ASHRAE Journal - June 2021 - 67

COLUMN HVAC APPLICATIONS
well regarding daily maintenance tasks, infi ltration,
source control and introduction of additional airborne
particulates that may compromise indoor air quality and
reduce the ability to disinfect. A common term used to
describe these return air systems is " wild, " since they are
so diffi cult to control.
With a ducted return-instead of a ceiling or attic
plenum- a testing, adjusting and balancing (TAB) technician
has the capability to maintain the duct as well
as adjust room pressures, modify airfl ow patterns and
stay safe from exposure to biologically active pathogens
within the airstream and on the surfaces around them.
Besides providing a level of thermal control, return
ducts also help avoid reducing building occupants'
immune responses by providing conditioned air at a
controlled indoor temperature and relative humidity
optimal for occupant health.
ASHRAE reported in an April 2020 position document
that viruses such as the one that causes COVID-19 can
spread through the air in two ways: larger droplets can
travel 6 ft to 7 ft (1.8 m to 2.1 m) before dropping to the
ground, but smaller droplets can mix with room air,
remaining airborne for extended periods of time.2
While a ducted return can be cleaned and disinfected
using established methods, a ceiling plenum cannot, leaving
facilities personnel working inside it at risk of exposure.
" Airborne transmission of SARS-CoV-2 is signifi cant
and should be controlled, " ASHRAE released in a statement.
" Changes to building operations, including the
operation of heating, ventilation and air conditioning
systems, can reduce airborne exposures. " 3
Because of this, ceiling plenums are a health concern
for anyone working inside them. They also limit the
ability to create pressurized rooms during a pandemic,
introduce potential infi ltration, reduce source control
and lack the ability to be cleaned or disinfected.
During a pandemic, when medical offi ce buildings,
wellness centers and school nurse stations are being
transformed into alternate care sites for COVID-19
patients, a TAB technician has to be able to adjust a room
between positive, neutral or negative pressures. Return
ductwork allows room pressure differentials to be accurately
adjusted to maintain a consistent setpoint. Ceiling
plenums simply cannot be controlled in this manner.
Ceiling plenums also represent a hazard besides that
posed by COVID-19. In buildings constructed before
1980, asbestos, polychlorinated biphenyls and dust and
particulates containing heavy metals may be among
the hazardous materials introduced to workers inside
ceiling plenums. Ceiling plenums also can have microorganisms
develop over time through leaks and condensation
from uninsulated pipe or duct. Any maintenance
activities-even by a worker in proper protective gear-
can agitate particles and fungal spores that can enter the
airstream.
Given the health and safety benefi ts of return ductwork,
ASHRAE Technical Committee 5.2 supports the
white paper's recommendation that ducted return systems
be used in all new buildings, core renovations and
additions as prioritized by occupancy classifi cation and
use.
Due to the urgency to provide health-care environments
that do not increase the spread of COVID-19,
medical facilities are recommended as the fi rst priority
to install ducted returns for any new construction, core
renovations and building additions. Education facilities
are recommended as a second priority, due to the
volume of occupants that would use those facilities.
Assembly areas, business, factories, commercial and
residential facilities are third priorities.
Some benefi ts exist to a ceiling plenum return, such as
energy effi ciency and a reduction in labor and materials
costs. The decrease in HVAC system costs is due to the
lack of return ductwork, including labor, time and materials.
Compared to a ducted return, plenum returns can
result in reduced fan energy consumption due to lower
plenum return resistance, as well as with the use of passive
relief or relief fans instead of return fans.4
The need to balance these benefi ts with health and
safety concerns has never been more important than
it is today. Adoption of these recommendations will
address this need, making buildings more resilient to
biological and indoor environmental hazards.
References
1. ASHRAE. 2021. " Return Ductwork Requirement for Airborne
Pathogens Through the Airstream. " ASHRAE TC 5.2 White Paper.
https://tinyurl.com/cmhwksh3
2. ASHRAE. 2020. " ASHRAE Position Document on Infectious
Aerosols. " ASHRAE. https://tinyurl.com/vhr2jes8
3. ASHRAE. 2021. " ASHRAE Epidemic Task Force Releases
Updated Airborne Transmission Guidance. " ASHRAE. https://
tinyurl.com/ecjd99bs
4. Kettler, J. 2004. " Return fans or relief fans. " ASHRAE Journal
46(4):28 - 33.
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https://www.tinyurl.com/cmhwksh3 https://www.tinyurl.com/vhr2jes8 https://www.tinyurl.com/ecjd99bs https://www.tinyurl.com/ecjd99bs https://www.ashrae.org/
ASHRAE Journal - June 2021

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