ASHRAE Journal - July 2014 - 53

Technical FeaTure
Fea

Ventilation Benefit of DOAS
To realize the ventilation benefit of DOAS the OA must
be supplied to each occupied space, or either directly, or
in conjunction with, local units. The details of different
configurations are further discussed in the following
sections of this article.
One of the difficulties of achieving good ventilation
performance with central HVAC systems (e.g., VAV systems) serving multiple zones is that the individual zone
sensible loads do not necessary vary with their ventilation requirement. Therefore, increased system total
ventilation intake airflows which are often required to
ensure the proper ventilation for each zone at all operation conditions is in compliance with ASHRAE Standard
62.1.8
Another difficulty is over-ventilation of non-critical
zones. In a multiple-zoned recirculating ventilation
system, a single air handler supplies the mixture of OA
and recirculated return air to more than one ventilation
zone. Since the system delivers the same air mixture to
each zone; proper ventilation to the critical zone (the
zone requiring the highest OA fraction) will generally
result in over-ventilation for other zones.1
When DOAS supplies required ventilation air to each
zone, either directly or in conjunction with local unit(s),
such system configurations overcome the previously
listed problems. Allowing the DOAS to be sized separately ensures proper ventilation in each zone at design
zone population.8

Humidity Control Benefit of DOAS
There is also humidity control benefits with a DOAS
approach, albeit a secondary goal. The various humidity control benefits can only potentially be achieved if
the described system configuration associated with each
benefit is applied.
Since the 1989 edition of ASHRAE Standard 62
increased earlier mandated OA requirements, proper
indoor humidity control is even harder to achieve with
some types of HVAC systems, because bringing more
OA into the building can sometimes result in raising
indoor humidity levels in non-arid climates.10 Also,
this requirement brings more moisture into the system since OA accounts for a large portion of the overall
latent load in most commercial buildings.8 Rather than
using one system to control both indoor humidity and
temperature, DOAS uses two systems to control them

separately.11 The DOAS unit (the unit conditioning the
ventilation air separately) can be sized to supply the
exact amount of required ventilation air at a dew point
low enough to also offset the indoor latent load, thus
controlling the indoor relative humidity level without
relying on local HVAC units. When humidity control
becomes a goal of DOAS, a total-energy recovery device
is generally required by ASHRAE Standard 90.1. In locations with cold winter, a pre-heat coil is suggested near
the OA intake for frost prevention at the total-energy
recovery.
Humidity control requires engineers to pay more
attention to part-load operation. Popular VAV systems
usually reduce the amount of total air delivered to the
space during part-load conditions to achieve energy
efficiency. However, when the space loads change,
this can result in loss of indoor humidity control, even
though indoor space temperatures remain at acceptable levels. DOAS overcomes this problem. During
part-load conditions, with local unit(s) running
under reduced capacity mode to fulfill the reduced
sensible loads, the DOAS unit continues to supply air
at a low enough dew point to maintain acceptable
indoor humidity levels. This humidity control benefit
is potentially available for almost all DOAS system
configurations.

Other Benefits of DOAS
DOAS also may allow broader local unit selection
options when the DOAS unit alone controls the indoor
humidity and supplies OA to the occupied spaces, either
directly or in conjunction with local HVAC unit(s). Also,
if the DOAS unit controls all of the latent loads, the high
efficiency sensible-only HVAC unit may be effectively
used as the local HVAC unit. Using high efficiency local
HVAC equipment can potentially increase the overall
system energy efficiency.2
Because a DOAS approach generally results in conditioning less OA than with a VAV system approach, there
is an increased potential to reduce energy usage associated with the ventilation air components, air conditioning, and fan operation. Under part-load conditions, local
unit(s) may be able to switch off to save energy, when the
DOAS delivers ventilation air directly to each zone or to
the supply-side of local HVAC units.
There is a view among certain professionals that a
key fundamental change has to be made to achieve
J u ly 2 0 1 4

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ASHRAE Journal - July 2014

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