ASHRAE Journal - August 2014 - 71

Column Hvac applications

Fan Powered Terminal
Unit With ECM Motor

Sensible Cooling Coil
Optional MERV 8 Filter

Chilled Beam

Perimeter Slot Diffuser
DOAS
Fan Powered Terminal
Optional Hot Unit With ECM Motor
Water Heat

Hot & Cold Water

Standard Ceiling Diffuser
Figure 1 Chilled fan powered terminal unit.

these new systems cannot be realized nor operated in
economizer mode.

DOAS Unit: What It Is
Having already been used in a number of projects,
including the Pentagon, a variant of the series fanpowered terminal unit has been able to effectively provide control of dynamic ventilation requirements while
offering additional energy savings. The modified unit
is the same as the regular series fan-powered terminal
unit, with the exception of a cooling coil added to the
induction opening (to handle sensible as opposed to
latent loads). Once installed, the water temperature just
needs to be controlled so that it stays above dew point. In
fact, the mechanics of the DOAS fan-powered terminal
unit are not that different from induction terminal units
under windows or chilled beams in the ceiling.
When the unit is specified with an electrically commutated motor (ECM) and a VAV damper (usually smaller
than with a standard design) that is connected to DOAS
ductwork system, it can offer a number of additional
control strategies that are not available with other types
of air delivery systems. Such strategies can be employed
to produce the best combination of airflows and discharge temperatures to meet both comfort and aesthetic
requirements. While an ECM typically requires an analog signal for setting the airflow rate, there are many low
cost, direct digital control (DDC) suppliers that can provide combinations of digital and analog outputs that can
be easily configured.
Some may see similarities between the DOAS terminal unit and a fan coil, but the variable DOAS damper
control and ECM variable fan operation makes it quite
different. Another differentiating feature is how the

DOAS
Figure 2 DOAS terminal unit used in perimeter application: Chilled beams placed
within the interior and linear slot diffusers placed on the perimeter.

units are rated. Items such as octave band sound power,
motor amp draw, and other AHRI ratings are not yet
required for fan coils. As with other terminal units, this
certified data provides engineers with the assurance that
their systems will perform as designed.

DOAS Unit Benefits
Many other benefits of a DOAS terminal unit include:
* Delivering air by traditional means to several
spaces from a single cold water, power, and DOAS connection simplifies installation and allows for greater
flexibility than alternative systems that require piping
to each zone.
* By using standard, proven, and well-documented
ceiling air outlets, ADPI (occupant comfort) can be
predicted and compliance with ASHRAE Standard
55 (Thermal Comfort) can be proven. In an earlier
article, "High Performance Air Distribution Systems,"
March 2014, we discussed how a properly designed
array of air outlets can manage both sensible and latent loads at far lower room loads and corresponding
air supply rates.
* The large sensible cooling coil (required to both
maximize the output of a coil with 55°F to 60°F [13°C
to 16°C] water and to minimize pressure drop through
the coil), allows the addition of a MERV 8 filter for use
during construction. While there is a LEED point for
installing a filter during construction, it is not required
for later building operations; most designs recommend
its removal after construction is complete.
* There are opportunities to deliver conditioned air to
spaces at a lower installed cost compared to other technologies that are labeled as "energy efficient." (While
dependent on many different factors significant savings
Au g ust 2014

ashrae.org

ASHRAE JouRnAl

71
ASHRAE Journal - August 2014

Table of Contents for the Digital Edition of ASHRAE Journal - August 2014
