Gas-Fired Water Heaters
Gas-fired automatic storage water heaters are those in which the hot-water storage tank, the gas burner assembly, the combustion chamber and necessary insulation, and the automatic controls are combined in a single self-contained, prefabricated unit or package. Size limitations, resulting from the necessity of such heaters being readily portable, generally restrict the storage tank capacity to approximately 75 gallons.
In this type of water heater, the heat of the gas flame is trans- mitted to the water by direct conduction through the tank bottom and flue surfaces. Some heaters have multiple central flues, while in other designs the hot exhaust gases pass between the outer surfaces of the tank and the insulating jacket. In either case, these areas become radiating surfaces serving to dissipate the heat of the stored hot water to the flue or chimney when the burner is off. This is particularly true if the flue or chimney has a good natural draft.
The most commonly used gas-fired water heater is the underfeed type. If properly maintained, gas underfeed water heaters will have a long service life. They are generally inexpensive to purchase and install. The older gas underfeed models were not especially efficient, but design improvements, such as greater tank insulation and improved heat transfer surfaces, have improved their efficiency. Locating the gas burner and flue outside the storage tank has resulted in still another type of gas-fired water heater. These units (sometimes called sidearm heaters) provide indirect heating of the water, which allows the use of plastic-lined storage tanks and reduces standby losses.
Storage Capacity
The average ratio of hourly gas input to the storage capacity in gal- lons of water (for gas-fired automatic storage water heaters of the so-called rapid-recovery type) is such that the recovery (heating) capacity in gallons of water raised 100°F in one hour, in most instances, approximately equals the storage capacity of the tank in gallons.
Where the water must be raised 120°F, the recovery (heating) capacity in gallons per hour will be approximately 83 percent of the storage capacity in gallons.
Where the water must be raised 140°F, the recovery (heating) capacity in gallons per hour will be approximately 71 percent of the storage capacity in gallons.
Gas Burners
The burners used in gas-fired water heaters must be provided with nlet gas orifices and some means of air intake. These conditions are necessary to provide the required air-gas mixture for the flame. Beyond these two basic requirements, gas burners will vary widely in both design and construction. These variations in design and construction are generally concerned with providing good flame pattern and ignition. Flame characteristics are affected not only by the design of the ports (raised, drilled, ribbon, slotted, or flush) but also by their number, distribution, depth, and spacing. The gas input rating is an important factor in determining the number, dis- tribution, and size of the ports. Proper spacing is generally deter- mined by observation. Some common types of gas burners used on water heaters are shown in Figure 4-34.
The purpose of the gas orifice is to provide the proper input for the type of gas (for example, natural, LP) and the normal range of gas pressures. The gas passes into the mixing tube of the burner where it mixes with the air. Air is generally admitted through adjustable air shutters located around the gas orifice. The design and arrangement of the burner ports control the burning characteristics and distribution of the flame.
The size of the ports and their distribution affects the flame char- acteristics. If the ports are too large (both individually and in their distribution), the flame may flash back to the burner orifice. On the other hand, blowing flames can result from porting that is too small. As can be readily understood, good flame patterns are in part determined by proper porting. The number and size of ports neces- sary to give proper flame characteristics must be calculated.