ASHRAE Journal - August 2014 - 70

Column Hvac applications
Dan Int-Hout

Variable Volume DOAS
Fan-Powered Terminal Unit
By Dan InT-HouT, FeLLoW aSHRae; anD LILLI WILBaR

ASHRAE, LEED, and local codes advocate, if not require, that energy use in buildings be reduced. Of course, before there can be any marked reductions, there must
first be a point of reference by which to run a comparison. As outlined in ASHRAE/
IES Standard 90.1 for air-distribution systems in large buildings, a traditional overhead system that employs well-mixed VAV terminal units and ceiling diffusers is the
baseline. For smaller or more specialized applications, it could be something entirely
different. However, once established the challenge becomes how to best capture the
efficiencies while satisfying a number of other requirements.
DoaS as a Solution
Engineers, owners, and architects have begun to
consider alternate air-distribution systems (touted as
environment-friendly or green), fearing the obstacles
to achieving efficiencies using the baseline (overhead)
system. These include underfloor air distribution
(UFAD), displacement ventilation, variable refrigerant
flow, water source heat pumps and chilled beams. In
general, these technologies have been popular solutions to address the need for reducing energy consumption, but they do not always take into account
the requirements for increased quantities of outdoor
air. This is why using a dedicated outdoor air system
(DOAS) has become more popular in recent years. It
integrates well with a number of different devices, and
it can help meet the requirements for both ventilation
and humidity, while keeping the quantity of outdoor
air to a minimum (which helps control building energy
consumption).

Today's Building Facts, Standards and Requirements
Of course, when considering any system, much less a
DOAS integration, it is important to understand some
key facts about today's buildings and commonly referenced standards and building codes.
* For any system using overhead heating, one must
comply with the requirements for overhead heating in
Table 6.2, of ASHRAE Standard 62.1, where it outlines
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the requirements for additional ventilation air should
the heated air temperature be more than 15°F (8.3°C)
above the average room temperature.
* ASHRAE research has shown that many office
spaces tend to operate at far lower loads than designed
(6 W/ft2 [0.6 W/m2]), which equates to about 1 cfm/ft2(0.1
W/m2)(a common ratio found with overhead systems
that use 55°F [13°C] supply air).
* Quite often, interior zone loads are so low that the
ventilation rate alone can handle the comfort conditioning of the space, which makes these areas already at
100% outdoor air.
* Minimum ventilation requirements will vary
throughout a space and are likely change over time.
They can go from zero for areas that are rarely occupied
(e.g., mechanical room) to a space that may have a high
occupancy (e.g., classroom).
* Ventilation rates are dynamic and need to be managed, if not also measured. In fact, it is very possible that
in the future, there will be standards that will require
the ventilation rate to be measured. There may even be
building rating system credits offered for doing so.
* Modern buildings designed with alternative airdistribution systems that use traditional design practices
can lead to situations where energy-saving features of
Dan Int-Hout is a chief engineer and Lilli Wilbar is communications manager at Krueger
in Richardson, Texas. Int-Hout is a member of SSPC 55, SPC 129 and consultant to
SSPC 62.1.

ASHRAE Journal - August 2014

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