Demand water systems use a technique that heats water instantaneously, eliminating the need for a heated tank. Because water is only heated as needed, monthly electricity bills often are slashed by 10 to 15 percent. The systems are common in Europe and Japan, but they’ve only been in the United States for about 25 years. Demand water systems can be retrofitted around existing fixtures or be standalones for remote areas. The device—available in electric, propane or natural gas models—is activated when a hot water valve is opened. But the systems restrict the flow rate of water to allow time for heating, which makes them unsuitable for all residential or commercial places. The systems have an output of about 2 to 5 gallons per minute, which may require multiple demand water units for simultaneous uses. With a lifespan of more than 20 years, these “tankless” systems outlast conventional water tank systems, which last about five to 15 years.

Because demand water systems are dependent on the demand of use, the systems should be sized to the expected use. To do so, it’s advised that you add up the number of hot-water devices you anticipate having open at any one time and add in their flow rates. As with most water devices, low-flow shower heads and water-conserving faucets will save money and enhance the demand water system’s performance. Demand water systems are more expensive than conventional heaters, but the lower operating costs are easily returned—some manufacturers of the systems claim an annual savings of $200 on energy bills. Users should be aware that the pilot light in some systems is always on, which can deter savings. The costs of a constantly-running pilot light can range from $12 to $20 a year, and a common money-saving practice in Europe is to turn the light off after each use.

Drainwater heat recovery systems Drainwater heat recovery systems collect the heat from hot water used during such activities like showering and washing dishes. The systems, designed to decrease primary energy use, saves 60 percent of the hot water used. The saved hot water can go to a water heater, a water storage device or be used to preheat cold water for showers or laundry. The type of recovery system used depends on the size of piping and whether they run horizontally, vertically or other. Some DHR systems are storage units that have a tank with a supply of clean water. Drainwater flows through a heat exchanger within the tank, warming the water. The cold water supply to the water heater is preheated by a coil also inside the storage tank. Heat from the warm drainwater is transferred to the cold supply water without mixing. The system is also designed so it does not lose a lot of stored energy when cold wastewater runs through the drainwater heat exchanger.

Another system has a copper heat exchanger that replaces a vertical or horizontal section of a main waste drain pipe. As warm water flows down the waste drain, cold supply water flows through a copper tube. Energy is transferred from the warm drainwater to the cold supply water, which is directed to the cold side of the fixtures and/or to the water heater. Although the water heater effectively stores some of the useful energy collected by this type of DHR, it is not classified as a storage system. Solar water heating Solar water heaters have hot water storage tanks that warm water with sunlight from rooftop solar collectors instead of fuel or electricity. The solar collectors consist of a dark absorber plate under cold water pipes placed in an insulated glass box. The electricity gathered can either be used to heat water or to pump it through the systems.

Solar water heaters have been used in the United States, namely Florida, since the 1920’s, and by 1967, nearly 20 percent of Israel’s population used the systems. In 2005, Spain became the second country (behind Israel) to require the heaters.