Water Heaters & Energy Efficiency

Water heaters use about 20 percent of the energy consumed by an average American household, making them the second largest energy user in the home, after space heating.

Fifteen to thirty percent of the energy consumed by a standard water heater goes to keeping the water hot while it's not being used! This lost energy is called standby loss. Gas water heaters have higher standby losses than electric water heaters, because of the uninsulated flue running up the center of the tank.

Demand heaters (also called tankless or instantaneous heaters) reduce standby loss by heating water only as it flows through the heater on its way to a faucet actually in use. Demand heaters have their efficiency drawbacks as well. Gas demand heaters have pilot lights, limiting their savings to 15-20 percent over a standard water heater. Electric demand heaters require so much power to instantaneously heat water at a reasonable flow and temperature rise, they can actually be the most expensive type of water heater to operate. Applications for electric demand heaters are best limited to small flow, low temperature uses like bathroom sinks at the far end of the house from the main water heater.

Energy Ratings

Water heaters are rated for energy efficiency by a number which assumes a 64 gallon a day demand, and accounts for energy losses during the water heating process, standby losses, and pilot light losses. The ER number is a decimal value between 0.4 and 1.0 which reflects the percentage of total energy used which remains in the delivered hot water. Local public electric utilities offer a rebate to customers who purchase an electric water heater with an ER of more than 0.93.

Increasing Water Heating System Efficiency

Reduce standby losses from storage tank and pipes.

1. Set water heater temperature to 120 degrees. Use a thermometer; the numbers on the thermostat are usually not accurate at all. If the house has a dishwasher which does not have its own booster heater, the water heater temperature may have to be set a bit higher, until the dishwasher washes properly. Some soaps wash better than others at lower temperatures. For safety reasons, in no case should the temperature be set higher than 130 degrees! Every 10 degree reduction in water temperature results in about an 8 percent savings in energy use.
2. Insulate the storage tank. Install a water heater insulation blanket; the higher the R-value, the better. Use wire or twine or straps to insure that the blanket stays in place. Some new high efficiency heaters should not be insulated; consult the equipment manual provided by the manufacturer. Gas water heaters should not be insulated on top or within about 8" of the bottom of the water tank. Set an electric water heater on a rigid foam insulation board. This step is most critical when the heater sits on a concrete slab, but it's always a good idea. Install the water heater in a heated location. The colder the air surrounding the heater, the more the standby loss. Indoor gas heaters should be sealed combustion or fan-forced draft.
3. Insulate pipes and use heat traps. Insulate all exposed pipes. The R-value of pipe insulation is dependent on wall thickness; thicker is better. A 5/8" wall thickness should be considered minimum for foam insulation, while 3" is the minimum for fiberglass wrap. Heat trap nipples work best to eliminate convective losses from the tank into the plumbing, but pipe loops also work if the drop is at least 6".

Reduce distribution losses by insulating all hot water pipes anywhere they are accessible.

Reduce the use or waste of hot water.

1. Install low-flow shower heads and faucet aerators. Use shower heads with a flow of 2.5 gallons per minute or less. Bathroom faucets can be cut back to .5 to 1 gallon per minute, and kitchen faucets can be limited to 1.5 to 2.0 gallons per minute. Low-flow shower heads and aerators save on water heating costs, water bills, and sewage bills.
2. Purchase an efficient washing machine and dishwasher and use them efficiently. Horizontal axis, or front loading washing machines use 60 percent less energy than conventional vertical axis machines. Wash clothes with cold water, and wash full loads. Dishwashers should have booster heaters, and a no-heat drying cycle. Don't rinse dishes before loading them in, and wash full loads. It is possible to wash dishes most efficiently by hand if water is conserved.

Time clocks or timers are available, which turn an electric water heater off during periods when hot water demand is likely to be low. Savings from these controls are a maxiumum of about 5 percent, and they are probably not a good investment. Focus on reducing tank standby losses and you'll come out ahead.

Turn the water heater off during vacations or absences from home of more than two days.

Copyright Alan Van Zuuk