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Thread: Does pool chemistry change in midwest winter?

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    Does pool chemistry change in midwest winter?

    That's my question. I have a gunnite pool and need to know pool parameters that are best for closing. We cover the pool with the old-fashioned tarp method (not porous) and our water temperatures get as low as 45 degrees farenheit in the winter. Currently, I have:
    pH of 7.5
    TA of 90
    CH of 280
    Cya of 32
    The CSI re: the pool calculator with a current water temp. of 80 degrees is -0.02. What would be optimal closing parameters upon closing? ( I have a suspicion that both pH and TA rise and I know that Calcium becomes more soluble as the water temperature decreases.) But, we do not circulate our water during the winter, so I wonder if the parameters matter when the water is just still.


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    Mod Squad JohnT's Avatar
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    Apr 2007
    SW Indiana

    Re: Does pool chemistry change in midwest winter?

    If you only get down to 45, you may just consider not closing at all so you can circulate and make chemistry adjustments. My pH climbs during the winter, just as it does during the summer. I'd think CH would be your main concern at closing.

    You have my curiosity up though, as to where in the Midwest you live that only gets down to 45 degrees.
    TFP Moderator
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    Re: Does pool chemistry change in midwest winter?

    JohnT-We live in central Indiana where, yes, it does get below 45 degrees air-wise. However, I made an assumption that the pool water did not get much below 45 (this is a guess due to ground warming the sides of the pool). I have a small (100 gallon) pond as well as the pool, and at most it only gets an inch or so of ice on it, so I figured our 25,000 gallon pool probably had none. However, the water on the cover does freeze, but only, again, in a thin layer.

    If I left the cover off, I would have to deal with leaves in the fall and lots of debris from our strong winds in the spring.
    BTW, I did a little experiment with CH where I measured the level at 83 degrees to be 270. Then, I cooled down an identical sample by immersing the test cylinder in an ice bath to 42 degrees. The CH at 42 degrees increased from 270 to 320! So, I know CH definitely changes as the temps drop. However, the Pool Calculator indicates that changes in pH in cold water affect the CSI more than CH. At least the CH is predictable. I did read that it's best to close the pool with a pH of 7.0-7.2. I guess I'll try that and hope for the best. I will be talking to a lady I trust who has worked with pools in this area for 20 some years & get her opinion. Thanks for your reply.

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