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Thread: Sunny day heating - faster with or without cover

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    Join Date
    Jan 2010
    Northern Virginia

    Sunny day heating - faster with or without cover

    The rain has finally stopped and the sun is out, but the pool is cool. I was wonder if I'd get a higher rise temperature if I left the cover on the pool or took it off. In my mind I can see pro and cons of each, but I wonder if anyone has done any home testing on this?

    It's a blue "bubble" cover.

    700 sq. ft./30,000 gal, gunite pool; 48 SF DE filter; 1.5 HP pump; Salt Water Generator; Solar Heat

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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Sunny day heating - faster with or without cover

    On a bright sunny day with sunlight directly on the water you are almost always better off with the solar cover off. The solar cover blocks some of the sunlight, removing it allows more to reach the water. The only exception is if it is both very dry (no humidity) and very windy.
    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

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    gullzway's Avatar
    Join Date
    Nov 2009

    Re: Sunny day heating - faster with or without cover

    Thanks Jason, I was about to post this same question. I also have a blue solar cover and have been leaving it on unless swimming. Pool is still 70 degrees. I'll try leaving it off during the day, but with only 4-5 hours of direct sunlight I may be wasting my time.
    17,500 gallon 18x36 gunite pool
    Hayward super II 1.5 hp pump, Hayward cartridge filter
    Jandy AquaPure Ei SWG 35K, Raypak R6350TI-E HeatPump
    Solar cover, Loop Lok winter Cover
    Artisian Antigua hot tub, Polk Atrium 4's

  4. Back To Top    #4

    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Sunny day heating - faster with or without cover

    As described in this post, a typical white plaster pool can gain up to 0.7ºF per hour in temperature from direct noontime sun. However, as described later in that thread, typical 1/4" of evaporation will lower the temperature by about 5ºF or if consistent over 24 hours roughly 0.2ºF per hour. So there is a net gain so long as the cooling from evaporation isn't particularly high.

    As Jason noted, very dry air and wind can significantly increase evaporation rates. Specifically, for typical water/air temperature differences, 10% relative humidity evaporates about 50% faster than 66% relative humidity while as little as 5 MPH wind (at the pool surface) increases evaporation by 220%. So just a consistent light breeze could nearly wipe out all gain from direct sunlight.
    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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