Efficiency is the key word when it comes to RTUs, but it can take many forms. There are multiple opportunities to deploy efficient RTUs in new construction, replacement, and retrofit applications.
High efficiency. RTUs are available with many different levels of efficiency. Several national efforts are under way to increase the efficiency of new products and to develop and deploy retrofit options that will increase the efficiency of the large installed base. In the field, poor maintenance practices and the inability to control for part-load operating conditions have led to a generally low installed efficiency of RTUs. ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) and the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) evaluate RTU efficiency based on three primary performance metrics:
- Energy-efficiency ratio (EER): The ratio of the rate of cooling (in Btu/hour) to the power input (in watts) at full-load conditions. The power draw includes all inputs to compressors, fan motors, and controls.
- Seasonal energy-efficiency ratio (SEER): A seasonally adjusted rating based on representative residential loads. SEER applies only to RTUs with a cooling capacity of less than 65,000 Btu/hour.
- Integrated energy-efficiency ratio (IEER): A measure that expresses overall operating efficiency on the basis of weighted operation at different part-load capacities.
EER is the rating of choice when determining which RTU will operate most efficiently during full-load conditions. SEER and IEER are better indicators of which RTU will use less energy over the course of the year or cooling season.
Federal minimum standards. The current ASHRAE 90.1-2010 Standard requires a minimum EER of 11.0 and a minimum IEER of 11.2 for the typical commercial RTU.
High-Performance RTU Challenge. In an effort to move the needle on efficiency improvements and help companies capture significant energy savings, in 2013 the US Department of Energy (DOE) issued a challenge to equipment manufacturers to produce RTUs designed to reduce energy use by more than 50 percent over the ASHRAE 90.1 Standard. For RTU equipment to qualify, it must meet the Consortium for Energy Efficiency’s (CEE’s) Tier 2 standards, which vary by equipment size and technology type. The required minimum IEER for RTU models entered for consideration in the challenge is 18.0.
In 2012, the Daikin Rebel was the first RTU to meet the DOE’s High-Performance RTU Challenge requirements, with an AHRI-certified IEER of 20.6. In 2013, Carrier’s WeatherExpert also met the challenge’s performance requirements with an IEER of 20.8. Other companies working to develop commercial RTU models that meet the high-performance standards include Lennox, Rheem, and 7AC Technologies.
Advanced RTU Campaign. In an effort similar to the High-Performance RTU Challenge, the DOE’s Office of Energy Efficiency and Renewable Energy’s (EERE’s) Better Buildings Alliance is leading the Advanced RTU Campaign to help motivate companies to adopt high-performance RTUs and retrofit control devices. The campaign provides an easy-to-use checklist and decision tree (PDF) to assist participants in comparing retrofit versus replacement options and evaluating performance and savings potentials for various applications. In addition to the CEE Tier 2 performance standards, this campaign requires that qualifying equipment use ASHRAE 90.1–compliant economizers and that standard maintenance practices are followed and enforced according to ASHRAE Standard 180. Verification of program compliance is based on an honor system and participant-provided information.
Efficient compressor controls. Most RTUs use efficient reciprocating compressors that have several control options. RTUs generally handle part-load conditions with simple on/off switches that stage compressors with programmable timers. As an alternative to completely shutting off the compressor, high-efficiency units offer either staged-capacity or variable-capacity compressor-control options to reduce compressor energy usage during part-load operation.
Efficient condenser options. Nearly all RTUs under 20 tons have air-cooled condensers, which are about 20 percent less efficient than the evaporative condensers used in larger units. The efficiency of air-cooled condensers can be improved by incorporating “microchannels” and other advanced heat-exchange technologies into the design to increase the rate of heat removal, but at an additional cost.
Efficient fan motors and controls. Fans are used to move air across the RTU’s condenser and evaporator. The airflow across the evaporator is also typically used as 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 annual energy use because they operate for many more hours than the compressor does. Most manufacturers also offer units with high-efficiency fan motors that increase EER, as well as variable-speed fans that improve IEER.
Economizers. An economizer is a dampered cabinet opening that draws in air from outdoors when the 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 (including ASHRAE 90.1-2013). Economizers can reduce energy use by anywhere from 15 to 80 percent, depending on local conditions, and they are usually cost-effective given their relatively small added up-front cost. To ensure energy savings and proper operation, economizers should be checked regularly to be certain that they’re operating properly—the dampers can get stuck open, which leads to increased cooling loads as the economizer draws in outdoor air even when it’s warmer than the air inside.
Standard controls. Programmable digital controls, which offer flexible settings that can be tailored to the application, are increasingly available as standard equipment. A common example is a seven-day scheduler that consistently operates the RTU according to occupancy expectations 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 occupancy 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.
Retrofit controls. A number of retrofit control devices are now available as commercial products and can deliver significant energy savings, largely from the application of variable-speed fan controls to the majority of existing units that still operate at constant speed (Figure 3). One model (the Catalyst from Transformative Wave) allows for discrete setpoints at 40, 75, and 90 percent of full-load fan speed. In addition to the Catalyst, other available retrofit device models include the Enerfit V1 and the Bes-Tech Digi-RTU. Demonstrated energy savings from successful trials range anywhere from 25 to 70 percent, with the wide range due in large part to differences in the magnitude of energy waste in baseline operating conditions. Demonstrated simple payback periods on retrofit investments for commercial applications are approximately two years.
Figure 3: RTU retrofit controllers save energy
Retrofit controllers for rooftop units (RTUs) convert single-speed RTUs to variable-speed units, which can result in energy savings of as much as 70 percent. Fault detection and diagnostics, remote monitoring, and variable-speed condenser fan and compressor control are offered as additional features.
Fault detection and diagnostics. Over the past several years, growing emphasis has been placed on the need for fault detection and diagnostics (FDD), the ability to find and diagnose errors in RTU operation. Among the first commercialization efforts of equipment with FDD capabilities can be seen in the Catalyst retrofit controller, which currently includes anomaly detection and economizer failure alerts. The makers of the Catalyst have said that they intend to include several additional FDD features in the future, including condenser and evaporator coil fouling, excessively high or low refrigerant levels, liquid line restrictions, compressor valve leakage, and the presence of noncondensable gas.
Evaporator 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. Electronically controlled expansion valves are the best available technology for ensuring that refrigerant is efficiently metered between the evaporator and condenser and maintained at the proper temperature and state for the operating conditions.