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Multifamily Buildings Controlling Stack Effect-Driven Airflows The author is a proponent of the individual buildings and individual services and systems school. The most elegant argu-ment for compartmentalization of apartment buildings comes from Handegord.1 Stack effect-driven airflows in tall buildings compromise smoke control and fire safety, adversely affect indoor air quality and comfort as well as increase operating costs for space conditioning energy (Figure 1). By isolating the units from each other and from corridors, shafts, elevators and stairwells, stack effect-driven interior airflows can be controlled (Figures 2 and 3). To achieve compartmentalization unit airtightness should meet a
minimum resistance or air permeance of 2 L/(s· m2) at 75 Pa (0.4 cfm/ft2
at 0.30 in. w.g.), which is the recommended minimum resistance of
enclosure air barrier systems.2 This level of unit airtightness is
necessary to control stack effect air pressures and limit airflow
from adjacent units and cross contamination. Additionally, elevators
should be located in vestibules, lobbies and other “airlocks”
thereby isolating them from corridors. Unit doors should be
weather-stripped.
Distributed Ventilation Applying the compartmentalization principle further, ventilation is provided to each individual unit across exterior walls, not across interior pressure boundaries such as floors. Under the compartmentalization principle, ventilation is provided by ventilation systems unique to each unit rather than by central systems (Figure 4). In practice rooftop central exhaust systems are difficult and arguably impossible to balance and typically defeat measures to control the stack effect through compartmentalization. Balance is complicated due to the additive effect of the stack pressures to the fan pressures in the exhaust system duct risers. Since stack pressures vary with temperature, the flows in the exhaust systems also vary with temperature. The use of constant airflow regulators (devices that maintain constant airflow across variations in air pressure) can mitigate this effect somewhat, but are rarely used and require extremely high pressures within ducts to effectively operate. Apartments on upper levels tend to be overventilated during cold weather since they are closer to the fans and are most subject to the stack pressure. Shafts acting as exhaust chases or containing exhaust ducts are difficult to seal effectively and significantly complicate fire and smoke con-trol due to the development of complex three dimensional airflow pathways and interstitial pressure fields.3 Ventilation shaft construction typically is linked to mold and is the subject of frequent litigation due to these airflow pathways and the gypsum board used to construct the fire separations.3 Central systems are further limited as most installations do not allow individual unit control—the central fans are con-trolled by building management and are typically on all of the time or off all of the time. Units are either all ventilated or all not ventilated (resulting in overventilation in many units and significantly higher energy consumption) or underventilation in many units resulting in contaminant buildup and other complaints.
Distributed Heating, Cooling and Domestic Hot Water The compartmentalization principle also can be extended to heating, cooling and domestic hot water. Unit space heating is provided by sealed combustion gas furnaces and water heaters located in each individual unit (Photographs 3 and 4) that are vented either upward or downward through small diameter plastic ducts. Exhaust gases and combustion air are provided by separate ducts that follow parallel paths (Figures 5 and 6). Most systems can be routed upward or downward four to five floors.
Air conditioning is provided in a similar manner with individual exterior units located on roofs or at ground level in dry wells or in parking garages (Figure 7). Combined heating and cooling with heat pumps is executed in the same manner. Rooftop penetrations are collected and located in “doghouses” minimizing penetrations. All cable, duct and pipes are run through walls of doghouses (Photograph 5). The doghouse lids are removable, allowing for access. The distributed heating, cooling and domestic hot water components are arguably easier to service, and lower in cost to maintain by less skilled personnel than central systems. Problems with systems are limited to individual units rather than many units or entire buildings. Distributed systems are also less expensive to install in many regions.
This cost argument is supported on a per project basis on multifamily developments as the free market operates and developers adopt these approaches due to first cost reasons. Developers rarely select systems because they perform better, are more energy efficient, are safer or provide better indoor air quality. Developers select systems because they are cheaper. More of these systems are being adopted and displacing standard approaches principally for cost reasons. Central systems for heating, cooling and domestic hot water are not conducive to energy conservation since they do not encourage rational individual behavior unless individual metering is provided. It is the author’s experience that individual systems are the easiest to individually meter (Photographs 1 and 2). In hot, humid climates, part-load humidity control is almost impossible to control with central systems in apartment or condominium construction unless preconditioned makeup air or ventilation air is supplied to corridors through rooftop units. This begs the question whether makeup air and ventilation air can be supplied to individual apartments through corridor leakage or door undercuts. Most fire codes do not allow this approach (although in reading the codes it is not always clear) and require fully ducted supply with smoke dampers. In the distributed conditioning approach, individual dehumidifiers are provided in each unit, typically in the air-handling closet (Photograph 6). Individual apartment units in essence are treated identically to single-family detached houses with respect to heating, cooling and domestic hot water down to the types of equipment used. Since these systems are, for all practical purposes, residential systems, they can be installed by less skilled residential contractors, which is also an attractive feature to developers since more subcontractors are capable of both installation and service.
Ventilation System Configurations Ventilation of individual apartment units or condominium units using the compartmentalization approach should be pro-vided according to ANSI/ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. In the author’s opinion, the title of the standard is misleading and limiting. The author believes that the standard should also apply to apartments and condominiums regardless of height (low-rise or mid-rise or high-rise). The author’s preferred system configuration is presented in Figure 8. An outside air duct is directly connected to the re-turn side of an air-handling unit. An inline motorized damper and the air-handler blower is controlled by a programmable thermostat or other device to ensure minimum ventilation and prevent overventilation. Exhaust air is vented directly to the exterior from the kitchen and bath via an intermittently operated fan (or fans) controlled by the occupants. The author prefers that the exhaust fan (or fans) are controlled by a switch with an integral timer that shuts off the exhaust fan (or fans) after a preset time (e.g., 10 minutes), which is a practice common in the hotel and hospitality industry. Dryers should be vented directly to the exterior (Figure 10). A better approach, in the author’s opinion, is the use of condensing dryers that require no exterior vent. The water removed from the clothes is condensed and drained. No exhaust to the exterior occurs. This approach has obvious advantages with respect to makeup air (none is needed), and thus provides huge benefits in hot, humid climates from latent load perspectives and negative pressures, not to mention the benefits in cold climates arising from heat reclaim. In a hot, humid climate, a 94 L/s (200 cfm) exhaust flow results in approximately a 3.5 kW (1 ton) load. During part-load periods, the 94 L/s (200 cfm) exhaust is almost entirely a latent load dramatically skewing the sensible to latent ratio. Finally, there is one less hole in the building enclosure to worry about. Not all apartment or condominium units are designed and constructed with air-handling units. Many units have electric heat and no air conditioning, particularly in the Pacific Northwest. Others have through-wall packaged heat pumps that provide heating and cooling. Figures 11 and 12 illustrate the compartmentalization approach for such units. Exhaust is provided by continuously operating exhaust fan with outside air (makeup air) provided by an outside air duct. In the author’s opinion, this is not the best approach as it relies on induced infiltration. The air exhausted will be replaced by infiltration air through the duct. In the author’s experience this approach works best with effective compartmentaliza-tion (unit air tightness should meet a minimum resistance or air permeance of 2 L/(s·m2) at 75 Pa (0.4 cfm/ft2 at 0.30 in. w.g.), thereby limiting air drawn from neighboring units and the corridor, thusly favoring the outside air duct as a source of makeup air. A better approach is presented in Figure 13 where both supply and air exhaust is provided by a heat exchange ventilator. The installed cost of this approach is significantly more than the approaches described in Figures 11 and 12. However, the approach has the benefits of lower operating costs (particularly extreme climates such as severe cold and hot, humid climates) and not relying on induced infiltration.
Summary Compartmentalization of the building enclosure and apartment and condominium units to control stack effect-driven interior airflows can be extended to ventilation systems and systems for heating, cooling and domestic hot water. The approach results in distributed systems that allow for individual unit control, service, replacement and metering. The approach arguably results in buildings that are safer from a smoke and fi re perspective, more energy efficient, more comfortable and more durable with better indoor air quality.
Brian Simkins, LEED AP Article in: ASHRAE Journal, December 2005. Please see article for all references and credits. By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAEl
CTTC - Programs for Albuquerque and Beyond So, do you have an innovative project or an absolute disaster that you have worked on? Do you want to have your ASHRAE conference fees waived? If you give a presentation at the conference, you go for free! Here is how it works… Technical Program Preliminary program submissions include energy efficient design and operation of data centers, central chilled water plant innovations, and latest research on refrigeration system and components and design applications for sustainable buildings. Submission Types Technical Papers: Require double-blind review, approved by three reviewers, maximum length: 30 pages. Conference Papers: Abstract undergoes review for acceptance or rejection. Papers due 3 months after notification of abstract acceptance. Require single-blind review, approved by two reviewers, maximum length: 8 pages. Seminar: Submit abstract for presentation w/ chair and 2-4 speakers, each speak approximately 20 minutes. Forum: One moderator, no presentation, 60-minute open discussion. Tracks Track 1: What Is Sustainable Anyway? This track will discuss Sustainability and its overall affect on our environment as it pertains to all facets of energy consumption in the near term and the long term on new and existing building stock. How do present energy efficient standards compare to the proposed 189 and what is the additional cost? How will this affect us physically and financially? What are the short term and long term goals and benefits of sustainability? Are there simple and low cost intermediate steps as we progress? What can we do as ASHRAE members and how will other professions be affected? What will decisions by other professions (i.e., architects) have on our progress and ability to reach our goals? Case studies? How will IAQ be affected? What can be done to conserve water, sewer and electric uses in buildings? Track 2: Energy Facts and Simulation Track As building design criteria changes to include sustainability and energy use, it is important that consideration be given to building energy use simulation methods. This track is intended to include topics that address this particular issue including but not limited to: current and future tools for modeling energy use, the importance of building energy simulation, the influence and incorporation of energy models on the design process, the accuracy of energy use models, etc. This track will also explore the use of an energy model as a tool to provide energy savings opportunities. Track 3: Ventilation Systems Ventilation system design and maintenance is a wide-ranging topic. In this track, the objective is to address topics which are in significant interest at the present time, related to ventilation systems; starting with what’s new in ASHRAE Standard 62.1-2010, discussion of what indoor contaminants should be controlled in occupied spaces, operation and maintenance of ventilation systems and specialized ventilation system requirements for applications such as dry climates, hot, humid climates and variable air volume systems. Track 4: Refrigeration for the Future Track submissions are requested which address the need for cost effective and affordable advanced materials, components, refrigeration cycles, and system designs to improve energy efficiency of future refrigeration systems. In particular, submissions that address the energy consumption of entire systems through-out the entire operating regime to reduce annual energy consumption of the complete refrigeration system are re-quested. Furthermore, submissions that address the reduction of performance losses in the field due to installation deficiencies, operational effects and long-term degradation are sought. Track 5: Central Plant Systems This track will highlight presentations and case studies of energy efficient central heating and cooling plants. For this track, the definition of central plant includes plants serving single buildings and plants serving campus style groups of buildings. Plant sizes could range from very small to very large. Submissions should include discussions of both successes and challenges. Case studies of innovative central plant design are encouraged. Track 6: BIM/CAD/Paper and Pencils Track 7: Energy Conservation vs. New Generation In the United States, buildings consume over 40% of energy produced and contribute to over a third of our CO2 emissions. Current projections anticipate U.S. energy demands to in-crease by more than one-third by 2030, with electricity demand rising by more than 40 percent. As we continue to see a trend toward “net-zero” energy buildings, the debate continues as to what is the most effective method to curtail CO2 emissions and global climate change. While some experts have stated that improving energy efficiency in our buildings is one of the most constructive, cost-effective ways to address the challenges of high energy prices, energy security and independence, and global climate change, others experts predict that implementing new generations technologies such as solar and wind, in conjunction with smart grid technology, will reduce our dependence on fossil fuels, thus reducing CO2 emissions, and improving energy security. The series of sessions in this track illustrate the benefits of improved efficiency technologies verses new generation technologies and how the technologies promote net-zero energy buildings and contribute to lower CO2 emissions. Track 8: Living with HVAC&R Systems System designs can be advanced and efficient, but they are only as good as the actual operation after installation and start up. Uptime and simplicity often outweigh energy saving operation, even in green buildings. The series of sessions in this track will focus on the maintainability and optimization of installed systems. Potential topics include energy saving retro-fits, continuous commissioning, operations and maintenance and de-sign with operability in mind. Track 9: High Efficiency HVAC Systems As buildings are required to exceed ASHRAE Standard 90.1 by 30 percent or more and to meet LEED certification requirements, engineers are designing highly efficient HVAC systems. In addition to meeting ASHRAE's sustainability and net-zero energy goals, HVAC systems will have to be more energy efficient than previously designed. The series of sessions in this track illustrate ideas and designs for HVAC systems and their applications to various building types. New high performance building technology will be presented, integration approaches presented, and commissioning and operating strategies recommended. Track 10: Professional Skills Track 11: Data Center and High Density Cooling Data centers are very high energy facilities containing significant amounts of equipment gain in the space. What are the “state of the art” conditioning techniques and what is being done to minimize the HVAC energy use in these facilities. This track will explore energy savings opportunities associated with space conditions, cooling scenarios, cooling medium production and delivery. Track 12: Unassigned Deadlines Technical Papers: Due for review on April 9, 2010. Present at Las Vegas (January 2011). See LV tracks below. Conference Papers: Submit abstract online by April 9, 2010. Present at Las Vegas (January 2011). See LV tracks below. Seminar: Submit abstract with title for individual presentation by January 30, 2010, to Sarah Maston (smaston@rdkengineers.com). Also submit short biography. I will put submissions together with a chair and multiple speakers. Also email me if you are interested in chairing a presentation. Forum: Submit forum online. For Technical or Conference Papers, tracks for the Las Vegas Winter Meeting are below: Track 1 Impact of Code Requirements on the work of an ASHRAE Member Track 2 Integrated Design Process Track 3 Low Energy Hospitality Design Track 4 Is that Ammonia in your Refrigeration (or What Else Is New in Low-GWP/ODP)? Track 5 “Greening” of the Industrial Base Track 6 Real Cost of Zero Energy Buildings Track 7 Impact of ASHRAE Standards on the Contractor Industry Track 8 Professional Skills Links Albuquerque Technical Program Information: http://www.ashrae.org/events/page/2539 Papers and Programs Information: http://www.ashrae.org/events/page/1756
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