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performed by different parts of the systems, which
simplifies calculations.
With DOAS ventilators, heat recovery can be provided
internally, qualifying for exceptions from requirements
for outside-air economizers and further simplifying
units and controls. DOAS heat-recovery damper control
including bypass and defrost is a well-established routine
in packaged ventilator controls and much simpler than
the custom sequences of VAV control.
Energy recovery can be incorporated at the system
level with commercially available products, such as a
heat wheel or stacked-plate heat exchanger. In addition
to using local packaged controls, these commonly
incur maintenance costs lower than those of a larger,
centralized VAV air-handling unit (AHU). Another
advantage of heat recovery through DOAS is a
potential reduction in building cooling tonnage. If
this approach is conceived during design, savingsfrom the smaller chiller or VRF condensing unit can be
used to offset the cost of the heat-recovery equipment.
Also, because buildings with energy recovery are slower
to warm, during a load-shedding event as part of a
demand-response action, a chiller can remain off for a
longer period of time.
As in a VAV system, bypass dampers may be needed
at energy-recovery heat exchangers, but the dampers
will be smaller, and their design with regard to pressure
losses can be more flexible.
DOAS need fan power only for ventilation air, which
reduces fan horsepower and helps to meet energy
codes.System pressure losses (and resultant fan horsepower)
can be lower than with a VAV system because power is
not needed to move cooling air through terminal heating
coils. Also, less pressure generally is needed to operate
and control terminal units, which reduces parasitic losses
and helps to meet energy codes.
18 HPAC ENGINEERING APRIL 2014
Grant Bowers, PE, LEED AP, is a mechanical engineer with SSOE Group (www.ssoe.com), provider of architecture, engineer-
ing, construction-management, and specialized services worldwide. Over the last 20-plus years, he has developed expertise in the
design of mechanical systems and management of mechanical design and construction projects for manufacturing, cleanroom,
laboratory, office, government, medical, and education facilities. He can be contacted at 503-439-8777 or [email protected] .
DOAS and VRF,Separating cooling/heating, ventilation airflows simplifies system design, control
Combining
PART 2 OF 2
VBy GRANT BOWERS, PE, LEED AP
SSOE Group
Hillsboro, Ore.
Variable-air-volume (VAV) systems with air terminal
units have been used extensively in commercial and
institutional buildings in the United States for decades.
Unfortunately, optimized design of a VAV system with
terminal heat is difficult at best because of limitations
inherent in VAV and complications posed by design
standards and regulations. One new approach involves
the pairing of a dedicated outdoor-air system (DOAS)
with a variable-refrigerant-flow (VRF) system. By
separating the goal of achieving ventilation rates from
the goal of maximizing thermal comfort, we can avoid
situations in which the two goals are in conflict and
efforts suffer from the resulting compromises. Whats
more, we can simplify the design process and find systemefficiencies that go far beyond those commonly achieved
with VAV systems with terminal heating.
Part 1 of this two-part article (March 2014,http://bit.ly/
Bowers_0314) discussed the limitations of VAV design and
VAV systems. Part 2 will discuss the merits and challenges
of combining DOAS and VRF systems as an alternative to
VAV systems.
Advantages of DOAS
DOAS provide ventilation air directly to occupiable
spaces, usually separately from heating and cooling
functions. Thus, the central air-handling fan, ductwork,
and diffusers of a DOAS are smaller and use lesspower than those of a VAV system that provides
cooling. At the same time, separation of ventilation from
heating and cooling functions enables designers to
separate ventilation reset from heating turndown,
allowing supply-air-temperature reset, supply-air-
pressure reset, and dynamic-ventilation reset to be
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22 HPAC ENGINEERING APRIL 2014
COMBINING DOAS AND VRF, PART 2 OF 2
for which diffusers are selected for
more of the time. Similarly, VRF sys-
tems are closer to being constant
volume, helping to maintain diffuser
throw, particularly during heating
operation, increasing ventilation
effectiveness.
From an in tegrated-des ign
perspective, DOAS/VRF can aid
implementation of advanced air-
distribution strategies, including
displacement ventilation and under-
floor air distribution (UFAD). Dis-
placement ventilation and UFAD
require less energy and offer greater
ventilation effectiveness and highercontaminant-removal rates than
traditional full-room air mixing.
Displacement ventilation and UFAD
improve not only how ventilation
air gets to an occupied space, but
how it moves to and through the
breathing zone. Displacement-
ventilation and UFAD approaches
offer options in the areas of natu-
ral ventilation for entire buildings
or hybrid ventilation to move more
air when building delta-T is l ow.
Displacement-ventilation and UFADstrategies have higher contaminant-
removal rates, as well as better air-
distribution effectiveness. Further-
more, displacement ventilation and
UFAD increase energy effectiveness
by reducing fan-power use and
lowering heat-removal costs by
reducing the treatment of air above
the occupied zone of a room.
The capital cost of a DOAS with
separate heating/cooling system is
comparable to that of a VAV system.
The life-cycle cost of a DOAS with
separate heating/cooling system,
however, can be lower because of
greater system energy effectiveness,
venti lation effectiveness, reliability,
and maintainability.
Advantages of VRF
VAV systems often use heated
and chilled water to transfer heat
to and from air supplied for space
heating and cooling and ventilation
pre-treatment. With smaller systems,
terminal heating/cooling units often
employ traditional direct-expansion(DX) heat transfer using piped refrig-
erant. Traditional DX systems have
one evaporator coil per fan unit piped
to a single condensing unit located
outdoors. Sometimes, the DX can
be reversible, providing heat, rather
than cooling, to supply air.
A VRF system, on the other hand,
can have several evaporator coils
piped to a condensing unit (Figure
1), which allows the con-
densing unit to oper-
ate only at thecurrent
net total heating or cooling capac-
ity (whichever is greater) and en-
ables heat transfer from zones with
excess heat to zones with heat de-mand (Figure 2). The larger number
of evaporator coils leads to greater
individual space temperature con-
trol. Each conditioned zone has heat-
ing- or cooling-capacity modulation
via a refrigerant valve at the fan coil,
while the central system typically has
capacity modulation via a staged or
variable-speed compressor.
The heat-recovery function of a
VRF system can lower energy cost
directly by transferring excess heat
from spaces being cooled to spaces
needing heat. Power input to modern
VRF systems is considerably lower
than that to traditional DX systems,
with coefficients of performance
(COP) as high as 3.8, compared
with the COP of less than 3.0 of DX
systems.
VRF systems general ly have
smaller components to maintain and
better system redundancy. Thus,
maintenance is less expensive and
less impactful on building opera-
tions. VRF fan-coil units often are
designed to be maintained within anoccupied space, not requiring access
doors or ceiling-tile removal.
In VRF systems, piping from
evaporator coils to condensers
often is smaller because heat is
carried in specialized refrigerants,
instead of heating water or chilled
water. While VRF piping systems
may have a greater number of
individual lines, pairs of smaller
lines can be run next
to each other
Outdoor Unit 1 Outdoor Unit 2
Defrost
Liquid pipe
High/low-pressure gas pipe
Suction gas pipe
Branch-selector
unit
Heatinglow low
Heatinglow low
Heatinglow low
Heatinglow low
FIGURE 1. Variable-refrigerant-flow-system architecture.
FIGURE 2.
Variable-refrigerant-
flow simultaneous
heating and cooling.
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24 HPAC ENGINEERING APRIL 2014
COMBINING DOAS AND VRF, PART 2 OF 2
in piping racks, taking up less vertical
space than larger-diameter heating-
water/chilled-water piping.
VRF systems sometimes sufferfrom the stigma of cold DX cooling
air (maybe 45F to 50F, as compared
with 55F VAV air), but spaces can
be quite comfortable when cool-
ing air is distributed properly and
supply-air temperature is reset.
Additionally, supply air is necessary
in VRF systems only when a space
needs heating or cooling. Supply-air-
temperature reset can be granular to
the zone, rather than selected based
on system-level parameters, such as
outside-air temperature.
Unlike traditional DX systems,
VRF systems general ly do not
require a full periodic reversal for
defrost of outdoor coils during
heating season. DX-system defrost
cycles on traditional heat pumps can
inject cold air into occupied spaces
periodically when heat is needed,
which, of course, can lead to discom-
fort or added cost for reheat. Where
there is no ducted outside air (as in
an overlaid DOAS/VRF system),
some or all fan coils can be stopped
during defrost. Alternatively, sup-plemental heat can be provided at
individual fan coils, as opposed to
all coils, and some VRF systems use
split coils to avoid full defrost.1Even
better, with modular outdoor units,
a single unit can be taken offline and
reversed for defrost, while all fan
coils remain in heating mode (Figure
3). These VRF techniques for reduc-
ing (or eliminating) electric heat for
defrost can have a substantial impact
on energy-cost reduction and some-
times even reduce building electrical-service size.
Building heating/cooling loads
and fan-power requirements vary
simultaneously. These independent
variables contribute to the difficulty
of estimating real-time total build-
ing loads. Thus, VAV central equip-
ment can be oversized considerably,
resulting in equipment-efficiency
losses and extra facility costs, such
as additional support structure.
Sizing of a VRF system can be based
on building-envelope block (net)
loads plus peak internal loads, while
sizing of a DOAS can be based on
peak population. These are more
easily estimable.
With the combination of a DOAS
and a VRF system, venti lat ion
airflow, ventilation pre-cooling, and
ventilation pre-heating are handled
by the DOAS, while space heating
and cooling are handled by the VRF
system. This simplifies design, as
simultaneous pressure effects of
various overlapped control schemes
are avoided.
VRF systems are much less prone
to inefficient modes of operation,
such as reheat and recooling, than
are VAV systems. For example, witha VAV system in typical winter or
shoulder-season operation, air will
be cooled mechanically at the central
system, delivered to interior zones
with heavy lighting and people loads,
and then reheated by exterior termi-
nal units for spaces requiring heat.
With VRF systems, there often
is little or no ductwork at occupied
spaces. Larger areas may require
ductwork for local air distribution
from fan-coil units, but most spaces
can be served with ductless fan coils
mounted either on walls or within
suspended ceilings. Various types of
fan-coil units are available (Figure 4).
VRF heat transfer is accomplished
with copper tubing, installation of
which is relatively simple compared
with custom ductwork. The tubing
is relatively small, although circuits
can be numerous.
VRF systems offer greater flexibil-
ity for retrofits, including interior-
space remodels, because refrigerant
piping often just needs to be changed
back to a zone-level distribution
assembly. This can be much lessintrusive than modifying ductwork
all of the way back to distribution
mains or back to an AHU. Addition-
ally, VRF-system redundancy can
allow individual refrigerant circuits
to operate while one is undergoing
maintenance, repair, or replacement.
Because VRF outdoor units often
Outdoor unit Outdoor unit Outdoor unit
Heating
(not available in heat pump)
Defrosting Ex1 Defrosting Ex2
Heat
Heat
Ex1 Ex2 Ex1 Ex2 Ex1 Ex2
FIGURE 3. Variable-refrigerant-flow defrost cycle.
FIGURE 4. Variable-refrigerant-flow fan-coil types.
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APRIL 2014 HPAC ENGINEERING 25
COMBINING DOAS AND VRF, PART 2 OF 2
are modular, expansion of a system
to accommodate the expansion of a
building can be relatively painless.
Air-cooled condensing units forVRF systems can be located almost
anywhere outside (e.g., rooftops,
storage facilities, dedicated exterior
rooms), as long as refrigerant piping
is sized and routed as usual for DX
systems. In high-rise buildings, these
units can be located on each floor,
as opposed to a VAV system serving
the floor with limited load diversity
or piping and ductwork routed to
central mechanical rooms, to the
roof, or to grade.
Air-cooled condensing units
with inverter compressors (variable
speed) also are available for VRF
systems. These adjust central-equip-
ment capacity to system need more
efficiently than increasing/reducing
fan speed does.
VRF is mature technology, with
several major manufacturers world-
wide. VRF systems are in opera-
tion throughout Asia and Europe
and making significant inroads
into U.S. markets, often in green
buildings.2VRF controls with new-
generation user interfaces thatinclude Internet-browser screens
and even smartphone applications
to facilitate remote adjustments
and troubleshooting now are being
manufactured.
VRF systems can represent one-
stop shopping for designers and
contractors, as compared with built-
up VAV systems, which typically
require coordination of equipment
from several manufacturers.
Lastly, DOAS/VRF systems pay for
themselves. Conceptually, approxi-mately 20 percent higher capital cost
(mechanical only, ignoring structural
savings) can be expected. But con-
sidering energy savings of approxi-
mately 10 percent per year, payback
periods often are five years or less.
VRF Drawbacks (Real and
Perceived)
Though attractive from many
perspectives, DOAS/VRF systems
do have drawbacks. First, with
increased use of refrigerants, safety
is a consideration. Calculations are
required to verify occupants are notexposed to more than the allowable
concentration of refrigerant (see
Applying Refrigerant Codes by
Greg Cunniff, PE, December 2013,
ht tp ://b it .ly/Cunnif f_1213 ). Safety
measures, such as refrigerant moni-
tors, air-transfer ducts, and dampers,
may be required.
VRF systems generally are avail-
able for relatively small zones. For
relatively large spaces, such as
cafeterias, gymnasiums, theaters,
and conference rooms, separate
HVAC units may be required.
Copper refrigerant piping can
be expensive, particularly when
metals prices jump. However, some
VRF manufacturers are introducing
steel tubing, which has been used
in automotive air-conditioning
applications for many decades.
Condensate drains often are
needed for individual VRF cooling
coils. However, fan-coil units typi-
cally are available with low-cost ac-
cessory pumps. These pumps often
use flexible tubing for condensatedischarge, rather than expensive
gravity condensate piping.
A separate ventilation system
usually is required with VRF, but for
smaller commercial and residential
systems, outside air can be ducted
directly from the outdoors to fan-coil
units. Direct-ducted outside air often
is limited to a relatively low outside-
air/supply-air fraction, and using
MERV13 filters per the U.S. Green
Building Councils LEED (Leadership
in Energy & Environmental Design)rating system may require careful
analysis of the pressure capability of
fan-coil units.
Some designers have avoided VRF
systems because certified perfor-
mance dataoften required by build-
ing codeswere not available. ANSI/
AHRI Standard 1230-2010,Perfor-
mance Rating of Variable Refriger-
ant Flow (VRF) Multi-Split Air-Condi-
tioning and Heat Pump Equipment,1
standardizes energy-efficiency
ratings of VRF systems. As a result,
documentation is now available for
VRF systems. In addition to simpli-fying the permit process, this docu-
mentation helps to validate that VRF
systems provide energy-efficiency
ratios not available with built-up
VAV systems.
Summary/Conclusion
VAV reheat systems once were
convenient and made good economic
sense for use with high air-change
rates. Today, stretching VAV designs
to address the competing motives of
good ventilation and energy savings
can be complex and expensive.
By combining a DOAS with an
inherently efficient heating and
cooling system, such as VRF, a
system designer can find system
efficiencies that go far beyond those
commonly achieved with VAV with
terminal heat.
Designers would be well-served to
break through to the new DOAS par-
adigm and consider supplementing
their DOAS with VRF. Such systems
take advantage of VRFs inherent en-
ergy recovery, better controllability,ease of design coordination, and re-
duced space requirements. In addi-
tion to simplifying system design and
operation, the use of DOAS/VRF, in
lieu of old-style VAV reheat, provides
greener systems by reducing both
energy use and overall life-cycle cost.
References
1) AHRI. (2010). Pe rf or ma nc e
rating of variable refrigerant flow
(vrf) multi-split air-conditioning and
heat pump equipment. ANSI/AHRIStandard 1230-2010. Arlington, VA:
Air-Conditioning, Heating, and
Refrigeration Institute.
2) ASHRAE. (2012). AS HR AE
handbookHVAC systems and
equipment. Atlanta: ASHRAE.
Did you find this art icle useful? Send
comments and suggestions to Executive
Editor Scott Arnold at scott.arnold@
penton.com.
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