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m e r g i n g T e c h n o l o g i e s

D uctless Split System sBy Kurt Roth Ph .D. Associate Member ASHRAE; De tle f We stp ha len Ph.D .; and

Jam es Brodrick Ph .D. Member ASHRAEM ost U.S. res idences use central ducted sys tems todel iver cooling and heat ing. Du cted sy s tem s c onsis tof a network of ducts with a blower that deliversconditioned airtodifferent indoor sp aces . An outdoor condens-ing unit cools refrigerant, which then flows to a refrigerant coilm ounted in the duct . I f the outdoor unit is a heat pu m p, theindoor coil also transfers heat to the duct during the heatingseason. More commonly, however, the ducted airflow passesthrough a furnace that heats the air.

In contrast, a ductless split sy stem provides space conditioningbydistributing cooled(and, inthe case ofa heatpumpunit heated)refrigerant through a network of insulated reftigerant lines to oneormore(typicallyu p to four) fan-coil units located in conditionedspac es. A ductles s sp lit system with a single outdoor unit anda single indoor unit also is known as a m ini-split. Ultim ately,low-profile ce iling- or wall-m ounte d fan-coil units transfer heatbetwee n the refrigerant and the room air. Available in capac ityranges from approximately to 4 tons (3.5 kW to 14 kW), duct-less split systems are smaller and simpler variations of variablerefrigerant volume systems used in commercial buildings.'

Duct less spl i t sys tems of ten include features such as var i-able-spe ed fans, var iable-capaci ty com press ors , and wirelesscontrols. In addition, units with m ultip le e vaporators have in-dividual therm ostats to control the op erat ion ( e .g., tem pe raturesetpoint) of each indoor coil.Effect ive duct less sys tem performance depends upon properplacement of indoor coi ls . Duct less sys tems take in ambientroom air, condition the air, and then blow the air into the c on-dit ioned space . Consequently, the indoor coi l m ust be ins tal led

in a location where it can distribute the conditioned air evenlythroughout the spac e. Im proper placem ent can cause an indoorunit to recycle already conditioned airflow (short circuiting),resulting in ineffective space conditioning.^Duct less spl i t sys tems are the pr imary res idential coolingsys te m s in many par ts of Asia, and most m ajor m anufacturersare based inAs ia. However, they have y et to achieve significantm arket share in the U.S., pr imar i ly becaus e the i r s ys tem arch i -tec ture differs from the predominant approach (ducted sys tem s)and operat ional l im itat ions in colder c l im ates .

Energy Savings PotentialD uc t le s s s p l i t s ys tems can reduceHV Cenergy consum ption

relative to ducted s ys tem s in at leas t three ways. Firs t, duct less

systems avoid losses of conditioned energy via duct air leakage. Se cond, a significant p ortion of duct runs are place d unc onditioned sp aces, such as attics and basem ents. If they lacksufficient insulation, the ducts can exchange sizeable quantitieof energy with these spaces . Third, mult i-evaporator sys temsprovide space-by-space heating and cooling control, which enables them to save energy by only conditioning occup ied sp acesAlthough ducted sy s tem s also can em ploy zoning, us ing dam pers and variable air volume blowers, they typically do not.

It is difficult to quantify the energy savings potential fromzoning. Studies of residential duct leakage suggest that ductedsyste m s deliver less than 70 of the heating or cooling effecproducedtothe intended space s.For example,anevaluation oftdelivery efficiency of 24 houses with ducted sy stem s in Californifound an average therm al delivery efficiency of 64 . A s s uing that app roxim ately half of the heating or cooling ene rgy losfrom the ductsistransferred to unconditioned sp aces, the aversystem would have to run app roxim ately 20 longerconsuming 20 m ore energyto m eet cooling and heating loads. As resul t , duct less AC sys tem s with the sam e effic iency as ductedsplit system s could reduce residentialHV Cenergy consumptiby approxim ately 20 . While duct less sy s tem s do lose energythrough the insulated refrigerant lines, these have much lessurface areas than ducts, limiting their energy impact.

A s s u m i n g 2 0 s a v i ng s , r e p l a c e m e n t o f t y p i c a l d u c t esys tems with duct less sys tems could reduce the 2.1 quads ocooling e nergy consum ed by central res idential sys tems '* bapproximately 0.4 quads. Addit ional savings would accrufor central sy s tem s with heat pu m ps, as well as from reducedcondit ioning of unoccu pied spaces from zoned c ontrol.Well-sealed and well- insulated ducted sys tems have mucsmaller losses than typical res idential ducts , i .e . , the moseffic ient sys tem s evaluated by Jum p, e t al . ' had del ivery eff ic iencies of about 90 . Relat ive to effic ient ducted sys te m sduct less sys tems would real ize more moderate savings on thorder of 5 , or about 0.1 quad.Maritet FactorsAlthough duc t l e s s s p l i t s ys tem s have becom e m ore com m oin the U.S., they s t i l l have a small market share. Relat ive the typ ica l duc ted s ys tem s , duc t l e s s s ys tem s have two pr imardisadvantages. Foremost, for a new air-conditioning systemfor an ent ire home, duct less sys tems typical ly have a highe

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E m erg ing Techno log iesinstalled cost than du cted system s, in part because they are notconsidered typical practice in theU.S.This general unfam iliar-ity with du ctless units decreases the num ber of co ntractors thatinstall the systems, as well as the volum e of products sold. Forpotential replacement of ducted systems, the cost premiumgrow s substantially becau se a ducted system retrofit would onlyneed to replace the indoor and/or outdoo r units.

Furthermore, ductless systems in colder climates eitherrequire extensive use of backup electric resistance heating,which increases operating costs, or installation of a separateheating system (such as hydron ic radiant heating), which furtherincreases their first cost prem ium.In addition, ductless system s use refrigeranttodistribute heat-ing and cooling and, as a result, have larger refrigerant chargesand long refrigerant runs than ducted systems with the samecapacity. This increases the po tential for refrigerant leakage andthe volume of refrigerant lost when a leak occurs.On the other hand, ductless split systems offer at least threeadvantages relative to conventional ducted systems. First,ductless systems are easier to retrofit in many existing homesbecause they use small refrigerant lines instead of much largerducts to distribute heating and cooling. Second, as currentlydesigned, ductless systems have quieter indoor and outdoor

fans, which minimizes their acoustic impact on occupants.^

Third, ductless systems provide zone control, i.e., each indoorunit has its own temperature controls. This enables occup antsof different spaces to customize air temperature settings toreflect their personal comfort preferences. In contrast, space-level zoning is not typical practice for most ducted systemsbecause of tscost.References

1. Goetzler,W. K.W.Roth, andJ.Brodrick. 2004. Variableflowandvolume refrigerant system . /4 S/ /^ 'Jo ra/4 6(1):S 164 -S16 5.2.Turpin, J. 2002 . Mini-S plits are moving into the mains tream,TheAir Conditioning, Heating Refrigeration NewsJan. 9.3 . Jump, D.A., LS. Walker, and M.P. Mod era. 1996. Fieldmeasurements of efficiency and duct retrofit effectiveness in resi-dential forced air distribution systems. Proceedings of the ACEEESummer Study.4. El A. 2001 . Residential energy consumption surveys: 2001consumption and expenditures tableselectric air-conditioning con-

sumption tables. U.S. Departm ent of Energy, Energy Inform ationAdministration (EIA ).Kurt WRoth,Ph.D., is associate principal and DetlefWest-

phalen, P h.D., is principal in the HVA C and RefrigerationTechnologysectorofTIAX Cam bridge, Mass. James BrodrickPh.D., is a projectmanager. Building Technologies PrograU.S. Departmen t of Energy, W ashington, D.C.9

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