Almost all of a pool’s heat loss—about 95 percent—occurs at the surface, mostly through evaporation to the air and radiation to the sky. A pool cover is an effective means to keep heat (and water) in a pool by reducing evaporation of water from the pool when it is not in use, and reduces radiant heat losses. A pool cover can reduce water loss by 30 to 50 percent. Each gallon of 80-degree water that evaporates removes around 8,000 Btu from the pool. Reducing water loss also reduces the amount of chemical water treatment required.
Outdoor pools can gain a significant amount of heat from the sun, absorbing 75 to 85 percent of the solar energy striking the pool surface. Abubble cover (sometimes called a solar cover) is one of the least expensive covers made specifically for swimming pools. It’s similar to bubblepacking material except it has a thicker grade of plastic and ultraviolet (UV) inhibitors. Vinyl covers are made of a heavier material, which extends their use. You can also get vinyl covers with a thin layer of flexible insulation sandwiched between two layers of vinyl.
A transparent bubble cover may reduce solar energy absorption by 5 to 15 percent, and an opaque cover may reduce it by 20 to 40 percent. However, the decrease in solar gain can be balanced or more than offset by the cover’s retention of the pool’s heat, which depends on the air temperature and humidity. Generally, the drier and cooler the air, the greater the heating benefit from covering the pool during the daytime. Of course, a cover should always be used at night to prevent losses when there is no solar gain.
A cover also helps you keep the pool clean and extend the life of the chemicals in your pool. At a cost of 20 to 60 cents per square foot, a pool cover may pay for itself in 1 year. Be aware, however, that UV radiation deteriorates the cover, requiring that you replace it every 3 to 5 years. Before you buy one, make sure the cover comes with at least a 2-year warranty. Also find out how easily you can place the cover over the pool, how to remove it, and how to store it. Systems are available that move the cover off and on the pool with a motor or hand crank.
Covers should always be installed and used according to the manufacturers’ guidelines. Always consider that people could use the pool unauthorized or unsupervised.
A windbreak around the pool can also reduce evaporation, and make pool “lounging” more comfortable and private. A7-mph wind at the pool surface can increase heat loss by 300 percent. A windbreak could include a fence or vegetation. It needs to be high and close enough to the pool so that it effectively blocks wind from moving across the pool surface, but doesn’t block beneficial solar energy.