Approximately half of all U.S. commercial floor space is cooled by self-contained, packaged air-conditioning units, most of which sit on rooftops (Figure 1). Also called unitary air conditioners or simply "packaged units," these mass-produced machines include cooling equipment, air-handling fans, and sometimes gas or electric heating equipment. Rooftop units (RTUs) are available in sizes ranging from 1 ton to more than 100 tons of air-conditioning capacity (1 ton of cooling capacity will remove 12,000 Btu of heat per hour).
The three main power consumers in a rooftop unit—the compressor, supply fan, and condenser fan—account for approximately 83, 10, and 7 percent, respectively, of the RTU's peak power (Figure 2, next page). However, because supply fans are often used to provide ventilation even when the compressor is not in use, the compressor's annual energy usage can be as low as 55 percent of the total energy use, with fans accounting for the remaining 45 percent.
Efficiency. RTUs of the same capacity are usually available with a wide range of efficiencies. The Air-Conditioning and Refrigeration Institute (ARI) defines efficiency in several different ways:
EER is the rating of choice when determining which RTU will operate most efficiently during full-load conditions. SEER and IPLV are better indicators of which RTU will use less energy over the course of an entire cooling season.
The cooling efficiencies of RTUs under 250,000 Btu per hour are certified according to standards published by ARI. (ARI standards also apply to RTUs of 250,000 Btu per hour and over, but ARI has no certification program and does not publish efficiency data for this size range.)
Federal minimum standards. The current U.S. federal standard requires manufacturers to produce equipment at minimum efficiencies as specified in Table 1. New standards created by the Energy Policy Act of 2005 will take effect on January 1, 2010. These will raise minimum efficiency levels by 25 to 30 percent for different RTU size categories.
Highest available efficiency. Manufacturers of RTUs continue to offer higher-efficiency units. As of 2008, the highest-efficiency RTUs on the market in sizes ranging from 65,000 to 135,000 Btu/h have EER values as high as 13.5; units from 135,000 to 240,000 Btu/h have EER values as high as 12.3. In January 2008, Lennox introduced a line of light commercial RTUs with high EER ratings, including a 3-ton unit with an EER of 14.3.
Compressor. Most RTUs use efficient reciprocating compressors, with several control options to consider. RTUs normally handle part-load conditions with simple on/off switches, operated by programmable timers, to stage compressors. As an alternative to completely shutting off the compressor, some units offer multiple valve-operated cylinders within the compressor that can be shut off individually. Effectively, shutting off cylinders creates a smaller cooling unit that is nevertheless operating with the original heat exchangers, and the result is a more efficient RTU. Another option is hot-gas bypass, which allows the compressor to provide reduced cooling at low loads. However, this option reduces capacity without reducing energy consumption.
Condenser. Nearly all RTUs under 20 tons have air-cooled condensers, which are about 20 percent less efficient than the evaporative condensers used in larger and more efficient models. Because evaporating water can remove more condenser heat than a stream of ambient air, lower condenser temperature and pressure are attained, and the compressors can therefore run at lower power. For smaller units, however (below about 20 tons), the energy required for pumping and spraying the water can outweigh the compressor energy savings gained by evaporative cooling. Other potential drawbacks are that the savings from water cooling decrease in humid climates and that evaporative condensers require more maintenance than air-cooled condensers.
Fans. Fans are used to move air across both the condenser and the evaporator. The airflow across the latter is also the supply air for the building. Although fan power use is a small fraction of compressor power use, fans can account for approximately 45 percent of the annual energy use because the fan operates for many more hours than the compressor. Most manufacturers also offer units with high-efficiency fans that increase both EER and IPLV as well as variable-speed fans that improve IPLV.
Economizers. An economizer is an additional dampered cabinet opening that draws air from the outside when outside air is cooler than the temperature inside the building, thereby providing "free" cooling. Many codes, standards, and utility programs already require the use of economizers, and most RTUs have this option. Economizers can reduce energy use by anywhere from 15 to 80 percent depending on conditions, and they are usually cost-effective given their minimal additional cost.
Controls. Programmable digital controls offer flexible settings that can be tailored to the application and are increasingly available as standard equipment. A good example is a seven-day time clock that consistently operates the RTU according to occupancy schedules and nighttime temperature setbacks. Digital controls are also easily tied into a central energy management system for monitoring and control as part of an overall building-control strategy. In addition, many new RTUs come ready to accept inputs from carbon dioxide sensors. These can be used to implement demand-controlled ventilation, an energy-saving strategy that adjusts building ventilation as occupancy changes rather than assuming that the building is always fully occupied.
Cooling coils. RTUs normally use direct-expansion evaporator coils, in which air is blown over a fin-and-tube heat exchanger that carries the evaporating refrigerant. A key variable in coil design is the face area, which determines the air velocity over the coil. Most RTUs keep this face velocity below 600 feet per minute to prevent condensed water in the airstream from blowing off the coil and into the duct system.
Select the right size. An undersized unit won't be able to provide sufficient cooling, but if a unit is oversized (the more frequent occurrence), it not only costs more but will lead to higher costs for associated ductwork and other auxiliaries. Operating costs increase too, because oversized equipment spends more time at less-efficient part-load conditions. Specifiers and designers commonly overestimate loads because they fail to take into account the reduced air-conditioning loads that result from energy-efficient lighting, and they overestimate plug loads by using nameplate ratings of office equipment in the building, which are frequently overstated.
It is also critical to use diversity factors when calculating internal loads. For example, consider a school: Peak load for the classrooms occurs when the classrooms are full, peak for the auditorium happens during an assembly, and peak for a gym occurs during a basketball game with the stands full. However, peak load for the school is not the sum of these loads, because they do not all occur simultaneously.
Consider high-efficiency levels recommended by CEE. The Consortium for Energy Efficiency (CEE) offers a program known as the High-Efficiency Commercial Air Conditioning and Heat Pumps Initiative. The initiative's goal is to encourage the use of high-efficiency unitary central air-conditioning and heat pump equipment in commercial buildings (unitary equipment consists of both single-packaged units, which contain all major assemblies in one cabinet, and split systems, which have one or more of the major assemblies separate from the others). Utilities participating in the initiative use CEE's high-efficiency equipment specifications in their education and rebate programs. As of August 2008, CEE suggests three tiers of efficiency levels for commercial equipment larger than 65,000 Btu/h. Units smaller than 65,000 Btu/h only have two tiers because new federal standards that took effect June 16, 2008, replaced one of the tiers. CEE has yet to establish a Tier 3 for this equipment due to limited availability. Efficiency levels in Tier 1 are approximately 22 percent greater than those in the current federal standard and closely match the levels specified in the new federal standard that takes effect in 2010. Efficiency levels in Tiers 2 and 3 are even higher. All of the tiers will be revised before the new federal standard takes effect in 2010. The CEE has also produced a table showing the availability of models that meet each tier.
The Energy Star program, which is jointly operated by the U.S. Environmental Protection Agency and the U.S. Department of Energy (DOE), establishes an efficiency specification above the federal standards. Equipment that meets these specifications is awarded the Energy Star label, which helps consumers and others readily identify high-efficiency products. The current efficiency level for Energy Star was set in 2002 and is not scheduled to be revised in the immediate future.
Identify high-efficiency models. ARI is the main source of information about energy-efficient RTU products. The organization maintains directories (available in both print and electronic formats) on its web site that include products from all ARI member-manufacturers.
CEE also maintains an easy-to-use directory of ARI-verified equipment that lists RTUs with capacities less than 65,000 Btu/h using both three-phase (commercial) and single-phase (residential) power.
Evaluate high-efficiency models by performing a cost-effectiveness calculation. The cost-effectiveness of a high-efficiency RTU depends on several factors, including cooling loads, operating hours, and the local cost of electricity. The U.S. Department of Energy's Federal Energy Management Program offers an energy cost calculator that compares estimated lifetime energy costs between RTUs with various efficiency levels, sizes, and hours of operation.
Pay attention to design, commissioning, and maintenance. No matter what equipment you choose, it's also important to make sure that the overall system is designed to be efficient (Figure 3, next page), that it's commissioned to operate as planned, and that it is properly maintained. A low-static-pressure duct system will reduce control problems, noise, and the fan power required. Comprehensive testing, adjusting, and balancing of the installed unit and its controls will maximize installed efficiency and comfort. Conducting regular tune-ups, correcting refrigerant charge, cleaning and adjusting the system to correct airflow and improve heat transfer, verifying economizer operation, and repairing major duct leaks can yield surprising energy savings at low cost. CEE offers installation guidelines for commercial air-conditioning equipment. Testing, adjusting, and balancing contractors; general HVAC service contractors; and those that use specialized diagnostic products such as CheckMe!, Enalysis, or the Honeywell HVAC Service Assistant can perform many of these services.
In the future, federal standards will likely drive RTU efficiency levels higher. To achieve higher efficiencies, RTUs will be equipped with variable-frequency drives with staged compressors and several levels of airflow to optimize part-load operation. In addition, technologies such as evaporative cooling of the condenser and energy recovery devices may begin to be widely incorporated into RTUs.
The California Energy Commission's (CEC's) Public Interest Energy Research Program is developing sophisticated RTU designs through its Advanced Automated HVAC Fault Detection and Diagnostics Commercialization Program. This program seeks to develop fault detection and diagnostic capabilities for HVAC systems as well as more fault-resistant HVAC equipment and to work with manufacturers to implement these improvements into new products. By automatically notifying service personnel of broken or otherwise malfunctioning equipment as soon as problems occur, these advanced units will help prevent equipment from deteriorating further and from operating at low efficiency levels. Researchers have found that the annual net savings range from $400 to $1,000 and the estimated payback period is less than one year. Greater savings are possible in hotter climates due to larger cooling requirements. The savings would also be greater for heat pumps because they operate year-round.