Getting warmer: the ABCs of greener water heaters. (House & Home).

Americans love their hot water. But what most folks don't realize is that their hot water heater is the largest energy gobbler in the home, after their heating and cooling units. According to Jay Burch, a solar specialist at the National Renewable Energy Laboratory (NREL), "Running the electric water heater of a single-family home for one year creates more emissions than driving an automobile 12,000 miles." The average American household spends 20 percent or more of its energy bill on hot water, and much of what's paid for is heat lost through the thin walls of the storage tank in the basement or utility room. The waste of energy can hit the environment (and the pocketbook) hard.

"The greatest inefficiency of a hot water heater lies in the heat [the tank] loses during the time it sits around," says Yen Chin of Seattle City Light. A conventional system runs 24 hours a day in order to heat and store many gallons of water. Homeowners pay for all this energy, though no hot water is used while they're at work or asleep.

Even after water heaters have ceased to work, they continue to harm the environment. Because the life span of a tank heater averages about 10 years, homeowners must replace them frequently. According to Get Tankless, a water heater manufacturer, more than 7.3 million hot water tanks are landfilled in the U.S. every year.

Go Tankless

Fortunately, there are a number of heating alternatives for the energy-conscious consumer. A tankless water heater eliminates the need for a storage vessel entirely, heating water only as needed. When the hot water faucet is turned on, these small units heat water as it flows through either a gas burner or an electric element. The supply of hot water never runs out and no energy is wasted keeping it warm. The only drawback to the tankless water heater is its limited flow rate of two to four gallons a minute: Multiple hot water appliances cannot be operated simultaneously unless the homeowner installs several units. Super Supreme is one company that offers tankless units with three different levels of heating capacity.

Another efficient option is the heat pump unit, which takes heat from the surrounding air and transfers it to the water in the tank--the same principle that runs a refrigerator, but in reverse. Since much less energy is needed to move heat, these units use only half the electricity of conventional heaters. The unit remains efficient except when surrounding air temperature is very low, in which case a back-up heating element may be needed.

Hot Solar

Depending upon climate and water use, solar water heaters may be a smart idea. According to Renewable Energy of Texas, "Energy from the sun can provide more than half of a home's hot water needs." Solar units use the sun's rays to heat either water or a heat-transfer fluid in collectors mounted, in most cases, on a roof. The heated water is then stored in a tank. Most solar heating units act as preheaters for conventional units. Although the installation costs are high, owners save from 50 to 85 percent annually on their utility bill, according to the U.S. Department of Energy.

If the solar option seems too drastic, try using the house's heating system to assist in warming water. The tankless coil heater and the indirect (boiler-dependent) water heater utilize energy from the boiler. The tankless model op erates on-demand--whenever a hot water faucet is turned on, water flows through the boiler. The tankless device works best in colder dimates where the heating system is used regularly.

In warmer locales, an indirect heater, which uses a heat exchanger like the tankless unit but incorporates a storage tank, saves more energy because it allows the boiler to operate less frequently. An indirect water heater combined with an efficient boiler can be particularly cost effective.

For those not ready for the transition to tankless heaters, there are a few ways to increase the efficiency of the unit you already own. Nell Newman's The a Good Life advises homeowners to turn down the temperature on their heaters. Plumbers tend to set the temperature to 150 degrees, when turning it down to 120 degrees can save consumers up to 15 percent a month. Homeowners can also place an installation jacket on the storage tank to reduce heat loss. Insulating the tank and the first 10 feet of pipe can shave 10 percent off the energy bill. CONTACT: Get Tankless, [phone], www.gettankless.com; NREL, [phone], www.nrel.gov; Super Supreme, (888)5966444, www.supersupreme.com.

LAURA RUTH ZANDSTRA and JAIME DEBLANC-KNOWLES are E interns willing to wear sweaters around the house.