The most common cooling technology for US commercial facilities is a self-contained, packaged air-conditioning and heat-pump unit, and it’s usually on the roof (figure 1). These rooftop units (RTUs), also called unitary air conditioners or packaged units, are mass-produced machines that include cooling equipment, air-handling fans, and sometimes gas or electric heating equipment (figure 2). RTUs are available in sizes ranging from 1 to more than 100 tons of air-conditioning (AC) capacity. To determine the capacity your facility needs, you can use an HVAC vendor calculator such as Kobie Complete Heating & Cooling’s Btu and Tonnage Calculator, or contact a local HVAC specialist.
The compressor requires the most power in an RTU, accounting for about 80% of the peak-operation power draw. The supply and condenser fans draw 10% each. Despite the compressor’s high power draw, it consumes only about 55% of the RTU’s total energy use. The supply fans, which provide ventilation and deliver outside air even when the compressor isn’t in use, account for 45% of the RTU’s total energy use.
What are the options?
High efficiency RTUs are available in many efficiencies. Several national efforts are underway to increase the efficiency of new products and to develop retrofit options that will increase the efficiency of the installed units. In the field, poor maintenance practices and the inability to operate equipment at part-load have lowered the efficiency of installed RTUs. ASHRAE and the AHRI evaluate RTU efficiency based on three primary performance metrics:
- Energy efficiency ratio (EER) is the ratio of the rate of cooling (in Btu per hour) to the power (in watts) at full-load conditions. The power measurement includes compressors, fan motors, and controls.
- Seasonal energy efficiency ratio (SEER) is a seasonally adjusted rating based on representative residential loads. SEER applies only to RTUs with a cooling capacity of less than 5.42 tons.
- Integrated energy efficiency ratio (IEER) expresses overall operating efficiency at 100%, 75%, 50%, and 25% operating loads.
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 a commercial RTU.
High-Performance RTU Challenge In 2011, the US Department of Energy (DOE) issued a challenge to equipment manufacturers to design RTUs that use 50% less energy than the ASHRAE 90.1 Standard calls for. 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 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. Carrier’s WeatherExpert also met the challenge’s performance requirements with an IEER of 20.8.
The challenge successfully increased the EER of 5-ton RTUs by 16%, which is 4% more efficient than the ASHRAE 90.1 Standard. Assuming constant fan operation, these RTUs would consume 26% less energy than the baseline equipment.
Advanced RTU Campaign Similar to the High-Performance RTU Challenge, the Advanced RTU Campaign motivates companies to adopt high-performance RTUs and retrofit control devices. The DOE’s Office of Energy Efficiency and Renewable Energy’s Better Buildings Alliance led the campaign. In addition to providing an easy-to-use checklist on its Retrofit or Replace? page, the campaign posts its Decision Tree for RTU Replacements or Retrofits (PDF) to help participants select the right equipment for their application. In addition to the CEE Tier 2 performance standards, this campaign requires that equipment use ASHRAE 90.1–compliant economizers and that facilities follow the maintenance practices defined in ASHRAE Standard 180. The DOE relies on participant-provided information to confirm compliance.
Efficient compressor controls Most RTUs use efficient reciprocating compressors that have several control options. Switches use programmable timers to turn compressors on and off to meet the buildings’ changing cooling needs. As an alternative to completely shutting off the compressor, high-efficiency units can vary capacity to minimize energy usage during part-load operation.
Efficient condenser options Nearly all RTUs under 20 tons have air-cooled condensers, which are about 20% less efficient than the evaporative condensers used in larger units. You can improve the efficiency of air-cooled condensers by incorporating microchannels and other advanced heat-exchange technologies into the system. These strategies will remove heat faster, but they cost extra to implement.
Efficient fan motors and controls RTUs use fans to move air across the condenser and evaporator. The system typically uses the airflow across the evaporator as the supply air for the building. Although fans draw less power than compressors, fans can account for approximately 45% of annual energy use because they operate for many more hours than the compressor does. Most manufacturers offer units with high-efficiency fan motors and variable-speed fans that improve IEER.
Economizers An economizer is an opening in the RTU that draws in air from outdoors when the outside air is cooler than the temperature inside the building. Because the economizer uses cool outside air instead of conditioning warm air, it provides cooling cheaply and efficiently. Many codes, standards, and utility programs already require facilities to use economizers (including ASHRAE 90.1-2013). You can save 15% to 80% of cooling energy by using economizers, depending on local weather conditions. And because of their small up-front cost, they’re usually cost-effective. To ensure energy savings and proper operation, you should have trained maintenance professionals check your economizer regularly—the dampers can get stuck open, which leads to increased cooling loads because the economizer draws in outdoor air even when it’s warmer than the air inside.
Standard controls Programmable digital controls, which you can tailor to the application, are becoming standard in RTUs. A common example is a seven-day scheduler that operates the RTU according to expected occupancy and nighttime temperature setbacks. You can also tie digital controls into a central energy-management system. The controls will let you monitor and control RTU operation, and many new RTUs are compatible with carbon dioxide occupancy sensors. You can use these to implement demand-controlled ventilation, an energy-saving strategy that adjusts ventilation as occupancy changes rather than assuming that the building is always fully occupied.
Retrofit controls Retrofit control devices are another way to improve the efficiency of your RTU. Most of the energy savings from these devices come from variable-speed fan controls (figure 3). One model—the Catalyst from Transformative Wave—allows for setpoints at 40%, 75%, and 90% of full-load fan speed. Other available retrofit device models include the Enerfit V1 and the Bes-Tech Digi-RTU. Demonstrated energy savings range from 25% to 70%, depending on how efficient the original system was. The demonstrated simple payback period for this technology is about two years.
Fault detection and diagnostics Fault detection and diagnostics (FDD) finds and diagnoses errors in RTU operation. The Catalyst retrofit controller is one of the first commercialized controllers with FDD capabilities. This product detects anomalies in performance and sends alerts when the economizer fails. The makers of the Catalyst said that they intend to include several more FDD features in the future, including alerts for condenser and evaporator coil fouling, high or low refrigerant levels, compressor valve leakage, and the presence of noncondensable gas.
How to make the best choice
Consider retrofitting your existing equipment It can be difficult to justify the time and expense of replacing existing RTU equipment with advanced, high-efficiency models. This challenge can lead to inefficient units that limp along for years. Advanced RTU retrofit controllers save as much as 70% of cooling energy in field demonstrations but cost less than full equipment replacement. With demonstrated two-year simple payback periods in commercial applications, advanced RTU retrofit controllers have become a very attractive option for saving cooling energy. For more details on demonstrations and evaluation results, see the Advanced RTU Campaign’s Case Studies and Guidance page.
Size equipment appropriately An undersize unit won’t provide sufficient cooling, but if a unit is oversize, which is more common, it costs more to operate. Operating costs may increase too, unless the unit has variable-frequency drives, because oversize equipment spends more time operating at part-load, which is inefficient.
In the past, facility managers attempted to avoid cooling a building too much by rightsizing RTUs, but advanced units with variable-speed drives make rightsizing less important. In fact, researchers at the Western Cooling Efficiency Center tested the Daikin Rebel in the field and found that it was more energy efficient to oversize RTUs with variable-speed control capabilities than it was to undersize them.
Identify high-efficiency models AHRI is the main source of information about RTU product efficiency ratings. The organization maintains its Directory of Certified Product Performance that includes products from all AHRI-member manufacturers.
Another place to get information about product efficiency is the CEE’s High-Efficiency Commercial Air Conditioning and Heat Pump (HECAC) Initiative. The initiative’s goal is to increase the number of available high-efficiency packaged RTUs by promoting voluntary performance requirements for commercial AC. To achieve this objective, participating CEE member utilities encourage commercial facilities to adopt high-efficiency equipment by offering information and incentives on these products. To see a list of CEE-qualified equipment see its Directory of Efficient Equipment. The directory lists RTUs with capacities less than 65,000 Btu per hour that use three-phase power in commercial applications and single-phase power in residential.
The US Environmental Protection Agency and the DOE run a similar program called Energy Star, which establishes an efficiency specification that’s more efficient than the federal standards. The program awards the Energy Star label to qualifying equipment, making it easy to identify high-efficiency products. The 2002 Energy Star Program Requirements for Light Commercial HVAC (PDF) explains the specifications.
Calculate cost-effectiveness The cost-effectiveness of a high-efficiency RTU depends on several factors, including cooling loads, operating hours, and the local cost of electricity. To calculate equipment’s cost-effectiveness in your application, use the Rooftop Unit Comparison Calculator from the DOE and Pacific Northwest National Laboratory (PNNL). It estimates lifetime energy costs for RTUs with various efficiency levels, sizes, and hours of operation.
Pay attention to design, commissioning, and maintenance No matter what equipment you choose, make sure that the overall system is efficient (figure 4), and that trained personnel commission and maintain it. For example, attaching the RTU to a low-static-pressure air distribution system will reduce control problems, noise, and fan power draw.
Maximize your equipment’s installed efficiency and performance by comprehensively testing and adjusting the installed unit, its controls, and the air distribution system. Specifically, consider hiring a contractor for these relatively low-cost efficiency measures:
- Conducting regular tune-ups
- Correcting refrigerant charge
- Cleaning and adjusting the system to correct airflow and improve heat transfer
- Verifying proper economizer operation
- Repairing major duct leaks
Maintaining your RTU
RTUs can fall into disrepair over time, leading to energy waste and occupant discomfort. To ensure that the units cool buildings and occupants as intended without wasting energy, be sure to change filters and conduct inspections quarterly. We also recommend annually performing other maintenance tasks such as cleaning ducts and coils, clearing any obstructed air paths, confirming control operation, and cleaning drain pans. This will help to prevent, or quickly catch, malfunctions before they have a chance to degrade the equipment or waste energy. For example, regular maintenance could reduce the risk of an economizer’s damper getting stuck in the open position. In a humid climate such as Tampa, Florida, that malfunction could cause the HVAC system to use 50% more energy than a system without an economizer would use.
Because getting service personnel and equipment on-site is a large part of the cost of servicing an RTU, it doesn’t make sense to use a piecemeal approach that only addresses one or two maintenance measures at a time. Instead, use an annual service or maintenance program or contract to maintain the entire unit.
Clean condenser coils In an RTU, the condenser coil is exposed to unfiltered outdoor air, so it will accumulate dirt buildup at a much higher rate than the other coils. The performance penalty of a dirty condenser makes cleaning it one of the most important RTU maintenance measures. As dirt accumulates, the coil’s ability to exchange heat—and thus its cooling capacity—will decrease, and its power draw will increase. A dirty coil will also impede airflow, reducing cooling capacity. If your equipment is in a harsh environment or often runs at or near capacity, you will need to clean your coils more often.
The best tool for this maintenance job is a power washer that feeds cleaning solution into a high-pressure water spray. Spray-on cleaning solutions intended to be used with a brush and a hose won’t do a good enough job of cleaning the coils, even though they may brighten the outer surface. A thorough condenser cleaning involves before-and-after measurements of the temperature difference across the coil to verify the cleaning’s effectiveness. If done improperly, power washing can damage coils by bending the fins or even breaking them off if the coil is old. Because of this risk, you should hire trained professionals to complete this task.
Clean evaporator coils Dirt on the evaporator coil causes two problems: It reduces airflow and it directly degrades the coil’s heat-transfer efficiency, which significantly reduces cooling capacity. With good filtration, an RTU’s evaporator coil will stay fairly clean.
Some technicians claim that annual evaporator coil cleaning is an unnecessary expense, but others say that enough dirt gets around or through the filters to justify the cost. Those in favor of annual evaporator coil cleaning believe that as long as the cleaning equipment is on the roof for the annual cleaning, it’s worth cleaning the evaporator coil as well.
You, or trained personnel, should inspect the evaporator coil at least once a year to make sure that the filters have been doing their job. Shining a light through the coil is one way to inspect it, although wavy-fin designs can make this difficult. Another approach is to measure supply fan amperage and filter or coil pressure drop when you install fresh filters. If the amps are lower and the pressure drop is higher than last year’s measurement with fresh filters then you need to clean the evaporator coil.
Fix refrigerant leaks Fixing refrigerant leaks in RTUs is a tedious but important task. Refrigerant charge that’s too low can compromise efficiency and capacity. The following tips can help prevent leaks and simplify repair:
- Use brass caps with rubber gaskets for all connections and keep them wrench-tight (rather than just hand-tight). Have plenty of spares on hand because they’re easy to lose. Replace these gaskets or the complete caps periodically because they dry out and crack over time.
- Enforce a standard of cleanliness for all refrigerant lines. Maintaining clean copper pipes makes it easier to find oil, which can leak from refrigeration lines. Wipe the lines regularly with a clean rag.
- Look for leaks on flanged and screw fittings first, rather than on soldered joints.
- Minimize the number of connection points between refrigerant gauges and service ports—at each connection, the system loses some refrigerant.
- Repair a leaking coil only once; when you do, mark it with paint or chalk. The next time it leaks, replace the coil completely.
- To repair a leak, first find where it is in the system. Then solder the area of the leak or replace broken components. Remove all refrigerant from the system and repressurize the system with nitrogen to ensure there are no more leaks (repeat this cycle at least three times). Reintroduce refrigerant and recharge the system according to the manufacturer’s refrigerant pressure temperature chart specifications.
Maintain cabinet integrity Sometimes RTUs spill their expensive chilled air onto the roof because of poorly designed or maintained cabinet hardware (figure 5). Annual inspections should include a survey of air leaks, followed by repairs including replacing screws or latches and patching or replacing gaskets. Cabinet integrity is particularly important on the supply-air side, where the fan creates high pressure that can force considerable air out of a small crack. Losing 200 cubic feet per minute (cfm) from a 10-ton RTU reduces cooling and airflow capacity by about 5% and wastes more than $120 per year in energy costs (assuming an EER of 11.2 and 2,000 hours of operation at $0.12 per kilowatt-hour). Another potential source of air leakage is through the condensate drain pipe that comes out of the pan under the evaporator coil. A narrow pipe section or U-bend water trap can reduce or eliminate this type of leak.
Most RTUs are covered with access panels that are held in place by small sheet-metal screws. Using hand tools to remove and reinstall these small screws can be frustrating—which explains why many panels have only one or two screws left in place after a few service calls. But loose panels mean that a unit is leaking valuable chilled air onto the roof. A cordless drill with the correct bit makes panel access and screw replacement quick and easy. Make sure that the drill has a clutch to prevent overtightening or stripping the screws and keep a bag of replacement screws, including oversize ones for stripped holes.
The final step in checking the air side of the system is to make sure the airflow is between 350 and 400 cfm per ton of cooling capacity. Because airflow in the unit and ducts isn’t uniform, single-point air-velocity measurements are nearly worthless. This makes measuring airflow difficult. We recommend this three-step process:
- Measure total static pressure drop across the fan.
- Measure fan-shaft rotation with a tachometer.
- Look up the flow from the manufacturer’s fan curve based on fan speed and pressure.
The hardest part of this process is likely to be obtaining the fan curve. You can call the manufacturer’s technical support service with the model number and field measurements and have them read back the corresponding flow rate. Collecting and archiving complete performance documentation on an RTU when you installed it—or incorporating it into a comprehensive service program—makes this type of measurement much easier.
If the airflow isn’t within the expected range and the cabinet is properly sealed, investigate to find out why—ducts could be leaking or obstructed, dampers may not be set or performing properly, or the fan may be malfunctioning.
Change filters Filters play two important roles: They help maintain indoor air quality, and they protect the evaporator coil and fan from dirt accumulations that can cripple performance (figure 6). A dirty filter will slow down standard fans. Although this reduces the fan’s energy usage, it also decreases the capacity and efficiency of the system.
To determine when to replace your filter, measure the pressure drop across the filter, schedule a calendar reminder, or visually inspect the system. Scheduled intervals should be between one and six months, depending on the pollutant loading from indoor and outdoor air. Your system might require more-frequent changes during the economizer season because outdoor air is usually dirtier than indoor air.
Measuring pressure drop is the most reliable way to determine how dirty your RTU filter is. However, this approach requires some effort—most RTUs don’t have built-in pressure taps that let you check the pressure easily. You can make taps by drilling into the cabinet wall and installing quarter-inch tubing with removable caps. A technician can use a handheld pressure meter to check the filter’s status. To get accurate readings, shut cabinet access panels tightly and replace all screws. In facilities with predictable and regular filter loading, you can use pressure measurements to establish the proper filter-change interval and schedule future filter changes.
Automated pressure tracking makes sense for RTUs with 20 tons or more of capacity and for large custom-built air handlers. There are two ways to track pressure automatically:
- Install transducers that feed pressure-drop data to a building automation system. When pressure drop exceeds the design limit, it triggers an alarm. Building operators can also use this data to check filter status. Unfortunately, this strategy yields a long payback period when applied to small RTUs because pressure transducers cost about $260 (for a model with plus or minus 3% accuracy).
- Use a differential pressure switch that flips from off to on at a certain pressure difference across the filter. This is a less expensive option, but these switches require time and care to calibrate.
Maintain outside air dampers Dampers are notorious for falling into disrepair. If your RTU has an economizer, make sure you have it serviced and tested with the rest of your RTU system to ensure proper functionality.
What’s on the horizon?
In the future, federal standards will likely make new RTUs more efficient. To achieve higher efficiencies, RTU manufacturers will include variable-speed and staged compressors to improve part-load performance. In addition, manufacturers may start to incorporate technologies such as energy-recovery devices and evaporative cooling in the condenser.
To improve efficiency of RTUs, the California Energy Commission’s Public Interest Energy Research Program developed sophisticated RTU designs through its Advanced Automated HVAC Fault Detections and Diagnostics Commercialization Program (PDF). The DOE and PNNL run a similar program called Smart Monitoring and Diagnostic Systems. These programs seek to develop FDD capabilities and fault-resistant HVAC equipment. They also work with manufacturers to implement these features into new products.
Who are the manufacturers?
The major manufacturers of RTU equipment are:
The major manufacturers of RTU retrofit controllers are:
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