Moving ducts into conditioned space: getting to code in the Pacific Northwest.
In residential housing, central forced air distribution system efficiency is largely a function of duct leakage and location. When ductwork is located outside of the conditioned envelope of a house, leakage on both the supply and return side of the system loses (or gains) energy as a function of the temperature difference between the air inside the system and the temperature of the zone containing the ductwork resulting in increased heating or cooling loads. With ductwork outside the conditioned envelope additional losses or gains result from conductive and radiant heat transfer from the ducts to the surrounding zone. Residential distribution system losses in the Pacific Northwest have been shown to range to more than 30% (Francisco et al. 2006). Conversely, when the entire distribution system including all the ductwork and the air handler are contained within conditioned space, distribution losses approach zero.
HISTORY IN THE PACIFIC NORTHWEST
The Pacific Northwest can be roughly divided into two climate zones: the marine climate west of the Cascade Mountains and the colder dryer area east of the Cascades. Historically older homes in both regions were built on basements. As central forced air heating was introduced, the systems in new homes or retrofitted in existing homes were generally installed with the air handlers and most of the ductwork in the basements. The basements were generally used as at least partially conditioned space.
As construction practices evolved, first in the marine climate zone and later in the colder areas more and more homes have been constructed on vented crawlspaces or slabon-grade. A survey of new construction characteristics (RLW 2007) found that 87% of new homes in the region were constructed without basements and 94% of all new homes had central forced air heating systems. Consequently, the vast majority of new homes in the region are built with a duct system and often the air handler outside of conditioned space. The same report found that duct leakage to the exterior averaged 22% of total measured fan flow.
The Pacific Northwest historically has had some of the lowest energy costs in the country. Plentiful hydroelectric power and inexpensive natural gas from Canada have kept heating and cooling costs low. Changing market conditions in the last ten years have driven a 2 to 3 fold increase in natural gas prices and modest increases by comparison for electrical rates. Rising costs coupled with growing concern to develop more sustainable energy efficient buildings to minimize environmental impacts has resulted in the evolution of a number of voluntary programs focusing on improving overall home performance by improving central forced air distribution efficiency.
The Energy Star Homes Northwest (ESHNW) program was brought into the region in 2004 by the Northwest Energy Efficiency Alliance (NEEA) as a market transformation program to market new homes built to a performance level at least 15% better than a home built to the average base energy code in the region. The primary measures used by ESHNW are increased heating and cooling equipment efficiency and sealing ducts confirmed by testing of all systems with ducts outside of conditioned space. Regional energy codes have required prescriptive sealing of ducts outside of conditioned space since the early nineties but without testing to confirm tightness no improvement in overall duct tightness was seen (Hales 2003). Quality assurance testing in the Energy Star Homes Northwest program has confirmed compliance with the program standard resulting in significantly tighter systems.
Pressure to increase home performance beyond ESHNW levels has grown in response to the 2005 Energy Policy Act federal tax credit for new homes 50% better than the 2004 IECC. Based on Building America research and NEEA sponsored demonstration projects, builders in the northwest are accepting the benefits accrued to locating the ducts within conditioned space as a cost effective path to higher performance and tax credit qualification.
Deemed savings for bringing ducts within conditioned space has been established in the region by the Regional Technical Forum (RTF) allowing utilities to incentivize the process and accelerate adoption by builders. Recent changes in building codes in Washington and Oregon now also encourage locating the ducts within conditioned space. Oregon's current code following the prescriptive compliance path allows "All ducts and air handler are contained within the building envelope" to fulfill high efficiency duct sealing option #2 as one possible choice out of nine compliance options (Oregon 2008). Washington's code effective July 1, 2010 exempts ducts with air handler entirely within conditioned space from the new requirement to test all new duct systems to demonstrate compliance with maximum allowable leakage rates (WSEC 2009).
OBSTACLES FOR BUILDERS
When ductwork is brought into conditioned space, builders have been concerned with the loss of floor space to locate the air handler; design challenges created by dropped ceiling and soffits; sequencing and coordination of trades; and costs. Design charettes with builders and their subs have helped to resolve many issues. A variety of approaches adaptable to different floor plans integrated into the design process from the start has been able to resolve most problems at minimal cost. Successful strategies have included: minimized duct design (supply vents at inside walls); expanding the volume of conditioned space to include ducts in attics and crawlspaces that were previously unconditioned; dropped ceilings and soffits to create duct chases; and floor trusses to create open space in the floor cavity for duct runs (Kerr 2008).
TECHNICAL DEFINITIONS (WHEN ARE DUCTS INSIDE?)
What may appear as a simple question, "When are ducts inside?" can become problematic from a programmatic or code perspective. To attain the full benefit of ducts within conditioned space, all parts of the system should be entirely within the thermal boundary and air barrier determining the conditioned envelope. In real world situations, these boundaries may be ambiguous. Ducts within building cavities are often partially connected to the outside by series leakage paths. A soffit or dropped ceiling, for example, used as a duct chase under an unconditioned attic may be completely within the thermal boundary (insulation) but at the same time be all or partially outside the air barrier. In this situation energy transfer from the duct to the unconditioned attic may be possible and significant.
EPACT 2005 federal tax credit standards for new construction require leakage testing of duct systems. If the entire system with air handler is entirely within conditioned space and completely visible at the time of final inspection (as in an unfinished but conditioned basement), the system may be treated as within conditioned space and leakage testing of the system is not required. If however, the ducts are by design within the conditioned space but concealed in soffits or other building cavities, the leakage to exterior must be quantified by testing. The test result then becomes the leakage rate used for tax credit qualification.
Testing ducts to assure performance has become routine in the Northwest for Energy Star homes, utility incentive programs and soon for codes in much of the region but it is also often perceived by builders as a burden because of the added cost. An added incentive to induce builders to put ducts within conditioned space has been to exempt them from duct testing requirements. To facilitate this process while preserving confidence in overall system performance, the RTF has developed a regionally accepted consensus standard (RTF 2008). The Performance Tested Comfort Systems (PTCS) standard for ducts within conditioned space prescribes procedures and options for verification by either testing or inspection.
Keeping ducts within conditioned space in houses with basements generally only requires that the basement be included in the conditioned space of the house. With good building practices to insure proper installation and alignment of insulation and air barriers distribution system efficiencies are high. Without basements, strategies for keeping ducts within conditioned space fall into two categories: placing ducts within the existing conditioned space and expanding the conditioned volume of the house to include the zones containing the ducts.
In homes with ceiling heights over 8 ft, soffits and dropped ceiling combined with minimized duct design can easily adapt most floor plans to keep ducts within conditioned space. Two story homes can be adapted to ducts inside by keeping branch supply runs in the floor cavity between floors with supply vents to the second floor in the floor and supply vents to the first floor in the ceiling. With conventional floor joists supply trunks often must be in dropped ceilings or soffits. In two story homes, some Northwest builders have found it advantageous to use floor trusses between floors allowing all 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 problems associated with series leakage paths from the ducts to the cavity through the rim to the exterior.
Builders in the region have been experimenting with designs using conditioned attics and/or conditioned crawlspaces for various reasons. Among the perceived benefits is the assumption that ductwork placed in these now conditioned zones will accrue the benefits of ducts being within conditioned space. By expanding the conditioned envelope to include zones that contain ducts that otherwise would not normally be part of the conditioned space; the surface area of the building subject to heat transfer is increased. Where insulation levels in the zones are consistent with the rest of the envelope, the overall reduction in energy transfer from the ducts combined with the added transfer from the expanded envelope should normally result in significant reductions. Concern in the Northwest, however has focused on the expansion of the conditioned space to include the crawlspace containing ducts and sometimes air handlers. The most common practice in the region replaces floor insulation above a vented crawlspace with perimeter insulation surrounding an unvented crawlspace built to International Residential Code standards (IRC 2006). This practice now leaves the conditioned space of the house coupled to the ground through the un-insulated floor of the crawlspace. In cooling dominated climates this ground coupling can be advantageous but in the heating dominated climates of the Northwest analysis suggests that it has a significant adverse affect on the benefits gained by bringing ducts within the conditioned space (Lubliner 2007).
The analysis of the savings impact of the PTCS interior duct specifications was based in large part on the specifications developed for application in the ESHNW program. This specification allowed builders some flexibility in placing the ducts. Up to 5% of the total duct length was allowed to be located in exterior cavities or buffer areas. In the case of the exterior cavities, there was some provision for sealing the cavity and insulation at the required level in the code. This provision also allowed the use of short "jump" duct in attics or crawl spaces provided the overall leakage was essentially zero. These two provisions compromised to some extent the overall impact of the interior ducts but the resulting tightness requirements were thought to more than compensate.
The analysis of the energy savings from these specifications proceeded using the SEEM hourly simulation. This simulation was developed by Larry Palmiter of Ecotope with the express intention of implementing the duct efficiency calculation methods developed and published as the ASHRAE Standard 152. In addition, the SEEM program is an hourly simulation tracking both the loads and equipment performance in the house and the interactions between envelope components, building internal loads and the heating and cooling equipment as it maintains the specified comfort conditions in the home. SEEM was developed as a primary calculation tool for evaluating energy savings measures in the Pacific Northwest residential sector. It has been used extensively to develop deemed savings tables for various measures including duct sealing and placement, heat pump upgrade and commissioning, window, other building shell upgrades and overall improvements in whole house energy use.
The SEEM program is designed to model small scale residential building energy use. The program consists of an hourly thermal simulation and an hourly moisture (humidity) simulation that interacts with duct specifications, equipment, and weather parameters to calculate the annual energy requirements of the building. It employs algorithms consistent with current ASHRAE, ARI, and ISO calculation standards. SEEM is used extensively in the Northwest to estimate conservation measure savings for regional energy utility policy planners. SEEM takes a number of input parameters including those for occupancy, equipment, ducts, envelope components, foundation (and basements), and infiltration. The large number of inputs makes the program flexible and allows it to model a diverse set of building construction types such as split-level, heated basements, slab-on-grade, and cantilevered floors. The simulation itself uses a single conditioned zone and several buffer zones that are evaluated separately. This allows an accurate characterization of the heating and cooling system without the calculation overhead of a multi-zoned specification (which is often too detailed for most single family residential applications). SEEM generates a number of outputs including building UA, heating load, heating equipment input, cooling load, and cooling equipment input. A detailed explanation of the model and its operation was developed for the USDOE STAC program (Eklund 2008). A comparison between the SEEM model and several standard residential simulation models is presented in Lubliner et al. 2007.
SEEM offers a number of advantages over other simulation programs. The step-by-step hourly calculations model both air temperature and mean radiant temperature using a state of the art algorithm. Next, heat pumps and air-conditioners are modeled on real performance data from manufactures' catalogues. SEEM also provides the capability to use multiple control strategies and thermostat setups for the equipment. Further, SEEM closely tracks duct losses to user specified zones (inside, outside, crawl, attic) and accurately models their impacts. Lastly, SEEM contains a comprehensive below-grade heat loss algorithm to model building ground contact through slabs, crawl spaces, and basements.
To do this analysis a standard analytical prototype was used that reflected the "typical" single family construction in the Pacific Northwest. This prototype was 2200 [ft.sup.2] built to the current standards in the Washington State Energy Code. In general these codes exceed the efficiency requirements of the International Energy Conservation Code (IECC 2006) by 9 to 15%. The analysis used two different heating systems, a gas furnace with an AFUE of 0.80 and a air-to-air heat pump (HSPF 7.7 and SEER 13). Table 1 summarizes the assumptions used in the evaluation of the duct sealing and placement parameters.
Table 1. Analytical Prototype Description Component Area U-Value Btu/h * SHGC Other [ft.sup.2] [ft.sup.2] * [degrees]F Measure ([m.sup.2]) (W/ [m.sup.2] * K) Window 364 (33.2) 0.32 (1.82) 0.3 Wall (R-21) 2180 (202.5) 0.057 (0.32) Ceiling (R-38) 1764 (163.9) 0.031 (0.18) Floor (R30) 1764 (163.9) 0.029 (0.16) Infiltration 0.35 ACH (natural) Furnace 0.80 AFUE Heat Pump 7.7 HSPF Cooling SEER 13
Three levels of duct sealing or placement to establish comparative savings:
1. A baseline sealing assumption derived from the regional duct review (Baylon 1999), this system included a supply ducts system in the crawlspace and a return duct system in the attic. The total leakage to the unheated buffer spaces is 22% of furnace fan flow roughly equally divided between supply ducts and return ducts.
2. The second run is a well sealed duct system with similar placement in the crawl and attic spaces. This system had a total of 9% leakage to the buffer spaces with about 66% of that leakage on the supply side.
3. The final runs were done assuming an interior duct system with a total of 5% of the duct area in buffer spaces and a total of 2% of furnace air flow in leakage to these spaces. This leakage was equally divided between supply and return ducts.
Table 2 shows the results of these runs applied to the prototype home. This table summarizes the gas heating and requirements for each level of duct sealing in two Washington State climates, Seattle and Spokane. As can be seen the savings fraction improves with the colder climates as the temperature in the buffer spaces goes down. This analysis predicts about 10% savings in heating energy from duct sealing alone but more than twice as much savings from placement of ducts inside the heated envelope.
Table 2. Heating System Savings: Gas Furnace Energy Seattle Spokane System Duct System therm (kWh) Unsealed 626 (18,348) 966 (28,313) Gas Furnace: AFUE 0.80 Estar Sealing 569 (16,667) 868 (25,441) Interior 491 (14,391) 736 (21,572) Savings Seattle Spokane System Duct System therm (kWh) Unsealed Gas Furnace: AFUE 0.80 Estar Sealing 56 (1641) 98 (2872) Interior 135 (3957) 230 (6741) % Saved System Duct System Seattle Spokane Unsealed Gas Furnace: AFUE 0.80 Estar Sealing 9.0% 10.1% Interior 21.5% 23.8%
Table 3 shows the same heating information using a heat pump. This calculation assumes that the heat pump installation uses first stage electric resistance heating (especially in cold temperatures). As can be seen the duct sealing and placement has a larger percent savings with a heat pump system. This is largely due to the increased run time of heat pumps as they deliver lower temperature air (roughly 20 to 30[degrees]F) than the typical gas furnace. The SEEM simulation takes the difference in delivery temperature into account (as specified in Standard 152).
Table 3. Heating System Savings: Heat Pump Energy Seattle Spokane System Duct System (kWh) Unsealed 6604 13,292 Heat Pump: HSPF 7.7 Estar Sealing 5921 11,537 Interior 5130 9518 Savings Seattle Spokane System Duct System (kWh) Unsealed Heat Pump: HSPF 7.7 Estar Sealing 683 1755 Interior 1474 3774 % Saved System Duct System Seattle Spokane Unsealed Heat Pump: HSPF 7.7 Estar Sealing 10.3% 13.2% Interior 22.3% 28.4%
Table 4 shows the cooling savings in each of these climates. Cooling is not particularly significant in either of these climates but the savings in the cooling system are reduced over the heating system as the crawl space fraction of the duct system is actually in a more favorable zone than the living zone. Conversely, the attic zone is much less favorable in the cooling season.
Table 4. Cooling System Savings Energy Savings Seattle Spokane Seattle Spokane System Duct System kWh kWh Unsealed 421 846 Air Conditioning: Estar Sealing 389 770 32 76 SEER 13 Interior 360 702 62 144 % Saved System Duct System Seattle Spokane Unsealed Air Conditioning: SEER 13 Estar Sealing 7.7% 9.0% Interior 14.6% 17.0%
SUMMARY AND DISCUSSION
The review of demonstration projects throughout the region has suggested that the cost of duct placement is quite attractive. Payback periods of about 4 years are anticipated even in the absence of utility incentives. This includes the cost of testing and quality control. It is also apparent that with the addition of the duct design "charette" these costs may well be reduced to essentially zero as interior duct placement is integrated into the plans for the home.
The review and analysis of these various duct placement and sealing options suggest substantial savings. Moreover, the impact of proper duct placement is more than twice the impact of simple duct sealing. Both these measures are included as options in the Oregon and Washington codes. As the importance of added energy efficiency is demanded by both consumers and public policy duct placement and to a lessor extent duct sealing become an attractive option.
In the Pacific Northwest there has been utility and state programs to train and evaluate duct measures for the last ten years (Baylon and Davis 1999). We believe that the infrastructure necessary to implement effective duct measures across most single family housing types is available. Both Oregon and Washington have implemented these measures as an option in their energy codes. Indeed, the current Washington Energy Code proposals make duct sealing mandatory and interior duct placement optional for trade-off with other envelope or HVAC measures. We believe that once the building community realizes the importance and advantages of these duct measures they will become a standard part of the regional construction practice.
Baylon, D. and B. Davis. 1999, Redsidential duct sealing cost benefit analysis, Final report. Prepared for: Northwest Energy Efficiency Alliance and the Oregon Department of Energy.
Eklund, K. 2008. State technology advancement collaborative (STAC)-Residential heat pump and air conditioner research, demonstration, and deployment improving pacific northwest utility and state HVAC programs, Idaho Department of Water Resources, Boise, ID.
Francisco, P.W., J. Siegel, L. Palmiter, and B. Davis. 2006. Measuring residential duct efficiency with the short-term coheat test methodology. Energy and Buildings, Volume 38, Issue 9, September 2006, pp. 1076-1083.
Hales, D., A. Gordon, and M. Lubliner. 2003. Duct leakage in new Washington State residences: Findings and conclusions. ASHRAE Transactions 109(2).
IRC. 2006. International residential code 2006 Edition. International Code Council.
Kerr, R. 2008. Green production building-moving ducts inside. Home Energy May/June.
Lubliner, M., L. Palmiter, D. Hales, and A. Gordon. 2007. Crawlspace design in marine and cold climates. Thermal Performance of Exterior Envelopes of Whole Buildings X.
Oregon Code. 2008. Oregon residential specialty code.
RLW Analytics. 2007. Single-family residential new construction characteristics and practices study. Final report. Prepared for: Northwest Energy Efficiency Alliance by RLW Analytics, Inc., Sonoma, CA.
RTF. 2008. Specifications for ducts inside new single family homes. August 5, 2008. Regional Technical Forum, Portland, OR.
WSEC. 2009. Washington state energy code 2009 Edition.
David Hales is a Building Science and Energy Specialist with the Washington State University Extension Energy Program, Spokane, WA. David Baylon is a principal at Ecotope, Seattle, WA.
Associate Member ASHRAE
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|Author:||Hales, David; Baylon, David|
|Date:||Jan 1, 2010|
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