Did you ever put on your swimsuit on a blazing hot day and jump into your swimming pool, only to spend the next five minutes gasping from the shock of the freezing water? It isn’t really surprising that the water in swimming pools cools down so much: The entire surface of that body of water is exposed to the air, at least when the pool is in use, and it loses heat through convection, radiation, and particularly through evaporation. For every pound of water that evaporates, nearly half a ton of water is cooled by 1.0°F, requiring more heat to keep the pool water at the desired temperature.
What you want is to be able to enjoy your pool at a comfortable temperature, but without losing all the heat you’re paying for. A 1980 Pacific Gas and Electric study suggests five ways to conserve energy in a heated pool:
1. Lower the pool thermostat setting.
2. Narrow the pool-heating season to the warmest months in your area.
3. Turn off the heater when the pool is not in use.
4. If possible, keep wind and shading near the pool to a minimum.
5. Use a pool cover.
Heat Loss from an Uncovered Pool:
An uncovered swimming pool loses heat in three ways—by conduction to the earth around it, by radiation to the air, and by evaporation, which cools the heated water in the pool. A simple swimming pool cover reduces most of the evaporation, which is the primary source of heat loss. It also traps the heat, which slows the radiation of heat to the sky. |
A plastic bubble pack pool cover can be rolled up and stored out of
your way while your pool is in use (top). When you’re finished swimming for
the day, or if you plan to be away from home for a while, replace the cover
over the entire surface of your pool. You can accomplish this task with or
without a hand-winch mechanism (middle). The cover will retain heat and severely
reduce evaporation, yielding large savings.
• Pool covers: Using a pool cover, or pool blanket,” at night and on overcast days When the pool is not in use offers three advantages: (1) The pool cover retains the heat already stored in the water; (2) It retains the water itself, thereby preventing the massive evaporative heat loss; and (3) It can even raise the temperature of the water at the pool’s surface—the first 12 inches or so, and more if the filter is in operation—by several degrees. The PG&E study found that the cost of a pool cover often can be recouped in a single year’s savings.
Several kinds of pool covers are available, ranging in price from about 25 per square foot for simple plastic film, to about $1 per square foot for woven plastic blankets with ultraviolet inhibitors. If you are prepared to pay up to $4 per square foot for your whole installation, you can have an electric winch that rolls and unrolls your cover across the top of your pool for you, and stores it conveniently out of the way while you swim.
Whether or not they are equipped with such devices, all pool covers per form essentially the same function: They cover the pool when it isn’t in use. The most common covers are dark or transparent plastic film, clear bubble-pack plastic, and opaque insulated film.
If you expect to use the cover often during daylight hours, choose the clear bubble-pack, since it will allow the pool water to absorb solar heat, while simultaneously insulating it against the loss of that heat. If you expect your pool to be uncovered most of the time on most days, but covered at night, an opaque foam cover will provide the greatest protection against heat loss by convection and radiation. Plain opaque or transparent plastic film is less effective and less expensive, but far better than no protection at all; it will retard evaporation, particularly if it can be secured tightly around the pool’s edges.
Unlike the insulated covers, which should last several years, a plain sheet of film may last only a single swimming season and then have to be replaced. In some states, the additional expense of an insulated cover can be more or less balanced out by a tax credit that requires a three-year manufacturer’s warranty. Plain covers don’t have such a credit.
Once your pool cover is installed, treat it with the respect its manufacturer recommends. In particular, pay attention to the way you store the cover when it is not in use. It should be kept out of the sun at such times, or covered with some other insulating material.
When the cover is lying on top of your pool the water underneath absorbs and diffuses the sun’s heat and keeps the cover itself cool. But when the cover is off your pool, if it lies around in the sun for very long it is liable to crack and blister, which will severely reduce its effectiveness as well as the length of time it will be useful.
• Pool heaters: To create the heat that pool covers will conserve, you can use a conventional gas or electric pool heater. As with any heating unit, it is important to maintain it in good condition. All the guidelines for servicing your home heating unit apply to your pool heater. The efforts you make to be conscious of and conserve the energy you use in doors and out can save you money.
If, after you have taken all possible conservation steps, you want to explore further energy-saving options, you may want to consider a solar pool heating system to augment the water’s natural tendency to collect solar heat. Solar pool heating systems are discussed in a future section.
The expense of heating a swimming pool with fuels during high-sun
season (May September) is approximately equal to driving a 20 mpg car 15
thousand miles. A simple plastic vinyl pool cover can reduce this expense
to almost nothing. An electric mechanism such as the one shown here makes
the task of covering and uncovering the pool very simple.
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