATRIX 1.5 H.P. 2 Spd. pump
    Stenner Injection Pump System

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    Join Date
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    San Rafael, CA USA
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    Re: Can a week's worth of constant high FC burn off the CYA?

    Chlorine will oxidize CYA, but normally does so slowly -- usually around 2-3 ppm CYA per month in pools and 5 ppm CYA per month in spas. However, it happens faster at higher FC levels and at higher pH so with severe yellow/mustard algae shock levels it's possible to have the CYA drop faster, though I wouldn't expect it much more than around 20-30 ppm CYA over a month so I wouldn't expect to see such a large drop over one week. Others have shocked at yellow/mustard algae levels and not had their CYA disappear. Another possibility would be if the chlorine level got to zero at some point (possibly before you started shocking) and bacteria converted it to ammonia, nitrogen gas and nitrate.
    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
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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