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Thread: How long until FC returns to normal after shocking?

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    How long until FC returns to normal after shocking?

    First of all, I can't say enough good things about this forum. I've gone from completely ignorant to successfully running a community pool 24/7 completely on the advice available here. You guys have the patience of saints when it comes to dealing with newbies.

    Following your guidance, I have yet to shock this pool because running FC at target levels given in your FC/CYA table keep the water sparkling, particularly if I set a fixed min at FC~3.5 ppm.

    But I'm getting what may be black spot algae in the damaged areas of the plaster (pool has a history of running CH ~30ppm, make-up water levels). My brushing may have helped, but hasn't been 100% successful. I'm afraid I may be headed toward a need for shocking.

    I'd hate to take the pool offline for anything more than a day. And as far as I know what I may do is wait until closing for the season and then shocking. Or perhaps waiting until next spring to deal with this.

    But I realize that I have NO idea how long it takes a pool to return to swimmable FC levels after a shock. So that makes planning impossible. How long does that take? I'm sure there are chemicals that will knock FC down, but so far I've been able to follow this forum's advice and focus only on a very small number of chemicals.

    THANKS!!!!!
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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    Isaac-1's Avatar
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    Re: How long until FC returns to normal after shocking?

    This is going to depend on CYA level, sun exposure level, etc. If you are rushed though there are Chlorine reducer products.

    Ike
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    Re: How long until FC returns to normal after shocking?

    Thanks, Ike. I run CYA low. ~30ppm and sometimes lower. I plan to lose ~2ppm FC per day. I guess low CYA would mean FC would drop faster from shock levels. On the other hand, the pool is slightly shaded and in the mountains of western NC which have seen few sunny days this summer!

    Do I understand you to say that I may very well have to use FC reducing products if re-opening the pool within 36 hours is a priority?
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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    Mod Squad jblizzle's Avatar
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    Re: How long until FC returns to normal after shocking?

    Why the hurry? It is safe to swim up to shock level for your CYA. So once you pass the 3 criteria to stop, you can swim almost immediately.

    Although are there public regulations that you have to follow? Have you read the SLAM process?
    Jason, TFP Moderator
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    Re: How long until FC returns to normal after shocking?

    Thanks, Jason. Yes I've read the very helpful SLAM article. In this case, I pass all three criteria before I even begin as I'm only doing this to get at the black algae on the bottom of an otherwise pretty pool.

    The regulations appear only concerned with the 1ppm min, and are ignorant of everything else including CYA and chlorine lock.

    But is it really safe to swim in? Did NOT know that. Even the SLAM article mentions damage to equipment at extreme FC levels. Seems like people, their hair, and their clothing would be even more sensitive than equipment. So I assumed I needed to return to normal FC target levels before re-opening. ???
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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    Mod Squad jblizzle's Avatar
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    Re: How long until FC returns to normal after shocking?

    Extreme would be well above shock level. The CYA buffers the chlorine. So the active chlorine level at shock level is still lower than it would be in a pool with a FC of 2ppm with no CYA.

    Btw, the clear water criteria means you do not see any algae. You are seeing algae.

    And chlorine lock is a myth and a term used by people that do not understand the relationship between CYA and FC.
    Jason, TFP Moderator
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    Re: How long until FC returns to normal after shocking?

    North Carolina Pool Codes have the following for water quality standards (I've selected a subset of them):

    15A NCAC 18A .2535 WATER QUALITY STANDARDS
    Whenever a public swimming pool is open for use, water quality shall be maintained in accordance with the following:
    (1) The chemical quality of the water shall be maintained in an alkaline condition at all times with the pH between 7.2 and 7.8.
    ?(2) The clarity of the water shall be maintained such that the main drain grate is visible from the pool deck at all times.
    :
    (4) When chlorine is used as the disinfectant, a free chlorine residual of at least one part per million (ppm) shall be maintained throughout the pool whenever it is open or in use. Pools that use chlorine as the disinfectant must be stabilized with cyanuric acid except at indoor pools or where it can be shown that cyanuric acid is not necessary to maintain a stable free chlorine residual. The
    cyanuric acid level shall not exceed 100 parts per million.
    :
    (6) When chlorine or bromine are used as the disinfectant, automatic chemical feeders shall be used. Automatic chlorine or bromine feeders shall be manufactured and installed in accordance with NSF/ANSI Standard number 50. Automatic chlorine and bromine feeder pumps shall be
    automatically prevented from operating when the circulation pump is not in operation.
    :
    (10) Test kits or equipment capable of measuring disinfectant level, pH, and total alkalinity must be
    maintained at all public swimming pools. Pools using cyanuric acid or chlorinated isocyanurates
    must have a test kit capable of measuring cyanuric acid levels.
    :
    (12) Water temperature in heated swimming pools shall not exceed 90 degrees Fahrenheit (32 degrees
    Celsius) and in heated spas shall not exceed 104 degrees Fahrenheit (40 degrees Celsius).
    So interestingly they require a 1 ppm FC minimum, but no maximum.

    The higher chlorine level will be noticeable by some people, but technically when the FC is at regular shock levels (around 40% of the CYA level) the active chlorine (hypochlorous acid) is at the same level as a pool with 0.6 ppm FC with no CYA so lower than most commercial/public indoor pools that do not use CYA. Nevertheless, this is around a 10x higher active chlorine level than the minimum FC so will oxidize swimsuits, skin and hair faster. My wife has experienced this difference between a commercial indoor pool and our own outdoor pool. Though not dangerous, people may notice.

    If you need to get the FC down faster, you can add a chlorine neutralizer. An easy one to use with no side effects is to add hydrogen peroxide which you can get relatively inexpensively in Baquacil Oxidizer which is 27% hydrogen peroxide. You can calculate how much is needed by entering in the FC change (pretending it's an increase) in The Pool Calculator, but use 54% as the bleach concentration. To reduce the FC by 10 ppm in 10,000 gallons would take around 23 fluid ounces (around 3 cups) of the Baquacil Oxidizer.
    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: How long until FC returns to normal after shocking?

    Thanks, Chem. An amazingly complete answer. I find hydrogen peroxide much less intimidating than other more sinister-sounding chlorine neutralizing chemicals.

    Yes, the code and its enforcement is odd here, and hasn't helped my pool learning curve at all. The health inspector's battered old test kit gave wild results that worried me sick until I was able to verify that his numbers were in error and mine were correct. Oh well, he passed me. But he was obviously intimidated by even the simplest water chemistry questions, and clearly his primary motive for allowing pools to pass inspection was that if he closed them it would mean more work for him to come back out and inspect again to re-open them. Nice guy though.
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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