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water heater[′wȯd·ər ‚hēd·ər]
a heat exchanger for heating water by means of steam, hot water, hot gases, or electric current. Water heaters are used in hot-water supply and heating systems, for heating boiler feed water, and for domestic use. Water heaters of the surface type, in which heat is transferred to the water to be heated through the surface of metal tubes that are heated by steam or water, are the most widely used. Contact water heaters, in which the water to be heated comes in direct contact with the steam or hot gases, are less frequently used. Water heaters that are installed in boiler units for heating water by the heat of waste gases are called feed-water economizers. Geysers, coil pipes or hot-water boxes mounted in stoves, and water boilers are all types of domestic water heaters that operate on gas or solid fuel. Geysers are the most commonly used domestic water heaters. They may be of the capacitive or circulating types and may operate on gas, solid fuel, liquid fuel, or electric current. The heating power of geysers is up to 35 kilowatts (30,000 kilocalories per hour).