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Thread: Minimum FC vs. Zero and General Qs

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    Join Date
    Jun 2013
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    Minimum FC vs. Zero and General Qs

    Hi all,

    Since successfully completing my first slam a while back (MANY, MANY thanks Needsajet), i have some questions around minimum FC. Sorry if i choose the wrong forum section...

    1. For my CYA (60), FC chart says minimum is 5ppm, target 7ppm. Does this mean that when it drops below 5 i risk an algae outbreak, or is it just not as sanitized as it could be? If dropping below 5ppm isn't going to cause an algae outbreak, is it safe to say i am safe unless it drops to zero? Anecdotally, my FC was 4.5 the other day and i noticed the water was a little off colour. A quick dump of FC to bring it up to SLAM level, corrected that overnight. Thank goodness!!

    The next questions are more around general chemicals..

    2. Was at the pool store today (needed Chlorine) and was talking to their tech. He told me that pucks will help me keep my Chlorine up (going through 20L in just over a week) and admitted that the more pucks, the more CYA, the more FC i need. He commented that they add stabilizer to compensate for the CYA lost during a regular backwash. I tried the math, each 8oz puck adds 1.6ppm CYA to my pool. With my pump and 1.5" plumbing, each puck much would require 10 min of backwash to remove 2% of my water (according to Pool Math). Is my math sound? It seemed logical until i ran the numbers.

    3. This one is about a correlation between CYA and Chlorine demand. In previous years, my CYA was much higher. Last year was around 90ppm, don't even ask about before that! Last year, i never had a problem with Algae even though FC was under the FC chart minimum (kind of reason for Q1). I also have noticed a higher Chlorine demand and evaporation loss this year. So far i've used at least 3-4x more chlorine than all of last season (without counting SLAM). Are there any guidelines to when you want to have a higher CYA level? Could it just be this summer has been much sunnier and hotter than previous? Could it be simply the higher your fc the more normal daily loss to expect? I have seen some posts where it was mentioned because where they live, they keep their CYA higher than normal. Not sure if that applies to me. Is desired CYA level more of a "guess and check" type of exercise to find out what works best for my pool?

    Hopefully I'm not doing something wrong and thats why i've got these questions.

    Thanks all!
    77000L (20500 gallon) vinyl in-ground chlorine double roman pool, 35'x17', Hayward SP3200VSP, Jandy Lite2 LJ400N heater, Hayward S244TC filter, *deleted* Hayward chlorinator, Taylor CK1004FAS+CYA

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    Mod Squad pooldv's Avatar
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    Re: Minimum FC vs. Zero and General Qs

    When FC is below minimum for your CYA, FC/CYA Chart, the pool is not sanitary. The kill rate for bacteria, viruses and pathogens is not sufficient to prevent person to person disease transmission.

    The hotter the climate the higher you want CYA to be.
    TFP Moderator
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    Re: Minimum FC vs. Zero and General Qs

    Pooldv - thanks. What if I know there are no swimmers for a few days or week? Does it still need to be above minimum or just above zero?
    77000L (20500 gallon) vinyl in-ground chlorine double roman pool, 35'x17', Hayward SP3200VSP, Jandy Lite2 LJ400N heater, Hayward S244TC filter, *deleted* Hayward chlorinator, Taylor CK1004FAS+CYA

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    Mod Squad jblizzle's Avatar
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    Re: Minimum FC vs. Zero and General Qs

    Always above the minimum. Once below, the algae can start to get ahead.

    The higher the cya, the higher % of FC protected from the sun. At higher FC levels, more ppm of FC will be lost to the sun for the same % loss.

    Unless I have this backwards, our minimum recommendations for FC are to prevent algae. The pathogens could be eliminated at lower levels of FC.
    Jason, TFP Moderator
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