WARM-AIR HEATING SYSTEMS
Learning Objective: Identify the different types of
warm-air systems, gas-fired and oil-fired furnaces,
components, controls, and the procedures for
installation, operation, and maintenance.
Heating equipment for complete air-conditioning
systems is classified according to the type of fuel
burned, the Btu capacity of the furnace, and the method
of circulating the warm air. Warm-air systems are
generally identified as either a gravity-type or a
forced-air type system.
Gravity furnaces are often installed at floor level.
These are really oversized, jacketed space heaters. The
most common difficulty experienced with this type of
furnace is a return-air opening of insufficient size at the
floor. Make the return-air opening on two or three
sides of the furnace wherever possible. Provide heat
insulation above the furnace top to avoid a possible fire
Gravity warm-air heating systems operate because
of the difference in specific gravity (weight) of warm
air and cold air. Warm air is lighter than cold air and
rises when cold air is available to replace it.
The majority of the furnaces produced today are of
the forced warm-air type. This type of furnace includes
the elements of a gravity warm-air system plus a fan to
ensure adequate air distribution. It may include filters
and a humidifier to add moisture to the air. The
inclusion of a positive pressure fan makes possible the
use of smaller ducts and the extension of the system to
heat larger areas without the need for sloping ducts. It
is possible to heat rooms located on floors below the
furnace if necessary. Forced-air furnaces are
manufactured in a variety of designs. A typical
oil-fired furnace is shown in figures 4-15 and 4-16. A
typical gas-fired furnace is shown in figure 4-17.
In a forced-air system, the fan or blower is turned
on and off by a blower control which is actuated by the
air temperature in the bonnet or plenum. The plenum is
Figure 4-15.Cutaway view of a typical oil-designed furnace.