Water heating consumes 15 to 20% of a home's energy needs. Selection of an appropriate and efficient model can provide significant savings to a household.

TYPES OF WATER HEATERS
Most homes in the U.S. use conventional storage-type water heaters that hold 40 to 60 gallons of water and are heated by electricity, natural gas, oil, or propane. The tank is always kept full and is constantly heated. The hottest water (which is denser) rises to the top and is drawn off for household use and replaced by cold water entering the bottom of the tank. Because the unit is constantly heated, energy waste occurs through standby heat loss, which is minimized in modern energy-efficient models. Check the EnergyGuide label that is attached to new water heaters to determine their energy efficiency.

Demand water heaters, common in Europe and Japan, employ a gas or electric element to heat water only as needed, eliminating standby heat losses (except for the pilot light on gas models), and reducing energy consumption by 20 to 30% over storage heaters. Demand heaters may be installed at the point of use or centrally. However, this type of water heater has a flow rate of only 2 to 4 gallons per minute (gpm), so consumers need to be aware of their peak residential demands in order to select an appropriate unit. For example, standard shower heads use 2.5 to 3.5 gpm; low-flow shower heads use 1.2 to 2 gpm; faucets use .75 to 2.5 gpm, and clothes washer/dishwashers use 1 to 2 gpm. It is best to use low-flow shower heads and water-conserving faucets with demand water heaters.

Heat pump water heaters take heat from the surrounding air and transfer it to water in their tanks, a more efficient process than directly heating the water. They are effective only when installed in locations where the air is 40 to 90 degrees F year-round. These heaters have self-contained storage tanks or can be retrofitted to existing tanks. Comparatively quite expensive to purchase (around $600 to $800 installed), they heat water at a rate of approximately 15 gallons per hour, more slowly than electric or gas heaters. However, they consume only a third to half of the energy required by a typical electric heater.

Solar water heaters can supply 50 to 100% of a home's hot water needs and provide savings of 50 to 80% over electric heaters in the water heating portion of a home's annual energy bill. Most solar heaters feature a collector and storage tank; active systems also have an electric pump to circulate the heat transfer fluid (water or an antifreeze solution). Most systems use a roof-mounted collector and require a well-exposed roof that faces south.

SYSTEM CAPACITY
Water heater storage tanks generally hold 20 to 80 gallons of water. Of importance equal to volume, however, is the first-hour rating (FHR) of a water heater, which is a measure of peak hour demand capacity. The FHR is shown on the water heater's EnergyGuide label, and indicates how much hot water the system can deliver during periods of peak demand. Gas water heaters have higher FHRs than electric heaters with the same storage capacity; consequently, a household can meet its hot water needs with a gas unit that has a smaller storage tank.

HEATING EFFICIENCY
Energy Factor (EF) ratings for different models of the same type of water heater may be compared to determine the heater's efficiency. The higher the EF, the more efficient the model. The EF rating is based on the efficiency of heat transfer from the energy source to the water, heat losses per hour of the standing water, and cycling losses.

MAINTENANCE AND SETTINGS
Most household uses require hot water that is no warmer than 120 degrees. Dishwashers are an exception, but most modern models have an independent temperature booster. If you use a dishwasher has that feature, you can safely set the thermostat of your household water heater to 120 degrees, realize significant energy savings, and reduce the chances of accidental scalding. The thermostat on most storage-type models is beneath a cover plate, and has a knob or a screw that is turned to set the temperature.

By draining a quart of water from your water heater approximately every 3 months, you can reduce sediment buildup and related energy costs.

Hardware stores or the local utility company can supply pre-cut blankets to insulate your water heater and reduce energy costs. Check the user's manual to see what parts of the heater should not be covered, such as the thermostat, pilot light access, drain valve, or electronic controls.