energy design resources design brief ADVANCED VARIABLE AIR VOLUME VAV SYSTEMS Many large HVAC systems use Summary This Advanced VAV System Design Brief provides recommendations to help engineers improve the efficiency of large HVAC systems It focuses on built up variable air volume VAV systems in multistory office buildings significantly more energy than necessary Design engineers can improve the efficiency and cost effectiveness of built up VAV systems The recommended measures promote efficient practical designs that advance standard practice achieve cost effective by following recommendations that energy savings and can be implemented using current emphasize integrated design and technology Here are some of the key recommendations designing for the full range of Reduce design system static pressure Employ demand based static pressure reset Use low pressure plenum returns relief fans Employ demand based supply temperature reset to reduce system operation reheat energy and extend economizer effectiveness Design fan systems to turn down and stage efficiently contents Size terminal units to balance energy impacts of pressure Introduction 2 drop and minimum airflow control Early Design Issues 4 Set terminal unit minimums as low as required for Zone Issues 7 ventilation and use intelligent VAV box control schemes to VAV Box Selection and Controls 8 prevent stratification Duct Design 10 Employ demand based ventilation controls for high density Supply Air Temperature Control 14 occupancies Fan Selection 15 Design conference rooms to provide ventilation without Supply Air Pressure Reset 18 excessive fan energy or reheat Coils and Filters 20 Outside Return and Exhaust Air Control 21 Notes and Additional Information 25 Of all the recommendations in this Design Brief VAV box control and supply air pressure reset often have the largest impact on system efficiency Design engineers are encouraged to pay particular attention to these two issues Introduction This Design Brief provides an authoritative new resource for heating ventilation and air conditioning HVAC designers It presents the most current recommendations on variable airvolume VAV airside system design and provides brand new information on fan selection and modeling The recommended measures incorporate findings from a recent study of built up VAV systems conducted for the California Energy Commission That three year study which included field monitoring of five large office buildings in California resulted in the publication of the Advanced VAV System Design Guide which presents a more comprehensive version of the recommendations in this Design Brief 1 Both publications this Design Brief and the full Design Guide focus on built up VAV systems in multistory office buildings in California or similar climates California has 16 climate zones But much of the information is useful for a wider range of system types building types and locations The recommendations in this Design Brief address airside system design with an emphasis on getting the air distribution system components to work together in an integrated fashion Of all the recommendations in this Design Brief VAV box control section 3 and supply air pressure reset section 7 will have the largest impact on system performance Design engineers are encouraged to pay particular attention to these two topics Figure 1 provides an overview of all the topics covered in this Design Brief Market Share Over the next 10 years about 30 million square feet per year of large office buildings will be constructed in California equal to 20 percent of new construction in the state Approximately onehalf of those buildings will be served by VAV reheat systems Therefore the recommendations in this Design Brief will apply to roughly 150 million square feet of new buildings built in the 10 year period between 2003 and 2012 This equals roughly 10 percent of the total commercial construction forecast PAGE 2 ADVANCED VAV SYSTEM DESIGN BRIEF Figure 1 Overview of Design Brief Contents This Design Brief is organized around key design considerations and components that affect the performance of VAV systems 8 COILS FILTERS 7 SUPPLY AIR PRESSURE RESET Static pressure sensor location Demand based SP setpoint reset Fan operation and isolation 6 FAN SELECTION Fan selection criteria Visualizing fan performance Construction filters Pre filters Final filter selection Filter area Extended surface area filters Coil selection Coil bypass 9 OUTSIDE RETURN EXHAUST AIR CONTROL Control of minimum OSA for VAV Design of airside economizer systems Economizer temperature control Economizer high limit switches 5 SUPPLY AIR TEMPERATURE CONTROL Optimal temperature Sequence of operations 4 DUCT DESIGN General guidelines Supply duct sizing Return air system sizing Noise control 3 VAV Box Selection VAV box selection summary VAV reheat box control Minimum volume setpoints Sizing VAV reheat boxes Other box types Other issues 2 ZONE ISSUES Design of conference rooms 1 EARLY DESIGN ISSUES Role of simulation in design Location and size of airshafts Return air system Auxiliary loads Design airside supply temperature Determining internal loads Integrated design While chilled water systems account for only about 4 percent of the HVAC systems in commercial buildings they account for as much These practices can lead to major improvements in system as 45 percent of the statewide cooling capacity And chilled water systems with VAV reheat the type of system addressed by this performance energy efficiency Design Brief are estimated to account for slightly more than 20 and occupant comfort percent of all cooling capacity Clearly the performance of these systems has a tremendous ability to affect statewide energy use Following the practices in this Design Brief can lead to major improvements in system performance energy efficiency and occupant comfort ADVANCED VAV SYSTEM DESIGN BRIEF PAGE 3 TYPICAL VS BEST PRACTICE PERFORMANCE Significant fan and reheat energy savings are possible through the design strategies promoted in this Design Brief The potential savings are illustrated in the graphs below which present results of simulations conducted as part of the Advanced VAV System Design study mentioned above 2 In this example the Standard case is a reasonably efficient code complying system while the Best case includes a number of the improvements suggested in this Design Brief The result of this simulation show that fan energy drops by 50 to 60 percent
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