Figure 4-35. - An electric switch type of relay.

Figure 4-36. - Cutaway view of a typical humidifier.

and radiating chamber, and the blower compartment. Figure 4-37 shows a cutaway view of a typical oil-fired furnace.

Like gas-fired units, oil-fired units are also available with various airflow designs. The model shown in figure 4-15 is designed with both the return-air inlet and the warm-air outlet in the top. More compact models (fig. 4-37) are available with the return-air inlet at the side or bottom below the radiating and combustion area. The warm-air outlet is at the top.

A floor furnace is shown in figure 4-38. This type of oil-fired unit is smaller, lighter in construction, and is designed to be hung from the floor of the space served. Only a minimum of clearance is required below the floor.

Oil burners may be separated into various classes, such as domestic and industrial. Since domestic oil burners are used almost universally in warm-air furnaces, they are the only ones covered in detail in this chapter.

Domestic Oil Burners

Domestic oil burners atomize the oil and are usually electrically power driven and are used in small central heating plants. They deliver a predetermined quantity of oil and air to the combustion chamber, ignite it, and automatically maintain the desired temperature.

Domestic oil burners are classified according to various methods, none of which is entirely satisfactory because of the overlapping among a great number of models. Classification may be by type of ignition, draft, operation, method of oil preparation, or features of design and construction.

DESIGN AND CONSTRUCTION. - One of the most common types of domestic oil burners is the pressure-atomizing gun type of burner. Gun type