A water heater is an appliance designed to heat water for such pur- poses as cooking, washing, and bathing. This water is generally referred to as domestic hot water or potable water.
The most commonly used water heater is the vertical tank-type stand-alone unit used in residences and small commercial buildings. It is independent of the space heating system and is a separately fired water heater. Other water heaters depend on an external heat source such as a boiler. The domestic hot water is heated when it circulates through a heat exchanger inserted in the hot-water or steam heating boiler. These heat exchangers are used only with boilers. In a warm-air heating system, domestic hot water is pro- vided by an independent and separately fired water heater.
Types of Water Heaters
Water heaters can be divided into several groups or classes. The most common classifications are based on the following criteria:
1. Size and intended usage.
2. Heating method.
3. Heat-control method.
4. Fuel type.
5. Flue location and design.
6. Recovery rate.
Water heaters are classified as either domestic or commercial water heaters on the basis of their size and intended usage. Domestic water heaters are those with input rates up to 75,000 Btu per hour. Water heaters regarded as commercial types have input rates in excess of 75,000 Btu per hour. Temperature is another of the criteria used to distinguish between domestic and commercial water heaters. For commercial usage, hot-water temperatures of 180°F or more are generally required. The hot water used in residences does not normally exceed 180°F. For domestic usage, 140°F is generally considered adequate.
Another method of classifying water heaters is by how the heat is applied. Direct-fired water heaters are those in which the water is heated by the direct combustion of the fuel. In indirect water heaters, the service water obtains its heat from steam or hot water and not directly from the combustion process. The advantages and disad- vantages of both direct-fired and indirect water heaters are described elsewhere in this chapter.
The heat-control method in a water heater may be either auto- matic or manual (nonautomatic). All water heaters used in resi- dences are now of the automatic type.
Natural gas or propane, oil, coal, electricity, steam, or hot water can be used to heat the water in a water heater. Either steam or hot water can be used as the heat-conveying medium in indirect water heaters. Neither, of course, is a fuel. The fuels used to heat the domestic water supply are gas, oil, and coal. Gas is by far the most popular fuel. Oil is gaining some popularity, but it still falls far short of gas. The use of coal as a fuel for heating water is now found only in rare cases. Although electricity is not a fuel, in the strict sense of the word, it is generally used along with the three fossil fuels as an additional category for classification.
Quite often, water heaters are classified on the basis of flue location and design. This is a particularly useful criterion for classifying the various automatic storage-type water heaters.
Water heaters can also be classified on the basis of their recovery rate. Quick-recovery water heaters are capable of producing hot water at a more rapid recovery rate than the slow-recovery types. Quick-recovery heaters are often used in commercial structures where there is a constant demand for hot water.