Heating household water accounts, on average, for 25 percent of energy costs in the United States. The only way to be completely self-reliant for hot water is to use passive solar heating, where the sun and mirrors provide all heat.

In areas where there is not enough sun to rely on passive solar, active solar heaters are used. These require a pump generated by electricity or other energy.

But the most economical system is the on-demand water heater. It also requires energy input, but it heats only the water you actually use. The United States is the only country in the world that still relies on heating water in tanks. On-demand heaters are the norm almost everywhere else.

Solar Water Heating

With solar water heating, solar energy is first collected, then used to heat the water in a storage tank. Then the heated water is passed into a tank that holds the water until use. If you were to buy a unit, your decisions would be about the type of collector, mechanism of the storage tank, and the heat transfer system.

Active systems use electric pumps, valves, and controllers. They are more expensive than passive systems, but also more efficient. Active systems are also easier to retrofit into an existing home. Cost is from $2000 to $4000. If you live in a cold climate, you also will need to use a recirculation system to keep pipes from freezing.

Some companies now make small solar-electric panels to activate the pump, eliminating power costs and ensuring water supply during power outages.

Passive systems are less expensive than active systems, as they have fewer parts, but they are also less efficient. Installed costs range from $1,000 to $3,000. Passive systems use "batch heaters" — storage tanks placed in an insulated box facing the sun — but in some climates they are vulnerable to freezing. Systems using "thermosiphon heaters" are more expensive, as the tank is above the collector. Thermosiphon is a conduction method where hot water will rise and cooler water will fall, thus circulating the water.

Solar water heating is much less expensive than electric. Owners can save as much as 50 percent to 85 percent annually on their electric utility bills.

Solar water heating saves little compared to gas, however. You save some costs, especially over time, but because savings are lower, solar water heaters to replace gas heaters are practical only for new homes or major renovations.

Some communities, states, and electric companies offer tax rebates to those using solar water heating.

The long-term benefits of solar water heating, however, are more than economic. They reduce our dependence on foreign oil, and there is no carbon dioxide emission (50 tons over 20 years in a non-solar system), and so they do not exacerbate the greenhouse effect. They also reduce other air pollution. Solar water heating also improves the resale value of a home.

On-Demand Water Heating

Tankless water heaters — sometimes called on-demand water heaters — are another way to save on energy costs. They are referred to as on-demand, or instantaneous, water heaters because they heat water only when there is demand for it, rather than storing pre-heated water in a tank. They may be powered by propane, natural gas, or electricity.

Demand heaters are commonly used in Japan and Europe. They came to the U.S. about 25 years ago, but for some reason have not caught on, although not only are they much cheaper to operate, but also you never run out of hot water!

Demand water heaters come in different sizes and can be used at the point of service, or they can be installed to function as a whole-house system. Makers claim they can match the performance of a 40-gallon (151 liter) tank heater.

The principle behind a demand heater is that when a hot-water tap is opened, cold water enters the heater and triggers the heating elements or burners to turn on. Water is heated as it flows through the elements. Water valves or a thermostat control the elements or burner flame to maintain a constant temperature in the water that flows into the house pipe system. Temperature can be adjusted from 100 to 140 degrees F. When the tap is turned off, the system shuts down. Gas or electricity is being consumed only when you are using hot water.

We cannot always expect a whole-house demand system to be able to provide hot water to all our locations at once, however, especially if you are using the same power source for water as for heating. Generally, heating takes priority over other utilities, so if you turn the heat up and immediately try to take a shower before the house reaches the temperature set on your thermostats, your shower may be lukewarm.

Costs of demand water heaters range from $200 to $1,000, depending on how many gallons they deliver. They are usually rated at from 1 to 3 gallons per minute. Some require a pilot light that will use a few dollars worth of energy per year. However, they do save money over conventional water heaters, and it is convenient to have hot water whenever you need it. They also last tend to last longer — 20 years, as compared to 10 to 15 years for a conventional water heater.

References

"Solar Water Heating," a Consumer Energy Information EREC Fact Sheet. U.S. Department of Energy. Phone: 800-363-3732, eren.doe.gov/erec/factsheets/solrwatr/html. Contact your country, state, and local energy offices for region-specific information.

Consumer Guide to Solar Energy, S. Sklar & K. Sheinkopf, Bonus Books, Inc, 160 East Illinois Street, Chicago, IL 60611, 1991.

The Homeowner's Handbook of Solar Water Heating Systems, B. Keisling, Rodale Press, 1983.

"Demand (Tankless or Instantaneous) Water Heaters," Consumer Energy Information: EREC Reference Briefs, US Department of Energy, phone 800-363-3732.