Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 3 of 3

Thread: Need for CYA on cloudy days

  1. Back To Top    #1

    Join Date
    Jun 2008

    Need for CYA on cloudy days

    When I removed my winter cover I found, as expected, green algae. My initial water test results were

    FC = 0
    pH = 7.2
    TA = 70
    CYA = 0

    I decided last night to purchase CYA so that I could add some in the morning. I hit the pool hard with bleach last night starting at 5:00 pm. While I knew that I needed to add CYA I was pleased to be able to start the process without CYA since the Cl would be more effective without CYA.

    After adding more bleach this morning my water test results are

    FC = 14.5
    CC = 0
    pH = 7.2

    However, this is a very overcast day. There are very dense clouds and no distinct shadows. If the sun were out most of the pool would be in the shade now due to nearby trees.

    My question is the following: Is it the visible light that burns off Cl or ultraviolet light? Do you need CYA when the pool is in the shade or when there is heavy cloud cover? Specifically, if it remains overcast am I better off to add CYA or to shock without CYA until the sun comes out?
    30,000 gallon gunite in-ground with a spillover spa, Diving board and water slide
    Hayward 1 HP Super II filter pump, Hayward 24 " Sand Filter Pro Series using zeolite media
    Polaris 380 cleaner with ¾ HP booster pump, Hayward Heatmaster pool heater, AutoPilot DIG-220 Power Supply SC-60 cell
    Testing with the TF-100 Test Kit

  2. Back To Top    #2
    Mod Squad JohnT's Avatar
    Join Date
    Apr 2007
    SW Indiana

    Re: Need for CYA on cloudy days

    It's UV light that causes your FC to drop. It probably isn't going to matter either way, since it takes several days to a week for CYA to dissolve.
    TFP Moderator
    20K Gallon 20X36 Vinyl Inground
    Hayward S244T Sand Filter with 1HP Whisperflo Pump. Liquidator C-201 and Solar Heat

  3. Back To Top    #3
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Need for CYA on cloudy days

    You will lose FC to sunlight even on very cloudy days, just not as quickly. With CYA around zero it is probably best to shock a couple of times in the evening and leave things alone during the day. Maintaining shock level during the day will clear up the pool more quickly, but it will also use more chlorine than if you shock in the evening and overnight.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts