HVAC: Packaged Rooftop Air Conditioners

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 (energy-efficiency ratio): The ratio of the rate of cooling (Btu per hour, or Btu/h) to the power input (watts) at full-load conditions. The power input includes all inputs to compressors, fan motors, and controls.
• SEER (seasonal energy-efficiency ratio): A seasonally adjusted rating based on representative residential loads. SEER applies only to RTUs with a cooling capacity of less than 65,000 Btu per hour.
• IPLV (integrated part-load value): A seasonal efficiency rating method based on representative annual commercial loads. It applies to RTUs with cooling capacities equal to or greater than 65,000 Btu per hour.

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 (from 1992) requires manufacturers to produce equipment at minimum efficiencies as specified in Table 1. The Energy Policy Act of 2005 set new federal standards for commercial RTUs with a capacity greater than 65,000 Btu/h that will take effect on January 1, 2010. For three-phase equipment smaller than 65,000 Btu/h, a rule-making is in progress to determine whether new standards should be set for this size category.

Highest available efficiency. Manufacturers of RTUs continue to offer higher-efficiency units. As of 2007, the highest-efficiency RTUs on the market in sizes ranging from 65,000 to 135,000 Btu/h have EER values as high as 12.7; units from 135,000 to 240,000 Btu/h have EER values as high as 12.3.

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.