registers in the wall.) The air, after circulating
are shown in . The split tee  is used to direct
through the room and losing heat, is returned to the
the flow of air on the warm side of the system. On
heat plant via the cold-air return registers and ducts.
the cold-air return, the straight tee  may be used.
The return-air register is placed in the wall just below
Truck duct takeoffs are shown in  and . In the
the window and the return-air duct is installed in the
double branch connection, less air is present in the
crawl space. The warm-air distribution via the
main duct after some of the air has been channeled
branch ducts to the other rooms of the building would
into branch ducts. Therefore; the size can be reduced
be the same as the examples.
after the connection. The single branch connection
shows two methods of reduction. The first occurs at
The comfort zone
the connection in a horizontal direction; the second is
b. Design Principles.
concept is the basis for all heating design. The
effected by a vertical reduction in depth. In both
comfort zone is defined as the horizontal area from
double and single branch take-off the branch
the top of an average man's head to his knees. It is
connections form a natural air scoop to encourage
apparent that if the air from the supply registers were
air-flow in the desired direction. A boot fitting from
blown directly on a person, he would be very
branch to stack, the stack terminating at a warm-air
uncomfortable. To avoid this, the registers are placed
register is illustrated in  figure 4-10. Using a boot
either above or below the comfort zone; i.e., high on
is one method of changing the shape of a duct
the wall or in the baseboard. Warm-air systems are
without changing its equivalent cross section area or
laid out so that the warm air from the registers is
constricting the flow of air.
directed at the cold exterior walls. Therefore, the
warm-air registers are placed on interior walls or
c. Warm-Air Heating System Plans. Warm-
ceilings. The registers for the cold air return are
air ducts are indicated in the heating plan by solid
always located at baseboard height. The reason for
lines. Cold-air return ducts are indicated by dashed
this location is probably obvious to you. Cold air is
lines. See figure 4-11. Note that all the duct sizes
heavy and collects at the floor of the room; thus, the
are given. The duct sizes are given with the
registers located in the baseboard collect the air. The
horizontal or width dimension listed first. The
cold air is motivated through the return ducts to the
second dimension gives the depth of the duct which
furnace for reheating and recirculation. Furnace
is not shown. On the plan, you can locate the warm-
location is also important to proper warm-air heating.
air registers and obtain the sizes. When ceiling
It is good design policy to locate the furnace room
registers (diffusers) are used, the neck dimensions are
centrally in the building plan to equalize duct lengths.
shown on the plan (as in figure 4-11). When wall or
In addition, the main (trunk) ducts should run above a
baseboard registers are used, face dimensions are
central corridor to equalize branch duct lengths to
given. The height of wall registers above finished
individual rooms. See figure 4-10 for illustrations of
floor line would be given to you in the notes on the
some of the common rectangular duct connections.
plan. Cold-air return registers are shown recessed
Illustration 1 is a typical warm-air bonnet with two
into the wall. The face dimensions of the return
main supply ducts. Two possible elbow connections
register are noted adjacent to the symbol.