HPAC Combining DOAS & VRF

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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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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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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


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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