The method of heat development for water heaters may be direct (heat from combustion of fuels or electrical energy directly applied to water) or indirect (heat from a remote heat source utilizing some other medium, such as steam, to heat water).
Direct-heat-type water heaters are classified as follows:
1. Automatic storage heaters, which incorporate burners or heating elements, storage tank, outer jacket, insulation, and controls as a packaged unit
2. Circulating tank heaters, which consist of what is essentially an instantaneous heater and an accessory storage tank. Hot water is circulated through the heating section by means of a circulating pump
3. Instantaneous heaters, which have little water storage capacity and generally have controls that modulate the heat output based on the demand
4. Hot-water supply boilers, which provide high-temperature hot water in a manner similar to hot-water heating boilers Fuel for direct-fired water heaters is generally one of the fossil fuels, such as natural gas or oil, or electric power.
Indirect-type water heaters are classified as follows:
1. Storage type, which consists of a heat exchanger installed in a storage tank (Fig. 14.7) or in a separate storage tank and stand-alone heat exchanger provided with a circulating water system.
2. Indirect immersion type, a self-contained water heater, utilizing one of the fossil fuels as a heating medium for a horizontal fire tube containing a finned-tube bundle. Water, or some other heat-transfer fluid, is heated in the finned bundle in the burner section and is pumped to a water-heating bundle located in the shell or storage tank installed below the fire tube.
3. Instantaneous type, which is suited for facilities requiring steady, continuous supplies of hot water (Fig. 14.8). The rate of flow is indirectly proportional to the temperature of the water being supplied.
4. Semi-instantaneous type, which have limited storage to meet momentary hotwater peak demands. These types of heaters consist of a heating element and a control system that closely controls leaving-water temperature. A hot-water storage tank provides additional hot water when required during periods of peak momentary hot-water demand.
The heat-transfer media normally utilized for indirect domestic hot-water heaters are steam and heating hot water. The heat-transfer media use heat provided by boilers and, in some instances, solar collectors, which collect heat from the sun.
(For detailed guidance in the sizing of domestic water heating systems, see Service Hot-Water Systems, Chap. 4, ASPE Data Book, American Society of Plumbing Engineers, Westlake, CA 91362. Recovery versus storage curves that have been developed based on extensive research can be utilized to compare various combinations available.)
Plumbing designers should also assure that all required safety devices and controls have been provided to prevent an explosion of the storage vessel. There have been numerous instances of injury and death to occupants due to overfiring conditions caused by malfunctioning controls and safety-relief devices that did not operate properly. All storage vessels should be provided with AGA/ASME-rated pressure and temperature (P&T) relief valves, installed as directed by the vessel manufacturer. The rating of the P&T valves should meet or exceed the Btu input rating of the water-heating apparatus.
As water is heated, the volume required to contain the heated water increases.
In the past, the increased volume and resulting increased pressure was allowed to expand back into the domestic cold water system. With the increased use of backflow prevention devices in domestic water systems, the potential for expansion of hot water has been limited. In many instances, water heater tanks have failed due to variations in pressure associated with expansion during heating. Most plumbing codes now require the installation of an expansion tank on domestic hot water systems to prevent premature tank failure.
Most storage tanks are constructed of steel and therefore are subject to rusting when in direct contact with water. Various liners are available such as cement, glass, copper, and nickel. The designer should select a liner that best meets the needs of the building being designed. Storage tanks should be ASME certified.
The hot-water load for a given building is computed in a manner similar to that described in Art. 14.8 but with Table 14.6 and the tabulated demand factor for the particular building type. The heating-coil capacity of the heater must at least equal the maximum probable demand for hot water.
For storage-type heaters, the storage capacity is obtained by multiplying the maximum probable demand by a suitable factor, such as 1.25 for apartment buildings to 0.60 for hospitals. Table 14.7 lists representative hot-water utilization temperatures for various services. It should be noted that service-water temperatures in the 140F range should be provided, to prevent the growth of Legionella pneumophila bacteria which causes Legionnaires disease.