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Thread: What effect does having a cover have?

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    What effect does having a cover have?

    I'm thinking of getting a pool cover that doesn't let light through. What effect is this going to have on my chemicals? The cover will on the pool at nights and weekends, but off during the day.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    Re: What effect does having a cover have?

    The cover independent of whether it lets light through will cut down carbon dioxide outgassing so will reduce the tendency of the pH to rise if the TA is already high enough for that to be happening in your pool. With an opaque cover, you eliminate the breakdown of chlorine from the UV in sunlight. So you should see a far lower chlorine loss for the days you have the pool covered (sound like just the weekend days in your case). There should be no change with regard to any other chemicals or water parameters (except temperature, as noted below).

    As for whether you will get any additional net heat gain, this depends on the color of the cover and the amount of evaporation in your pool. A very dark cover will add some heat to the pool during the day while a light or reflective cover will not add any noticeable heat. The cover will help retain heat, especially at night.

    I have a mostly opaque pool cover on my pool and it is on all the time except 1-2 hours every day when the pool is used for swimming (somewhat longer on the weekends). The main effects are to reduce the amount of temperature loss at night and to reduce chlorine loss during the day. You will benefit from the former, but not so much from the latter except on the weekends.
    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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    Re: What effect does having a cover have?

    Thanks. All that sounds positive.

    I was wondering what effect it might have on combined chlorines. We always have some. It's an outdoor pool with heavy use so it's unreasonable to think CC will ever go away completely. I'm just wondering if the lack of UV to break down the CC might have a detrimental effect.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    JasonLion's Avatar
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    Re: What effect does having a cover have?

    Yes, if you leave the cover on all the time the CC level can go up. Some cover usage isn't normally a problem with CC. It is normally only an issue when the cover is on nearly all day every day. However, if you have a lot of people swimming, which will produce more CC than normal, you may need to leave the cover off more of the time to keep the CC level down.
    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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    Re: What effect does having a cover have?

    if the water is essentially clean, bar the odd beetle and tree seed that's blown in, and the bather waste that can be dealt with has been dealt with, what happens to the combined chlorine? is it so incredibly stable that it just sits there, while the tiny bit of remaining pollution contributes to an increase, or is there a reaction that means CC can start to decline even though its under a cover?
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    Re: What effect does having a cover have?

    For an outdoor pool exposed to sunlight, the CC from higher bather load is mostly intermediate compounds that haven't yet been fully oxidized. The largest component of sweat and urine is urea so chlorine combining with urea to form chlorourea is one possibility. Creatinine can also lead to some fairly persistent CC. Monochloramine would be another component of CC though that is the most easily eliminated. The UV in sunlight breaks down hypochlorous acid to hydroxyl radicals that may help to keep CC in check so that most low bather load residential pools have very low CC levels (<= 0.2 ppm usually; almost always <= 0.4 ppm).

    The thing is that not all CC are bad. The test doesn't distinguish between the different types. Many municipal water districts use monochloramine at around 1 ppm, for example. It is presumed that at higher CC levels in pools there will be more of the "bad" CC such as nitrogen trichloride which is very irritating or trihalomethanes some of which are carcinogenic, but this doesn't take into account the active chlorine (hypochorous acid) level. In pools with CYA, there should be less nitrogen trichloride produced and ironically the CC level will be higher since everything gets oxidized more slowly by chlorine.

    How high is the CC you are seeing? Are you using a FAS-DPD test kit for that determination? Does the water smell "bad" or does it have the faint smell of chlorine bleach or no smell at all?

    I keep the cover on my pool almost all the time except when it is in use, but that is pretty much every day for 1-2 hours, longer on weekends. I almost always measure <= 0.2 ppm CC. The bather load is usually 1 person in 16,000 gallons and 2 people on the weekend. Even after a pool party with around 6 people in the pool, I still only measured <= 0.2 ppm CC the next day (it might have been <= 0.4 ppm CC shortly after their swim). Maybe your FC level is too low for your CYA level. Can you please tell us those water parameters?
    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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    Re: What effect does having a cover have?

    I think my levels are good and my water is clear. The "problem" is the number of children getting in the water, but since that's why the pool is there, that's a problem I have to live with. I guess we have around 100 children per day in the water.

    Anyway, I'm keeping FC between 3 and 4 and it has never dropped significantly below this.

    Other numbers:
    pH - 7.7
    TA= 150 (last week)
    CH=200
    CYA = 40

    At lunchtime today, after half a days worth of children thorugh the pool, the CC level was about 0.8ppm. This evening the pool has just a slight chlorine smell, although I'd like to bet I'm the only one that can smell it - I'm very sensitive to chemicals.

    What I don't want is the effect of each day to accumulate, so that in a few weeks time I do end up with a smelly pool. I'm trying to work out what best to do to prevent that.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    JasonLion's Avatar
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    Re: What effect does having a cover have?

    At very high bather loads, the FC levels required to get rid of all of the CC are often higher than what it is safe to swim at. In that situation you need to supplement the chlorine with something else to be able to keep up. Using MPS can help a great deal, and a well designed UV system can also be a huge help at keeping CC levels under control.
    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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    Re: What effect does having a cover have?

    thanks Jason and chem geek for your help. i am going to go for a dark cover and we'll see how things work out.
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    Re: What effect does having a cover have?

    You will find that in the UK a cover helps a lot. Our weather tends to be breezy with low humidity and cool nights and widely varying daytime temperatures. If left uncovered my pool looses about 6 degrees c overnight with a nightime temperature of ~ 12 deg c if I dont add any heat. With the cover on that overnight loss drops to about 2 deg c. Being dark blue it does help with adding some heat back when (if) the sun does come out but I think it is the heat loss due to evaporation that helps the most. My cover is on for most of the time and cc can become a bit of an issue if we open it after a longish period so it is worht opening it up every few days when the sun is out. Overwintering is a lot easier with a cover.
    32x16 -16,500 gallon plaster pool with connected spa and spillover, Hayward RS 1 HP pump, 2 Hayward Cartridge filters. 10 hours of direct sun a day and a first time pool owner!!

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