A pool cover has several benefits, but it should be left open periodically.  Here's why!

A pool cover has several benefits:

• It virtually eliminates water evaporation lowering water usage and prevents an increase in Total Alkalinity and Calcium Hardness from fill water.
• It retains heat in the pool, especially at night, allowing for warmer pool water temperatures. Also, a bubble-type cover lets sunlight into the pool further heating the water.
• An automatic pool cover can act as a safety cover preventing drowning accidents (a bubble-type cover does not).
• Helps to keep debris out of the pool (though this depends on the cover if debris falls back into the pool when getting rolled up).

However, pool water needs to occasionally breathe and be exposed to direct sunlight, preferably once a week, for the following reasons:

•  The UV in sunlight breaks down chlorine to produce a powerful chemical to remove bather waste and other organics that get into the pool.
• Total Alkalinity can rise in a pool and to lower it requires outgassing of carbon dioxide from the pool along with addition of pH Down.

So give your pool the exposure it needs and uncover it at least one day per week. Better yet, get in and enjoy your pool!

DETAILS

Water Evaporation

When water evaporates from a pool it leaves non-volatile dissolved chemicals behind so their total amount remains constant.

While their concentration increases because the volume of water decreases, adding pure water will bring the concentration back to where it was. That is, evaporation and refill does not decrease the water chemistry parameters (e.g. FC, TA, CH, CYA).

When one adds fill water to the pool to make up for the evaporation, it adds not only water but also Total Alkalinity (TA) and Calcium Hardness (CH) in the fill water increasing these values in the pool.

Hard fill water can significantly increase the CH and often the TA. The use of a pool cover will significantly reduce this increase.

Retaining Heat

56% to 70% of heat loss in swimming pools is due to evaporation of the water from the pool. This is a significant amount that in an average pool losing 1""/week is 2.8°F per day drop in temperature.1

Conduction of heat from air and the pool surfaces will limit the loss when there is no direct sunlight.

In typical white plaster pools with direct sunlight, heating from absorption of infrared and visible light can heat the pool by up to around 6°F per day.2

Use of a clear bubble-type cover can increase pool water temperature above day/night average air temperature by 10-15°F while a solar system in conjunction with a cover can increase it by 20-25°F depending on sizing.

UV in Sunlight

The ultraviolet light in sunlight breaks down chlorine that is unbound to Cyanuric Acid (CYA). In pools with CYA and that have a pH near 7.5, half of the unbound chlorine is broken down every 37 minutes in direct noontime sun. With a 5% FC/CYA ratio, the loss rate is around 0.05 ppm per hour at noon or around 0.4 ppm (mg/L) per day (there are other chlorine losses not resulting in hydroxyl radical production). When chlorine breaks down from UV it produces hydroxyl radicals that are powerful oxidizers though very short-lived.

HOCl + hν à •OH + •Cl

Hypochlorous Acid + UV in sunlight à Hydroxyl Radical + Chlorine Radical

OCl- + hν à •O- + •Cl

•O- + H2O à •OH + OH-

---------------------------------

OCl- + H2O + hν à •OH + •Cl + OH-

Hypochlorite Ion + Water + UV in sunlight à Hydroxyl Radical + Chlorine Radical + Hydroxide Ion

The chlorine radical mostly forms more hypochlorous acid and chloride ion (salt). The amount of hydroxyl radicals at noon in a 16'x32'x4.5' pool is around 5.5 g/day (0.69 g/hour at noon).3 By comparison, typical residential ozonators produce 0.25 g/hour of ozone and advanced oxidation processes produce no more than 0.059 g/hour of hydroxyl radicals so less than 1/10th that from pool exposure to sunlight.4 Most hydroxyl radicals are scavenged by bicarbonate ion and result in water and oxygen gas.

O3 + H2O + hν à O2 + H2O2

2O3 + H2O2 à 2•OH + 3O2

----------------------------------------

3O3 + H2O + hν à 2•OH + 4O2

Ozone + Water + UV in sunlight à Hydroxyl Radicals + Oxygen Gas

Exposing a chlorinated pool to sunlight is the easiest way to break down bather waste and organics to keep the water clean.

Total Alkalinity and Carbon Dioxide

Pools are intentionally over-carbonated to protect plaster surfaces and grout and to provide buffering of pH. This over-carbonation is the primary source of rising pH in pools not using net acidic sources of chlorine. Total Alkalinity (TA) is primarily a measure of the amount of bicarbonate in a pool. The bicarbonate is in equilibrium with aqueous carbon dioxide in the pool and carbon dioxide outgasses from the pool when the pool is uncovered and does so more quickly when there are sources of aeration (e.g. waterfalls, spillovers, fountains, saltwater chlorine generators).

The outgassing of carbon dioxide causes the pH to rise with no change in TA. Adding acid lowers both pH and TA. The combination of acid addition with carbon dioxide outgassing lowers the TA.

When the pool has TA rising over time (from fill water or other sources), it is important to let the carbon dioxide outgas to remove the excess over-carbonation. Acid addition is then able to lower the TA to keep it stable.

1. (1 inch/week evaporation)/[(4.5 feet)*(12 inches/foot)] * (2429.8 kJ/kg water heat of vaporization @ 86°F) / (4.18 kJ/(kg•K) water isobaric heat capacity @ 86°F) = 10.76 K/week
   • (10.76 K/week) * (9°F / 5K) / (7 days/week) = 2.77°F/day
2. (70% sunlight absorption in white plaster pool)/100% * [(1000 W/m2) * (1 (J/s)/W) / (1000 J/kJ)] * (3600 s/h) * (8 equivalent direct noontime hours/day) * / (996 kg/m3 water density @ 86°F) / [(4.5 ft avg. pool depth) * (0.3048 m/ft)] / (4.18 kJ/(kg•K) water isobaric heat capacity @ 86°F) = 3.53 K/day
   • (3.53 K/day) * (9°F / 5K) = 6.35°F/day
3. http://www.nrcresearchpress.com/doi/abs/10.1139/s06-052#.W5ijsS2-LOR
   • [(0.4 mg Cl2/L/day) / (1000 mg/g)] * [(15 g/mol •OH) / (70.906 g/mol Cl2)] * (16*32*4.5 cubic feet) * (28.3168 liters / cubic foot) = 5.5 g/day
4. (0.25 g/hour O3) * [(17 g/mol •OH) / (48 g/mol O3)] * [(2•OH)/(3O3)] = 0.059 g/hour •OH"