Water heating is thermodynamic process using an energy source to heat water above its initial temperature. Typical household uses of hot water are for cooking, cleaning, bathing, & space heating. In industry, both hot water & water heated to condensation have many uses.
Domestically, water is customarily heated in vessels known as water heaters, kettles, cauldrons, pots, or coppers. These metal vessels heat a batch of water, but do not produce a continual supply of heated water at a preset temperature. The temperature will vary based on the consumption rate of hot water: the water becomes cooler as flow is increased.
Appliances for providing a more-or-less stable supply of hot water are variously known as water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, or geysers depending on whether they are heating filtered or non-potable water, in domestic or industrial use, their energy source, & in which part of the world they are found. In domestic installations, potable water heated for uses other than space heating is sometimes known as domestic hot water (DHW).
In many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil, or sometimes-solid fuels. These fuels may be addicted directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, & sometimes-geothermal heating, may also be used as available, usually in arrangement with backup systems supplied by gas, oil or electricity.
In some countries district heating is a main source of water heating in compactly populated urban areas. This is especially the case. District heating systems make it possible to supply all of the energy for water heating as well as space heating from waste heat from industries, power plants, incinerators, geothermal heating, & central solar heating. The actual heating of the tap water is performed in heat exchangers at the consumers' premises. Generally the consumer has no in-building backup system, due to the very high-expected accessibility of district heating systems.
Water enters residence in the US at about 10 °C (50 °F) (varies with latitude & season). Hot water temperatures 40-49 °C (104-120 °F) are preferred for dishwashing, laundry & showering; requiring the water high temperature to be raised about 30 °C (54 °F) or more, if the hot water is later mixed with cold water. The Uniform Plumbing Code situation shower flow rate is 2.5 US gallons (9.5 L) per minute; sink & dishwasher usages range from 1-3 US gallons (3.8-11 L) per minute.
Natural gas in the U.S. is measured in CCF (100 cubic feet), which is converted to a st&ardized heat content unit called the thermal, equal to 100,000 British thermal units (BTU). A BTU is the energy required to raise one pound of water by one degree Fahrenheit. A U.S. gallon of water weighs 8.3 pounds (3.8 kg). So, to raise a 40-gallon tank of 55 °F (13 °C) water up to 105 °F (41 °C) would require (40 × 8.3 × (105 - 55) / 100,000) BTU, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU/h heater would take 25 minutes do this, at 100% efficiency. At $1 per thermal, the cost of the gas would be about 17 cents.
In comparison, a typical electric water heater has a 4500 watt heating element,in which if 100% efficient results in a heating time of about 1.1 hours. Since 16,600 BTU is roughly 4.9 kWh, at 10 cents/kWh the electricity would cost $0.49. Operating a shower at 2.5 gpm & 104 °F (40 °C) is equivalent to operating a 19.8 kW appliance [ ref. w computes 13.2 kW, but that is for 20 degree C increase instead of 30 ]. In the UK, domestic electric immersion heaters are usually rated at 3 kilowatts.
Energy efficiencies of water heaters in residential use can vary greatly, particularly based on manufacturer & model. However, electric heaters tend to be slightly more efficient (if one omits the power station losses) with recovery efficiency (how efficiently energy is transferred to the water) reaching about 98%. Gas fired heaters have maximum recovery efficiencies of only about 86% (the remaining heat is lost with the flue gasses). Overall energy factors can be as low as 80% for electric & 50% for gas systems. Natural gas & propane tank water heaters with energy factors of 62% or greater, as well as electric tank water heaters with energy factors of 93% or greater, are considered high-efficiency units. Energy Star-rated natural gas & propane water heaters have energy factors of 67% or higher; electric tank water heaters are not included in the Energy Star program. Since electricity production itself today has efficiency levels ranging from only 15% to slightly over 55% (combined cycle gas turbine), with around 40% typical for thermal power stations, direct electric water heating is typically the least energy efficient option. However, use of a heat pump can make electric water heaters much more energy efficient & lead to a decrease in carbon dioxide emissions, even more so if a renewable source of electricity is used.
Long-term energy savings: Although a water heater might cost more initially it may result in both energy & cost savings in the long term. As water is heated only when it is needed, there is no storage of hot water. With a tank, water is kept warm all day even if it never gets used & heat loss through the tank walls will result in a continual energy drain. Even in homes or buildings with a high dem& for hot water, a water heater may provide some level of savings. In a typical home these savings are quite substantial. If instant hot water at the taps at limited hours is a priority, using an aquastat & timer in order to decrease the added heat loss from the recirculation system can accommodate a recirculation system similar to those in the tank-type systems. It has to be said though that if the storage in tank is highly insulated-a few tanks are available with excellent levels such as 100 mm / more polyurethane foam-the savings become minimal. For one consumer-grade electric storage water heater, the surface temperature was less than 1 °C higher than the air temperature.
Unlimited hot water: As water is heated while passing through the system an unlimited supply of hot water is available with a water heater. Although flow rate will determine the amount of hot water that can be generated at one time it can be generated indefinitely. However, this can also be a disadvantage, as running out of hot water self-limits use while a heater has no such limit. Less physical space: Most water heaters can be mounted on a wall or even internally in a building's structure. This means less physical space has to be dedicated to heating water. Even systems that can't be mounted on walls take up less space than a tank-type water heater. Reduced risk of water damage: No stored water means there is no risk of water damage from a tank failure or rupture, although the risk of water damage from a pipe or fitting failure remains. Improper piping in either the hot or cold water lines to the water heater can result in water damage though. Safety: Water Heaters can precisely control the temperature of the treated water, which means dangerous temperature levels & spikes are no longer a problem.