A double duct system generally consists of a
blowthrough fan unit discharging filtered air
through stacked or adjacent heating and cooling
coils into separate plenums and ductwork with
thermostatically controlled mixing dampers at
various room locations.
The inherent advantage of a double duct
system is that individual room conditions can be
maintained from a central system, within the
limitations of supply air temperatures. This is
done by the blending of hot and cool air through
automatically controlled mixing devices. Another
important credit is flexibility. In this regard, in-
dividually controlled rooms can be easily
incorporated, at modest cost, after the building
In modern buildings of multiple exposures de-
signed for variable functions and changing oc-
cupancy, individual room control is essential and
a double duct system should be seriously
Double duct systems for low pressure are
usually tiered hot and cold ducts within the furred
space. They are generally located above corridors.
The manner of distributing proper temperature air
to the room is through right angle, interlinked
mixing dampers operated by motors controlled
through thermostats. In general, this type of
system uses the same corridor plenum area around
the ducts for conveyance of return or exhaust air.
The residual volume of space left for this purpose
is too often neglected. Inevitably, this results in
insufficient relief for the rooms.
The main disadvantage of a double duct
system is lack of stability of air quantities supplied
to areas (rooms) because of varying duct static
All duct elbows, including supply, exhaust,
and return, should be made with a center line
radius of 1.5 times the duct width, parallel to the
radius wherever possible. In no case should the
center line radius be less than the width of the duct
parallel to the radius. Where space does not per-
mit the above radius, or where square elbows are
indicated on plans, turning vanes of an approved
type should be used.
Additionally, there are numerous adaptations
and modifications of duct systems. Figure 13-1
shows a residential duct system with the furnace
and central air unit located in the basement.
Figure 13-1.Residential duct system.