Hot water heaters
Heating water for household use accounts for 15% of your total energy bill.
Be aware that the most energy-efficient water heating systems may tend to be more expensive. But once you do purchase an energy-efficient model you can expect to save more money over the long run on your heating bills.
Conventional storage water heaters - These hot water heaters are standard for most homes. A storage water heater holds holds about 20 to 80 gallons of hot water and the fuel sources include natural gas, propane, fuel oil, and electricity. Although these systems are popular they also can waste energy because water is constantly heated in the tank even when a hot water tap isn't running. The best way to combat this negative is to choose models with tanks that have a thermal resistance (R-Value) of R-12 to R-25. Wrapping the tank in a thermal blanket will help as well.
Heat pumps or heat reclaimers - A heat pump is actually an air conditioner that works in reverse. In the summer the heat pump functions like a normal air conditioner and in the cold months the heat pump can be operated as a whole house heating system. As long as the outside temperature is above freezing the heat pump can extract heat from the air but below freezing the heat pump relies on an internal electric heating coil to create heat which can be expensive.
Waste water heat exchanger - Hot and warm water from various sources in your home goes down the drain and carries away heat energy. Drain-water or greywater (from sinks or showers) heat recovery systems captures energy that can be used to preheat cold water entering the water heater or going to other water fixtures. U.S. Department of Energy - diagram
Tankless water heater - Demand (tankless or instantaneous) water heaters provide hot water only when it is needed. They don't heat and then store the hot water as the conventional storage water heater. These systems can be expensive and will only serve single locations so multiple units might be necessary. U.S. Department of Energy - diagram
Solar water heaters - These systems can be used in nearly any climate and can be a cost-effective way to generate hot water for your home. Solar water heating systems include solar collectors, a circulating pump and a storage tank. In one-tank systems, the back-up heater is combined with the solar storage in one tank. In a two-tank systems where the solar water heater preheats water before it enters the conventional water heater. U.S. Department of Energy - diagram
