ASHRAE Journal - August 2022 - 20

TECHNICAL FEATURE
Condensing things slightly, we now use the highest
zone fraction value from earlier, that is, the one corresponding
to the critical zone, to calculate Ev, the system's
ventilation efficiency:
Ev = 1 + Xs - Zp
This will be a value between zero and one. It represents
how well the system is distributing the outdoor air that
it is bringing in, with a value of one implying perfect
ventilation efficiency. Finally, the required outdoor air
intake volume is found by dividing the uncorrected
value by the efficiency:
Vot = Vou/Ev
This is the minimum amount of outdoor air that
must be introduced at the air handler according to the
standard, though more than this amount is permissible,
such as when economizing. This set of equations
supports two use cases. First, system designers may use
this calculation process with conservative assumptions
for the purposes of system sizing, and second, it can be
implemented into the control scheme to use real-time,
zone-level data to provide ventilation in accordance
with what is occurring within the building during
operation. Such real-time reset of ventilation air is now
required prescriptively by Standard 90.1-2019 for VAV
systems.4
Minimizing Outdoor Air Requirements
Given that outdoor air is expensive to condition, it is
desirable to minimize the volume required. Walking
backward through the ventilation equations shows us
how. To start, we can decrease Vot by either lowering Vou,
or by raising Ev. The value of Vou can be adjusted according
to the principles of demand-controlled ventilation
(DCV). This involves monitoring or estimating the
population in the zones using equipment such as CO2 or
occupancy sensors (see RP-1547, for example5), which
can allow ventilation requirements to be lowered, even
to zero.6,7 These principles are well-known, and adding
such sensors has a rapid payback.
Instead, let us focus on increasing Ev to get it closer
to a value of one, since this is applicable regardless of
whether DCV is present in the system. We can do this
by raising Xs or by reducing Zp. First, Xs can be raised
20
ASHRAE JOURNAL ashrae.o rg
A U G UST 2022
TABLE 1 An increase in discharge airflow to the critical zone here results in a
15% reduction in the outdoor air requirement.
ZONE NAME
Voz, MINIMUM VENTILATION (cfm)
Vdz, DISCHARGE AIRFLOW (cfm)
Zpz, ZONE FRACTION
CRITICAL ZONE
Zp, ZONE FRACTION OF CRITICAL
ZONE
Vou, UNCORRECTED OUTDOOR AIR
REQUIREMENT (cfm)
Vps, SYSTEM PRIMARY AIRFLOW
(cfm)
Xs, AVERAGE OUTDOOR AIR
FRACTION
Ev, VENTILATION EFFICIENCY
Vot, OUTDOOR AIR REQUIREMENT
(cfm)
A
INITIAL STATE
B
C
IMPROVED EFFICIENCY
C
A
B
200 300 200 200 300 200
400 900 1,000 600 900 1,000
0.5
0.33
A
0.5
700
2,300
0.30
80%
870
0.2
0.33 0.33
A & B
0.33
700
2,500
0.28
95%
739
slightly by reducing the discharge airflow in any of the
non-critical zones, since doing so will shrink its denominator
without affecting the numerator. The second,
better option is to reduce Zp by increasing the discharge
airflow in just the critical zone. Increasing the discharge
airflow to the critical zone can dramatically reduce the
outdoor air requirement. This fact is even called out in
an informative note in Appendix A of the standard. To
illustrate, Table 1 shows an example for a simple system,
where providing an additional 200 cfm (94.4 L/s) of
discharge airflow to the critical zone results in a 131 cfm
(61.8 L/s) drop in the outdoor air requirement. Readers
are encouraged to recreate this example using numbers
from their own systems.
This example can be taken even further. For instance,
once the zone fraction of Zone A has been reduced to
0.33, both Zones A and B together become the critical
zones. To continue increasing the ventilation efficiency
of the system then, they can have their discharge airflows
increased simultaneously, such that their zone
fractions drop at the same rate. Once they reach zone
fractions of 0.2, Zones A and B will have discharge airflows
of 1,000 cfm and 1,500 cfm (492 L/s and 708 L/s),
respectively, and the system would have a ventilation
efficiency of 100%. At this point, it would only require
700 cfm (330 L/s) of outdoor air.
Diminishing returns to this process exist, however.
Going from the state of improved efficiency to one of
0.2
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ASHRAE Journal - August 2022

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