APPENDIX C (Continued)
exterior spaces with fan-powered boxes, the location depends on
than 250 Btuh per linear foot of exterior wall (including
infiltration), the diffuser should again be located centrally and
blow in all directions. If the heating load is between 250 and
400 Btuh per linear foot of exterior wall, the diffuser should be
located near and parallel to the exterior wall and blow
horizontally back into the room.
b) Diffusers which have operable internal dampers for
varying airflow should not be used in lieu of conventional
duct-mounted terminal units in a VAV system design.
Locate the static pressure sensor, for modulating fan
capacity, out in the supply duct system, not at the fan
discharge. Expect some field adjustment to be required to find
the best location. In many systems, the first location is
two-thirds the distance from the supply fan to the end of the
main trunk duct. Sensors shall have proper static sensing
Provide protection against overpressurization of the
supply duct. This should be accomplished by a high limit duct
static pressure sensor located at the fan discharge. This sensor
should turn off the supply fan if the duct static pressure rises
above setpoint and require manual reset of the supply fan.
Balancing dampers should not be necessary for VAV
systems. If the supply ductwork is designed properly using the
static regain method (refer to par. C-2.05) and VAV terminal
units are properly sized/selected (refer to par. C-2.11) and set
Use round ducts wherever space availability permits.
Round ducts are acoustically superior to rectangular ducts and
normally cost less. In high velocity systems, the additional
friction losses of duct walls and balancing dampers of
rectangular ducts cause the system to be inherently less stable.
Round ducts also produce less noise than rectangular ducts.
To save construction expense, it is now common to serve
ceiling outlets with short runs of flexible duct. Limit the
length of flexible duct and avoid bends to minimize duct friction
drop. Figure these pressure drops, and do not show longer runs