ASHRAE Journal - December 2023 - 35

TECHNICAL FEATURE
for Interpretation (IC-170-2013-6), advising that
designers could assume an area ventilation rate (Ra)
to use the Standard 62.1 Ventilation Rate Procedure.
As shown in Figure 1, the VRP in Standard 62.1 has
three corrections:
1. Ventilation Effectiveness (Ez): An adjustment for
how well ventilation is distributed in the zone itself.
2. Diversity (D ): Accounts for the fact that there may
be fewer occupants in the area served by a ventilation
system than the sum of all individual zones.
3. Ventilation Effi ciency (Ev): An adjustment based
on how well a system is distributing ventilation to all
the zones. (Note: As of the 2019 version of Standard
62.1, Ev is calculated from the diversity [D]).
As of the 2019 edition of Standard 62.1, all
ventilation rates calculated based on " a standard
other than Standard 62.1 " needed to be considered
as the uncorrected outdoor air intake fl ow or Vou.
(This requirement is in clause 6.2.4.1.3 in Standard
62.1-2019).
All these requirements created confusion for
designers attempting to use Standards 62.1 and 170 on
the same system. Since it was fi rst published in 2008,
Standard 170 requires any of its outdoor airfl ows be
considered as Voz; thus diversity (D) would be applied
to the result. Adding further confusion, the standard
did not stipulate ventilation rates per person, so how
diversity was supposed to be incorporated was not clear.
In contrast, the 2019 and 2022 versions of Standard 62.1
require the same airfl ow to be considered as Vou; thus,
diversity (D) should not be applied.
To resolve this confusion, a working group of various
SSPC 62.1 and SSPC 170 members was considered
in 2020. The group convened in early 2021 and met
monthly throughout 2021 with constant reporting
back to both committees.
Considered Methods
The working group considered three possible
methods for doing a combined Standard
62.1/Standard 170 ventilation calculation.
* Method 1: Combined Diversity Ev. This method
looks at the system as one. Designers obtain occupant
diversity (D) by looking at Standard 62.1 and
Standard 170 spaces together and derive a single Ev,
which is applied to both Standard 62.1 and Standard
170 spaces.
FIGURE 1 The existing confusion in doing combined Standard 62.1/Standard 170
calculations.
Vbz = Ra × A + Rp × P
How well is air distributed
in the zone? Ventilation
Effectiveness (Ez) Table in
Standard 62.1
Voz
Diversity in Occupants
Vou = DΣall zones (Rp × Pz) +
Σall zones (Ra × Az)
Vou
How well is air distributed
by the system? Ventilation
Effi ciency (Ev) (Ev is, as of
2019, calculated out of
occupant diversity)
Total Required OA
* Method 2: Individual Additive Ev-less for 170. In
this method, no Ev is applied to Standard 170 spaces.
Designers apply an Ev correction for Standard 62.1
spaces. Then, they simply add the outdoor airfl ow
from Standard 170 spaces to the result.
In equation form, System OA fl ow = OA(Vou62.1)/
Ev62.1 + OA170) .
* Method 3: Most Conservative Ev . This method
looks at occupant diversity for Standards 62.1 and 170
separately. Designers obtain an Ev for the Standard
62.1 spaces and Ev for the Standard 170 spaces; then
they use the most conservative of the two values for
the entire system.
The Selection
The working group evaluated the pros and cons of
each method. The SSPC 170 group provided two major
items of feedback:
1. Since the outdoor air requirements in Standard
170 are volumetric-based (air changes) rather than
people or area based, diversity (D) cannot be applied
to any ventilation rate calculated per Standard 170.
There is no diversity in building volume.
2. Standard 170 does not consider either ventilation
effectiveness (Ez) nor ventilation effi ciency (Ev ).
The group concluded that both Methods 1 and
3 would require determining diversity (D) and
applying ventilation effi ciency (Ev ) to outdoor airfl ow
D E C EM B E R 2 0 2 3 ashrae.org ASHRAE JOURNAL
35
Standard 170-2017
Par.7.1.a.6.ii requires
Standard 170's
OA rates to be
considered as Voz
Designer OK to
assume Ra based on
170 Interpretation
IC-170-2013-6
Standard 62.1-2019
Par.6.2.4.1.3 required
adding non-Standard
62.1 airfl ows here
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ASHRAE Journal - December 2023

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