1、2010 ASHRAE 507ABSTRACTChanges in building practices in the Pacific Northwesthave led to the installation of most components of centralforced air heating systems outside of the conditioned enveloperesulting in an overall degradation of distribution efficiency.Increases in fuel costs and efforts to r
2、educe environmentalimpacts have resulted in an effort to encourage builders toplace duct work within the conditioned envelope and regainlost distribution efficiency. Through a voluntary processsupported by research and training, builders have learned toovercome obstacles and accept placement of duct
3、s withinconditioned space as a cost effective way to reach tax and util-ity incentive levels for energy efficiency. Growing acceptancehas resulted in new compliance measures for ducts withinconditioned space in Oregon and Washington energy codes.Modeling with SEEM software indicates substantial savi
4、ngsfor the region across climate zones and system fuel types rang-ing from 9.0 to 28.4% system savings in heating and 7.7 to17.0% savings in cooling.INTRODUCTIONIn residential housing, central forced air distributionsystem efficiency is largely a function of duct leakage andlocation. When ductwork i
5、s located outside of the conditionedenvelope of a house, leakage on both the supply and return sideof the system loses (or gains) energy as a function of thetemperature difference between the air inside the system andthe temperature of the zone containing the ductwork resultingin increased heating o
6、r cooling loads. With ductwork outsidethe conditioned envelope additional losses or gains result fromconductive and radiant heat transfer from the ducts to thesurrounding zone. Residential distribution system losses in thePacific Northwest have been shown to range to more than 30%(Francisco et al. 2
7、006). Conversely, when the entire distribu-tion system including all the ductwork and the air handler arecontained within conditioned space, distribution lossesapproach zero.HISTORY IN THE PACIFIC NORTHWESTThe Pacific Northwest can be roughly divided into twoclimate zones: the marine climate west of
8、 the Cascade Moun-tains and the colder dryer area east of the Cascades. Histori-cally older homes in both regions were built on basements. Ascentral forced air heating was introduced, the systems in newhomes or retrofitted in existing homes were generally installedwith the air handlers and most of t
9、he ductwork in the base-ments. The basements were generally used as at least partiallyconditioned space.As construction practices evolved, first in the marineclimate zone and later in the colder areas more and morehomes have been constructed on vented crawlspaces or slab-on-grade. A survey of new co
10、nstruction characteristics (RLW2007) found that 87% of new homes in the region wereconstructed without basements and 94% of all new homes hadcentral forced air heating systems. Consequently, the vastmajority of new homes in the region are built with a ductsystem and often the air handler outside of
11、conditioned space.The same report found that duct leakage to the exterior aver-aged 22% of total measured fan flow.The Pacific Northwest historically has had some of thelowest energy costs in the country. Plentiful hydroelectricpower and inexpensive natural gas from Canada have keptheating and cooli
12、ng costs low. Changing market conditions inthe last ten years have driven a 2 to 3 fold increase in naturalMoving Ducts into Conditioned Space: Getting to Code in the Pacific NorthwestDavid Hales David BaylonMember ASHRAE Associate Member ASHRAEDavid Hales is a Building Science and Energy Specialist
13、 with the Washington State University Extension Energy Program, Spokane, WA.David Baylon is a principal at Ecotope, Seattle, WA.OR-10-053 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. F
14、or personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 508 ASHRAE Transactionsgas prices and modest increases by comparison for electricalrates. Rising costs coupled with growing concern
15、to developmore sustainable energy efficient buildings to minimize envi-ronmental impacts has resulted in the evolution of a number ofvoluntary programs focusing on improving overall homeperformance by improving central forced air distribution effi-ciency.The Energy Star Homes Northwest (ESHNW) progr
16、amwas brought into the region in 2004 by the Northwest EnergyEfficiency Alliance (NEEA) as a market transformationprogram to market new homes built to a performance level atleast 15% better than a home built to the average base energycode in the region. The primary measures used by ESHNW areincrease
17、d heating and cooling equipment efficiency and seal-ing ducts confirmed by testing of all systems with ductsoutside of conditioned space. Regional energy codes haverequired prescriptive sealing of ducts outside of conditionedspace since the early nineties but without testing to confirmtightness no i
18、mprovement in overall duct tightness was seen(Hales 2003). Quality assurance testing in the Energy StarHomes Northwest program has confirmed compliance withthe program standard resulting in significantly tightersystems. Pressure to increase home performance beyond ESHNWlevels has grown in response t
19、o the 2005 Energy Policy Actfederal tax credit for new homes 50% better than the 2004IECC. Based on Building America research and NEEA spon-sored demonstration projects, builders in the northwest areaccepting the benefits accrued to locating the ducts withinconditioned space as a cost effective path
20、 to higher perfor-mance and tax credit qualification.Deemed savings for bringing ducts within conditionedspace has been established in the region by the Regional Tech-nical Forum (RTF) allowing utilities to incentivize the processand accelerate adoption by builders. Recent changes in build-ing codes
21、 in Washington and Oregon now also encouragelocating the ducts within conditioned space. Oregons currentcode following the prescriptive compliance path allows “Allducts and air handler are contained within the building enve-lope” to fulfill high efficiency duct sealing option #2 as onepossible choic
22、e out of nine compliance options(Oregon2008). Washingtons code effective July 1, 2010 exemptsducts with air handler entirely within conditioned space fromthe new requirement to test all new duct systems to demon-strate compliance with maximum allowable leakage rates(WSEC 2009).OBSTACLES FOR BUILDERS
23、When ductwork is brought into conditioned space, build-ers have been concerned with the loss of floor space to locatethe air handler; design challenges created by dropped ceilingand soffits; sequencing and coordination of trades; and costs.Design charettes with builders and their subs have helped to
24、resolve many issues. A variety of approaches adaptable todifferent floor plans integrated into the design process fromthe start has been able to resolve most problems at minimalcost. Successful strategies have included: minimized ductdesign (supply vents at inside walls); expanding the volume ofcond
25、itioned space to include ducts in attics and crawlspacesthat were previously unconditioned; dropped ceilings andsoffits to create duct chases; and floor trusses to create openspace in the floor cavity for duct runs(Kerr 2008).TECHNICAL DEFINITIONS (WHEN ARE DUCTS INSIDE?)What may appear as a simple
26、question, “When are ductsinside?” can become problematic from a programmatic orcode perspective. To attain the full benefit of ducts withinconditioned space, all parts of the system should be entirelywithin the thermal boundary and air barrier determining theconditioned envelope. In real world situa
27、tions, these boundar-ies may be ambiguous. Ducts within building cavities are oftenpartially connected to the outside by series leakage paths. Asoffit or dropped ceiling, for example, used as a duct chaseunder an unconditioned attic may be completely within thethermal boundary (insulation) but at th
28、e same time be all orpartially outside the air barrier. In this situation energy transferfrom the duct to the unconditioned attic may be possible andsignificant.EPACT 2005 federal tax credit standards for newconstruction require leakage testing of duct systems. If theentire system with air handler i
29、s entirely within conditionedspace and completely visible at the time of final inspection (asin an unfinished but conditioned basement), the system may betreated as within conditioned space and leakage testing of thesystem is not required. If however, the ducts are by designwithin the conditioned sp
30、ace but concealed in soffits or otherbuilding cavities, the leakage to exterior must be quantified bytesting. The test result then becomes the leakage rate used fortax credit qualification.Testing ducts to assure performance has become routinein the Northwest for Energy Star homes, utility incentive
31、programs and soon for codes in much of the region but it is alsooften perceived by builders as a burden because of the addedcost. An added incentive to induce builders to put ducts withinconditioned space has been to exempt them from duct testingrequirements. To facilitate this process while preserv
32、ing confi-dence in overall system performance, the RTF has developeda regionally accepted consensus standard (RTF 2008). ThePerformance Tested Comfort Systems (PTCS) standard forducts within conditioned space prescribes procedures andoptions for verification by either testing or inspection.PRACTICAL
33、 APPROACHESKeeping ducts within conditioned space in houses withbasements generally only requires that the basement beincluded in the conditioned space of the house. With goodbuilding practices to insure proper installation and alignmentof insulation and air barriers distribution system efficiencies
34、are high. Without basements, strategies for keeping ducts 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in
35、either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 509within conditioned space fall into two categories: placingducts within the existing conditioned space and expanding theconditioned volume of the house to include the zones contain-ing the d
36、ucts.In homes with ceiling heights over 8 ft, soffits anddropped ceiling combined with minimized duct design caneasily adapt most floor plans to keep ducts within conditionedspace. Two story homes can be adapted to ducts inside bykeeping branch supply runs in the floor cavity between floorswith supp
37、ly vents to the second floor in the floor and supplyvents to the first floor in the ceiling. With conventional floorjoists supply trunks often must be in dropped ceilings orsoffits. In two story homes, some Northwest builders havefound it advantageous to use floor trusses between floorsallowing all
38、branches and trunks to be within the floor cavity.In either case careful detailing of the air sealing (air barrier)and insulation at the rim joist is important to prevent problemsassociated with series leakage paths from the ducts to thecavity through the rim to the exterior.Builders in the region h
39、ave been experimenting withdesigns using conditioned attics and/or conditioned crawl-spaces for various reasons. Among the perceived benefits isthe assumption that ductwork placed in these now conditionedzones will accrue the benefits of ducts being within condi-tioned space. By expanding the condit
40、ioned envelope toinclude zones that contain ducts that otherwise would notnormally be part of the conditioned space; the surface area ofthe building subject to heat transfer is increased. Where insu-lation levels in the zones are consistent with the rest of theenvelope, the overall reduction in ener
41、gy transfer from theducts combined with the added transfer from the expandedenvelope should normally result in significant reductions.Concern in the Northwest, however has focused on the expan-sion of the conditioned space to include the crawlspacecontaining ducts and sometimes air handlers. The mos
42、tcommon practice in the region replaces floor insulation abovea vented crawlspace with perimeter insulation surrounding anunvented crawlspace built to International Residential Codestandards (IRC 2006). This practice now leaves the condi-tioned space of the house coupled to the ground through theun-
43、insulated floor of the crawlspace. In cooling dominatedclimates this ground coupling can be advantageous but in theheating dominated climates of the Northwest analysissuggests that it has a significant adverse affect on the benefitsgained by bringing ducts within the conditioned space(Lubliner 2007)
44、.SAVINGS ANALYSISThe analysis of the savings impact of the PTCS interiorduct specifications was based in large part on the specifica-tions developed for application in the ESHNW program. Thisspecification allowed builders some flexibility in placing theducts. Up to 5% of the total duct length was al
45、lowed to belocated in exterior cavities or buffer areas. In the case of theexterior cavities, there was some provision for sealing thecavity and insulation at the required level in the code. Thisprovision also allowed the use of short “jump” duct in attics orcrawl spaces provided the overall leakage
46、 was essentiallyzero. These two provisions compromised to some extent theoverall impact of the interior ducts but the resulting tightnessrequirements were thought to more than compensate.The analysis of the energy savings from these specifica-tions proceeded using the SEEM hourly simulation. Thissim
47、ulation was developed by Larry Palmiter of Ecotope withthe express intention of implementing the duct efficiencycalculation methods developed and published as the ASHRAEStandard 152. In addition, the SEEM program is an hourlysimulation tracking both the loads and equipment perfor-mance in the house
48、and the interactions between envelopecomponents, building internal loads and the heating and cool-ing equipment as it maintains the specified comfort conditionsin the home. SEEM was developed as a primary calculationtool for evaluating energy savings measures in the PacificNorthwest residential sect
49、or. It has been used extensively todevelop deemed savings tables for various measures includingduct sealing and placement, heat pump upgrade and commis-sioning, window, other building shell upgrades and overallimprovements in whole house energy use.The SEEM program is designed to model small scale resi-dential building energy use. The program consists of an hourlythermal simulation and an hourly moisture (humidity) simu-lation that interacts with duct specifications, equipment, andweather parameters to calculate the annual energy require-ments of the building. It employs algorithms cons
