ASHRAE Journal - December 2021 - 23

COLUMN HVAC APPLICATIONS
makes it a series air-handling system, not a parallel system,
and thus not a DOAS or a DOAU.)
Second-generation DOAU/DOAS helped meet the
requirements found in the 2007 to 2013 range of
ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable
Indoor Air Quality, and ANSI/ASHRAE/IES Standard 90.1,
Energy Standard for Buildings Except Low-Rise Residential
Buildings.
Third-generation DOAU is a single unit with the components
needed to meet the latest ventilation requirements
in the ventilation Standard 62.1 and the energy
Standard 90.1.
Third-generation DOAS/DOAU controls ventilation,
humidity and building pressurization and is often
today's DOAS system of choice. This DOAU typically has a
supply and exhaust fan, fi ltration, dampers, cooling and
heating components, parallel air-to-air recovery device
and a packaged unit control all in one unit.
Additional components can be added to this thirdgeneration
DOAU to further reduce energy and/or
handle specifi c outdoor air requirements. These components
may include-but are not limited to-compressors,
condensers, series or additional parallel desiccant components
around the cooling coil, VAV fan operation, air
cleaning beyond fi ltration and pumps.
Third-generation DOAU has three basic functions:
* Provide clean ventilation air to building spaces;
* Control the building dew point to the 60°F (16°C)
maximum as required in the latest version of Standard
62.1; and
* Control building pressurization for envelope infi
ltration control and proper operation of the air-to air
exchanger and remote exhaust.
The shift to third-generation units occurred when
ASHRAE Standard 62.1-2019 required mechanically
cooled spaces to limit the indoor humidity to a maximum
dew point of 60°F (16°C) during both occupied
and unoccupied hours whenever the outdoor air dew
point is above 60°F (16°C). The dew point maximum is
intended to help maintain good indoor air quality and
minimize, if not eliminate, mold damage.
In summary, to control space dew point, a thirdgeneration
DOAU needs to clean and dehumidify the
outdoor air and, ideally, offset indoor internally generated
humidity loads and control space pressurization.
Sensible heating/cooling (H/C) units installed in the
Three Generations of DOAU
First Generation (Pre-1990)
* Makeup air unit with optional series heat transfer
component around the cooling coil to provide
enhanced dehumidifi cation.
Second Generation (Popular from 1990 - 2015)
* Outdoor air unit with an exhaust air recovery
component and a system exhaust fan.
* Can signifi cantly reduce system energy use and
meet requirements from earlier editions of Standard
62.1 and Standard 90.1.
Third Generation (2013 - Present)
* Has a supply and an exhaust fan, fi ltration,
dampers, parallel energy recovery (air-to-air) recovery
device, optional cooling and heating components
and unit control in one unit.
* Controls building ventilation, humidity and
building pressurization. Has the components
needed to meet the latest ventilation requirements
in Standard 62.1 ventilation and energy standards in
Standard 90.1.
space cool or heat the space to the right temperature
level.
Some DOAUs can be certifi ed under ANSI/AHRI
Standard 920, 2015 Standard for Performance Rating of
DX-Dedicated Outdoor Air System Units. If the unit is not
AHRI certifi ed, the components inside the DOAU can be
and should be individually AHRI certifi ed. Properly confi
gured, commissioned and operating DOAS/DOAU can
be a key part of a high-performance building.
More Resources
Resources are available to learn more about DOAS and
DOAU in their previous and current iterations.
These resources include ASHRAE's Advanced Energy
Design Guide-Achieving Zero Energy series; 2020 ASHRAE
Handbook-HVAC Systems and Equipment, Chapter 51,
" Dedicated Outdoor Air Systems " ; ASHRAE Learning
Institute course, " Latest in High Performance Dedicated
Outdoor Air Systems (DOAS) " ; and ASHRAE eLearning
course, " Essentials of Dedicated Outdoor Air Systems
(DOASs). "
D E C E M B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
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ASHRAE Journal - December 2021

Table of Contents for the Digital Edition of ASHRAE Journal - December 2021
