Heating, cooling, and ventilating commercial buildings is responsible for more than half of commercial building energy use and costs. Commercial systems differ from residential ones in several ways: While most houses and apartments rely on operable windows and structure-related infiltration (air leaks) for fresh air, commercial buildings often have mandatory ventilation requirements. As the size of a commercial building increases relative to a house, its heating, ventilating, and air conditioning (HVAC) needs are increasingly dominated by air conditioning to reject heat from lighting systems, equipment, and people working in the space. Core areas of skyscrapers commonly require some cooling in every month of the year. On the other hand, the ability to introduce large amounts of outdoor air with the mechanical system allows using outdoor air instead of mechanical air conditioning whenever interior loads heat the building but it is cool outdoors. This "economizer" mode uses the fan more and the refrigeration compressor less, and can save serious money, particularly in regions with large daily temperature swings.

Buildings smaller than 10,000 to 20,000 square feet usually use factory-built, air-cooled "unitary" (packaged) equipment. Buildings larger than 100,000 square feet and multi-building campuses generally use water-cooled "built-up" or site-assembled systems that are engineered for the specific situation. In between, buildings may employ multiple large packaged units (for example, one unit per wing of an office building) or small built-up systems. The use of very large packaged units (20 to 60 tons) of cooling capacity is growing.

The packaged air conditioners used in smaller buildings should incorporate a way to introduce and condition ventilation air from outside. Most are roof-top units (RTUs), which often contain a gas furnace section ("year-round" unit) or function as a heat pump in winter. Packaged units are used in two-thirds of all commercial floor space in the United States and dominate the market for small projects and low-rise construction [U.S. Department of Energy. 2001. Energy Consumption Characteristics of Commercial Building HVAC Systems. Volume 1: Chillers, Refrigerant Compressors, and Heating Systems. Prepared by Arthur D. Little, Inc. Washington, D.C.: U.S. Department of Energy].

In general, central equipment systems are more costly to install and maintain than packaged equipment systems. Because water-cooled chillers can be very efficient, the conventional wisdom is that chilled-water systems are more efficient than unitary systems. However, "parasitic" (non-chiller) loads of pumps and fans in central systems can be as high as the chiller's peak demand [U.S. Department of Energy. 1999. Energy Consumption Characteristics of Commercial Building HVAC Systems. Volume II: Thermal Distribution, Auxiliary Equipment, and Ventilation. Prepared by Arthur D. Little, Inc. Washington, D.C.: U.S. Department of Energy]. Thus, performance comparisons between unitary and built-up systems, or among systems of either type, must examine the performance of the entire system, not just the chiller or the condensing unit. The table below gives an overview of the systems we mention.

Choosing the right HVAC system can be a daunting task. For many building owners, comfort is even more important than building energy costs or efficiency. Thermal discomfort, noise, and ventilation problems can affect worker productivity and tenant satisfaction. Poorly designed systems that do not respond to occupant needs can lead to occupant distraction, loss of business, and even lawsuits. In an office tenant study conducted by the Building Owners and Managers Association International (BOMA), HVAC systems were identified as the worst management, operation, and design problem (Pens, Alton J., and Sandy Beard. 1988. Office Tenant Moves and Changes. Washington, D.C.: Building Owners and Managers Association International). In this context, operating costs include both energy costs and system maintenance. Unfortunately, the present literature on maintenance costs does not support strong inferences about relative cost advantages of any particular technology, even at the level of unitary versus built-up systems. What is known is that costs vary enormously from building to building. It seems possible to operate a building well, saving money, by adopting a disciplined approach.

We provide information here on how to select, operate, and maintain specific types of heating and cooling equipment, including packaged equipment, central cooling equipment (e.g., central chiller systems), and heating equipment.

Maximizing HVAC System Performance

Reducing heating and cooling loads is the first step in maximizing HVAC system energy performance. For example, more efficient lighting systems and office equipment, as well as better window and roof systems, can significantly reduce cooling loads. By carefully managing building loads, you can reduce the size and cost of the HVAC equipment. In addition, a more appropriately sized system HVAC system will provide improved thermal comfort and may achieve better ventilation. High-performance HVAC systems require commitment from the building owner and careful coordination of those involved in system design, specification, installation, and operation and maintenance. Facility managers need to appreciate the entire process. Increased attention to design at the outset of a project can result in systems that cost less to install and less to operate, and a regular maintenance routine can facilitate high performance throughout the life of the equipment.

Notes on HVAC Efficiency and Programs

A wide range of efficiencies is available in packaged equipment, chillers, boilers, and furnaces. In most cases, however, there are legal minimum energy efficiency standards. For example, the National Appliance Energy Conservation Act (NAECA) and the Energy Policy Act (EPAct) established minimum-efficiency standards for furnaces, boilers, and packaged equipment to which manufacturers must comply. In general, NAECA applies to smaller equipment and EPAct governs larger equipment. Many of the EPAct standards are based on those developed for the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) Standard 90.1. The ASHRAE standard also includes efficiency requirements for many types of equipment not covered by EPAct, such as chillers. This standard has been adopted as the commercial building energy code in many states and localities.

Several groups have defined higher energy efficiency target levels for utility incentive programs and federal facility managers to use. For example, the Consortium for Energy Efficiency (CEE) has established peak efficiency tiers for packaged air conditioners and heat pumps, which its utility members promote to their customers. And the Federal Energy Management Program (FEMP), in response to regulations directing federal agencies to purchase products that fall in the upper 25 percent of energy and water efficiency, has developed purchasing recommendations for large, water-cooled chillers, commercial unitary air conditioners and heat pumps, and commercial boilers. The New Buildings Institute's "E-Benchmark" gives prescriptive recommendations for high-efficiency equipment (New Buildings Institute. 2003. Energy Benchmark for High Performance Buildings. White Salmon, Wash.: New Buildings Institute). Utilities in California, Minnesota, New Jersey, and the Northeast all promote high-efficiency equipment. The slist of utilities with programs changes regularly-check with your local utility for the latest information.

Again, equipment efficiency must be considered in the context of the whole HVAC system. In a chilled-water system, for example, although the chiller is at the core of the system and typically is the single largest energy user, simply selecting a high-efficiency chiller does not guarantee high performance. Auxiliary equipment (such as fans and blowers) and design decisions (such as "approach temperatures") can have substantial effects on efficiency. Thus, attention to overall system design and auxiliary components is critical to achieving optimal performance and comfort. In packaged air-conditioning systems, leaky ductwork, improper sizing, refrigerant charge, and air flow rates can considerably affect energy performance.

Finally, remember the importance of the economizer for "free" cooling-cooling by circulating outdoor air when conditions allow. The value of the economizer varies with climate, being greater where there are larger daily temperature swings and generally cooler temperatures. Where humidity at moderate temperatures may be very high (e.g., the Southeast), "enthalpy" controls prevent the introduction of relatively cool but humid air that will make the indoors uncomfortable.

• HVAC Controls for Temperature, Comfort, Ventilation, and Efficiency
• Air-Handling Systems: Moving Heating and Cooling Energy
• HVAC for Smaller Buildings: Packaged Systems
• Design and Installation Considerations
• Serving Larger Buildings and Facilities: Central or "Built-Up" Systems
• Summary
  
• Resources on High-Performing HVAC