ASHRAE Journal - November 2022 - 49

TECHNICAL FEATURE
the inadequacy of lower rates, so the 1989 edition
increased offi ce ventilation rates to 20 cfm/person (10
L/s·person).3,4 Subsequent editions of 62.1 have required
roughly the same rates, albeit calculated somewhat
differently.5,6
Underventilated Offi ce Buildings
About 164,000 U.S. offi ce buildings in use today were
constructed in the 1980s, representing 17% of all offi ce
buildings.7 While many have been renovated over the past
40 years, a signifi cant number still rely on constrained
ventilation systems that fall far short of today's standards.
Many ventilation-constrained buildings in the previously
referenced study2 had similar system confi gurations,
with a central OA shaft providing minimum ventilation
air to air-handling units (AHUs) on each fl oor
(Figure 2). In some buildings, OA is simply induced by the
AHUs; in others, fans pressurize the
OA shaft.
Increasing Ventilation in Constrained
Systems
Increasing the amount of OA in
a ventilation-constrained building
presents several challenges.*
Potential solutions range from
equipment replacement to full
renovation of the ventilation system.
These modifi cations require signifi
cant capital investment and will
typically increase ongoing energy
use and operating costs. Options
generally fall into three categories:
upsizing the existing equipment,
splitting into high-rise and low-rise
systems or an incremental fl oor-byfl
oor approach.
FIGURE 1 A history of ASHRAE Standard 62.1 minimum ventilation rates for
offices, based on default occupant densities, with a significant reduction to
5 cfm/person from 1981 to 1989. While the calculation methods changed in later
editions (and expanded to two procedure options in 2019), the effective ventilation
rates remain roughly the same since 1989.3-6
30
25
20
15
10
5
Standard 62.1-2019
Standard 62-89
Standard 62-73
Standard 62.1-2004
1965
Standard 62-81 (Nonsmoking)
1975
1985
15
12
9
6
3
1995
2005
2015
2025
FIGURE 2 Typical constrained ventilation system
configuration for 1980s buildings.2
OA Fan, OA AHU,
or OA Intake
AHU
AHU
AHU
AHU
AHU
AHU
Other Capacity Considerations
For many retrofi t options, additional OA will require
more heating, cooling and exhaust air (EA) capacity. In
some cases, the changes to OA may incorporate energy
recovery (whether implemented by choice or required by
energy codes), which will lower the additional cooling and
heating loads imposed by the additional outdoor air. If the
existing central plant equipment is suffi ciently oversized,
then the retrofi t evaluation can focus on the ventilation
system alone. Otherwise, chillers, cooling towers, boilers
and pumps all may need to be upgraded, too.
The additional loads (sensible and latent) from OA
may require each AHU to have additional
cooling and/or heating capacity.
Typically, this would mean the replacement
of the AHU cooling coil and/or
the addition of heating capacity on
the fl oor, both of which are expensive
options. Instead, a dedicated outdoor
air system (DOAS) could precondition
OA, which would require more fl oor or
roof space and new electrical service
but adds fl exibility.
More airfl ow in requires more airfl ow
out, so in addition to increased thermal
capacity, the building will need provisions
for increased exhaust. Similar
approaches to the ventilation air can
be used: upsizing equipment, splitting
high- and low-rise or incremental
fl oor-by-fl oor changes.
Upsized Equipment
A system that requires a modest increase in OA may
be upgraded simply by increasing the existing OA fan's
*Small adjustments are unlikely to bring systems designed to meet
the nonsmoking rate of Standard 62-1981 into compliance with
Standard 62.1-2019; the case studies presented here need at
least twice their current ventilation rates. However, the rest of the
building stock likely has many low-/no-cost options for increasing
ventilation if systems fall short of today's standard, such as
increasing OA damper positions or changing OA fan setpoints to
meet the current ventilation standard, while still meeting most, if
not all, cooling and heating loads.
N O V E M B E R 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
49
cfm Per Person
L/s Per Person
https://www.ashrae.org/
ASHRAE Journal - November 2022

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