Water Heaters

Water heating is a thermodynamic process using a heating source to heat water above its initial temperature. Potable water is usually heated by a device known as a water heater or hot water system (the term 'hot water heater' is an incorrect, redundant term). more...

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Water heaters for nonpotable use, both industrial and domestic, are also called hot water boilers. When a shell-and-tube heat exchanger is used for potable or nonpotable water heating, it is commonly called a hot water generator.

Most commonly, human-induced heating processes, such as combustion or electric-resistance, are relied upon to heat the water, but solar energy, or where possible, geothermal power may be used instead. Heat pumps and heat recovery may be used as well. Sometimes a combination is used, such as solar preheating, and then conventional combustion or electric heating.

In English-speaking countries, except in North America, water heaters are usually known as boilers, or "geysers" (though the latter term originally applied to a brand of tankless heaters). In cold climates, the water heating and hydronic space heating are commonly combined in one boiler; in much of North America, the water- and space-heating functions are through separate pieces of equipment.

Water heating in the United States

Water enters residences in the US at about 50 deg F. An adult generally prefers a shower temperature of 105-120 deg F, requiring the water temperature to be raised about 50 deg F (or more, if the hot water is later mixed with cold water). The code reference shower flow rate is 2.5 gpm (gallons per minute); sink and dishwasher usages range from 1-3 gpm.

Natural gas in the US is measured in CCF (100 cubic feet), which is converted to a standardized heat content unit called the therm, equal to 100,000 British thermal units. A BTU is the energy required to raise one pound of water one degree. A gallon of water weighs 8.3 pounds. So, to raise a 40 gal tank of 55 deg water up to 105 deg would take 40 x 8.3 x 50 / 100,000 therms, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU (per hour) heater would take 25 minutes to do this, at 100% efficiency. At $1 per therm, the cost of the gas would be about 17 cents.

To heat the same quantity of water with electricity in the same amount of time would require about 28KW (120 amps at 240 volts) for 25 minutes, at 100% efficiency. This is about 12 KW-Hrs, which at 10 cents per KWH would cost $1.20.

A tankless water heater operating at those same power levels (at 100% efficiency) would be able to supply 1.6 gpm continuously, raising the temperature by 50 deg F. Or supply 1.3 gpm raising the temperature by 60 deg F. To be able to handle a full house load of multiple uses (at least 5 gpm) at the same time with a centralized water heater would require about three to four times this power level -- somewhat difficult to achieve with natural gas, and very difficult to achieve with electricity.

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