ASHRAE Journal - August 2022 - 19

TECHNICAL FEATURE
measures, variable air volume systems are somewhere
in the middle. Because of their relative simplicity and
good energy performance, VAV systems are a popular
choice for applications that have many zones, such as
offi ces and academic buildings.
VAV Drawbacks
VAV systems have a couple of limitations, however.
One is that a single air handler is tasked with ventilating
multiple zones within the building. This is problematic
because not all zones are created equal. Some zones,
like lecture halls, have large fl oor areas and high occupancies;
thus, they have high ventilation requirements
when in use, while others may be small single-person
offi ces. Since the outdoor air is evenly mixed into the
supply air, though, a VAV system cannot direct that
ventilation precisely where it needs to go. Inevitably, to
meet the requirements of zones that need a higher fraction
of outdoor air, zones with lower relative requirements
may become overventilated, especially if they
have high sensible loads. This means that more outdoor
air than necessary is cooled and dehumidifi ed, at considerable
expense.
A second drawback is that the air handler can only provide
supply air at a single temperature. This temperature
must be low enough to wring moisture out of humid air.
It also must be low enough to satisfy zones' positive sensible
loads without exceeding their maximum designed
discharge airfl ow rates. Cold supply air leads to another
challenge, though. Some zones may have low or negative
sensible loads. In that case, VAV terminal boxes can
clamp down on how much air they deliver to avoid overcooling
the zones. However, this is limited by ventilation
considerations such that after a certain point, a minimum
amount of airfl ow is provided, and the discharge
air is reheated. (Volume is increased again while reheating
with dual maximum logic.)1
This simultaneous cooling at the air handler and heating
at the terminal boxes represent wasted dollars. One
goal then for an existing VAV system should be to minimize
the impact of these inherent ineffi ciencies by making
improvements to its controls. For instance, supply
air temperature reset attempts to address simultaneous
heating and cooling by seeking a setpoint that balances
both needs.2 Addressing low ventilation effi ciency,
though, requires additional creativity. This will lead to a
solution that can tackle both.
The Ventilation Calculation
Appendix A of ASHRAE Standard 62.1-20193 defi nes
an alternative series of equations for determining how
much outdoor air is required at the air handler, based
on the ventilation requirements and discharge air volumes
at each of the zones. It recognizes that outdoor air
is not precisely distributed, and so some air returning
from overventilated zones can be considered unused
and credited toward the outdoor air requirement as it
makes another pass through the system. The result is
that the outdoor air volume requirement at the air handler
is greater than the sum of the volumes needed by
each zone individually, but the outdoor air fraction can
be somewhat less than the fraction required by the most
demanding zone.
Let's explore the equations to see both why this is, and
how that requirement can be manipulated in accordance
with the standard.
Each zone has a minimum outdoor airfl ow requirement,
Voz, based on parameters such as occupancy and
fl oor area. The discharge airfl ow to each zone is Vdz, and
dividing these two values gives the primary outdoor air
fraction, Zpz (referenced here simply as zone fraction):
Zpz = Voz / Vdz
If Zpz is ever greater than one, this would mean that the
zone cannot be properly ventilated, as even 100% outdoor
air would not be enough to meet its requirement.
So, Vdz must always be kept greater than or equal to Voz.
Among all the zones, the one with the highest Zpz value is
known as the critical zone, and we call its zone fraction
Zp. Next, summing up all the Voz values
VV
ou =∑
all zones
oz
gives the uncorrected outdoor intake volume, Vou. If
the system could perfectly distribute its ventilation air,
this would be the total amount of outdoor air it would
need, but since it cannot, the fi nal corrected value will
be higher. Likewise, summing all the discharge air values
gives Vps, the system primary airfl ow:
VV
ps =∑
all zones
dz
This is the total amount of supply air fl owing through
the system. Dividing these latest two values gives the
average outdoor air fraction:
Xs = Vou / Vps
A U G U S T 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
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ASHRAE Journal - August 2022

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