ASHRAE Journal - December 2013 - 16

technical feature

inside the mixing box is not as negative
Figure 4 Conditioned OA delivered directly to each zone.
when the terminal fan slows to reduce supply airflow; so outdoor airflow will decrease.
dedicAted OA unit
An added challenge is that outdoor air is
OA
delivered to this mixing box by a separate
cA
fan in the dedicated OA unit, and the static
eA
pressure inside the ventilation system ductwork is influenced by the changing operasA
cA
cA
sA
teRminAl
RA
tion of all the terminal unit fans served by
unit
that dedicated OA system.
teRminAl units cAn OpeRAte with vARiAble
Direct OA flow measurement and conspeed fAn cOntROl, withOut impActing
OutdOOR AiRflOw deliveRed tO the zOnes.
trol. One approach to address this challenge
(pRessuRe-independent dAmpeRs might
could be to install pressure-independent
still be used tO implement dcv).
teRminAl unit
dampers (such as VAV terminals) in the OA
ductwork to ensure that the required outRA
door airflow is delivered to each zone (Figure
3). As the various terminal fans (or the fan
inside the dedicated OA unit) change speed,
this damper modulates to maintain the same quantity of dampers could be used to implement demand-conoutdoor air delivered.
trolled ventilation and reduce outdoor airflow delivered
Deliver OA directly to each zone. The approach preto a zone during periods of reduced population.
ferred by this author would be to deliver the conditioned
This "direct-to-the-zone" approach also affords the
1,2 In this
outdoor air (CA) directly to each zone (Figure 4).
opportunity to deliver dehumidified outdoor air at a
case, the terminal unit conditions only recirculated air
cold temperature, rather than reheated to neutral. This
(RA). Since the outdoor air is not distributed through
has several installed cost and energy-saving benefits (see
the terminal unit fan, it can operate with variable speed sidebar, "Cold Versus Neutral Air Delivery").
fan control (or even cycle off when the zone requires no
Demand-Controlled Ventilation
cooling or heating), without impacting how much outThe control strategies discussed thus far attempt to
door air is delivered to the zone; and pressure-indepenensure that code-required outdoor airflow (design
dent dampers may not be necessary.
outdoor airflow) is delivered to the zone as the termiWhile maybe not necessary in all cases, HVAC design
engineers may still choose to include pressure-indepen- nal fan speed changes. In some zones, demand-controlled ventilation (DCV) might be used to reduce outdent dampers in the dedicated OA system to allow outdoor airflow during periods of reduced population, in
door airflow to be shut off to zones that are unoccupied
an effort to reduce energy use associated with cooling
while other zones are still occupied. In addition, these

Conditioned OA Delivered to the Open Ceiling Plenum, Near Each Terminal Unit
In some DOAS projects, the conditioned outdoor air
is ducted to discharge into the ceiling plenum, near the
intake of the terminal unit, where it mixes with recirculated air before being drawn in through the intake.
This approach avoids the cost and space needed to
install additional ductwork, separate diffusers, or mixing boxes, but it makes it more difficult to ensure that
the required outdoor airflow reaches each terminal unit.
In addition, the conditioned outdoor air may not be able
to be delivered at a cold temperature, due to concerns
16

ASHRAE JouRnAl

ashrae.org

D ecem ber 2013

over condensation within the ceiling plenum.
While changing the terminal fan speed may not have
much effect on the static pressure in a large, open ceiling plenum, balancing the system to ensure that the
required outdoor airflow reaches each terminal unit gets
complicated when lots of terminal units, each equipped
with variable speed fan control, are drawing air out of the
same ceiling plenum. Pressure-independent dampers
in the OA ductwork are likely needed to ensure that the
required outdoor airflow reaches each terminal unit.

ASHRAE Journal - December 2013

Table of Contents for the Digital Edition of ASHRAE Journal - December 2013