Figure 4-16.Cutaway view of a horizontal stowaway oil furnace.
Figure 4-17.Gas-fired vertical warm-air furnace.
that part of the furnace where it joins the main trunk
duct (fig. 4-18). The blower control starts the fan or
blower when the temperature of the heated air rises to a
set value and turns the fan or blower off when the
temperature drops to a predetermined point. Thus the
blower only circulates air of the proper temperature.
A knowledge of air distribution principles is
important when dealing with central warm-air heating
systems. Satisfactory heating from warm-air systems is
absolutely dependent upon proper distribution of warm
air from the heat source to all portions of the space
served. Warm air must be distributed in quantities that
are required to offset the rate of heat released to each
room. With radiator systems, distribution is primarily a
problem of getting enough hot water or steam to each
radiator to be sure the radiator heats to its rated capacity.
It is not possible to deliver more heat through steam or
hot water than the radiator is designed to transmit. With
warm-air systems, however, the rate of air delivery and
the temperature of the air delivered to the room
determine the amount of heat reaching each room.
Temperature balance, therefore, is primarily a problem
of controlling air distribution.
Factors, such as velocity, volume, temperature,
and airflow direction, play an important part in
temperature balance. In addition and for human
comfort, space-temperature variations and noise levels
must also be considered. Convection currents result
from the natural tendency of warm air to rise and cold
air to fall. Examples are the temperature variations
near doors and windows, and when dense, cool air is
drawn away quicker than warm air. Objectionable
noise will result at supply diffusers if room velocities
exceed 25 to 35 feet per minute (fpm). Air stratification
and cold floors may also result when supply diffusers
are not properly located within the space.