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A flash, tankless, or on demand water heater is one that heats water as needed, rather than storing it in a tank. In general, a flash water heater will be more efficient than a tank water heater, and will yield hot water consistently when it is required. Many energy-efficient homes and businesses use them to bring energy costs down, incorporating multiple heaters into the designs for a structure with potentially heavy water demand.
In a house with a flash water heater, turning the hot water tap on turns the heater on. Water from the household pipes enters the chamber of the heater and is quickly warmed and routed to the tap. When the tap is turned off, the heater turns itself off, and will wait in standby mode until it is called for again. In some cases, a separate one may be installed to provide hot water to large appliances like dishwashers and washing machines, so that people can take showers and use the sinks without fear of straining the water heater's capacity.
A number of sources can be used to power a flash water heater. The cheapest and the most common is gas, which can also be used to power heating systems and stoves. Oil and kerosene are also sometimes used, and in rare situations, electricity will serve as the energy source. Electricity can actually be more energy efficient, especially if it is derived from clean power sources, but it is usually more costly to use. In a totally solar home, of course, electricity will be used to power the water heater.
In some instances, a passive solar heating system will be used to make the water heater more efficient. In a passive heating system, water is pumped through solar chambers, which attract heat and warm the water while it sits in tubes. The already warm water is pumped through the flash heater, which does not need to expend as much energy heating it as it might otherwise.
The installation of a flash water heater is relatively simple, and can be undertaken by people with even minimal mechanical ability. People who are uncomfortable with connecting the power source can find a contractor to perform that portion of the job, which will also ensure that it is done properly and safely. Make sure to follow all advisories on the appliance about clearance, as the extreme heat can start fires in flammable materials situated too close to the heater. If you are concerned that one will not be enough for your household, ask about the capacity of the flash water heater you are interested in, and try to get an accurate estimate of the peak water usage in your home to gauge whether or not you need two.