Packaged rooftop units (RTUs) are the workhorses of the commercial market, providing cooling to approximately 60% of U.S. commercial building floor area. While it is impossible to know exactly how many are currently in operation, some estimate that there are about 15 million RTUs installed in commercial buildings in the U.S.
RTUs often lose efficiency over time, particularly if they are older or have not been properly maintained, leading to higher energy bills and lower occupant comfort. Replacing RTUs can be an expensive proposition, though, and building owners and managers may be hesitant to make such large investments. In this case, the best solution may be for contractors to suggest retrofitting these units with energy-saving upgrades, which can save money as well as improve occupant comfort.
Understand The System
Before deciding whether it even makes sense to retrofit an existing rooftop unit, there are several factors to consider. First, understanding the age of the equipment is critical, said Nathan Cazee, project development engineer at Daikin Applied. Many of today’s direct-expansion (DX) systems have a lifespan of 15 to 20 years, so if a system is 12 or 13 years old, making costly upgrades may not deliver the greatest ROI, he said. Second, an assessment should be made regarding whether any changes have been made to how the equipment and space are being used.
“For example, we recently worked with a school district that purchased an old office/warehouse building with packaged roof equipment, and they planned to convert the space into a teacher training and special education facility,” said Cazee. “Given the changes, we knew the heat loads were going to be totally different. Instead of replacing the equipment, which is what the customer initially thought would be required, we recommended some minor adjustments to ducts and dampers, as well as a new control system that offered variable volume functionality to improve both efficiency and comfort.”
That is why before any retrofit measures are undertaken, contractors should first completely understand how the current installed system operates. This may involve consulting with the manufacturer and/or an engineer in order to perform various analyses to determine if the retrofit measures will be effective, said Sankar Padhmanabhan, application expert of rooftops and packaged systems at Danfoss.
“The analysis may include investigation of the current installed unit features, building load analysis, energy simulation with current installed features and proposed retrofit features, and subsequent ROI analysis,” he said. “Using energy simulation such as EnergyPlus and DOE-2 in combination with analysis tools provided by the equipment manufacturer will provide a detailed energy consumption pattern. It’s important to remember that every retrofit will be unique to each building and its location.”
Contractors should listen to the end user as well in order to learn of issues that may include excessive energy usage, constant volume fan operation, oversized equipment, limited part-load operation, code minimum energy efficiency, and lack of advanced features, said Steve Hueckel, market manager of commercial a/c at Emerson.
“Contractors should also consider operating issues such as service deficiencies, deferred maintenance, improper set points, duct leakage, and the system’s inability to adapt to changes in building load realities,” he said. “In addition, contractors should evaluate if preventative maintenance will be ineffective, preventable repairs will be costly, and if premature replacement programs will be expensive versus upgrade options.”
TWO OPTIONS: If a retrofit is deemed appropriate, then end users will have two options to consider: a standard retrofit or a deep energy retrofit. (Courtesy of Lysair)
If a retrofit is deemed appropriate, then end users will have two options to consider: a standard retrofit — where economizers, variable-frequency drives (VFDs), or controls are added to a unit to reduce energy and maintenance costs — or a deep energy retrofit (DER), also known as retrocommissioning, said Jason Carter, senior product manager of ducted systems at Johnson Controls.
“A DER is completed with a thorough analysis of building systems in order to maximize energy savings, such as integrating BACNet or another building automation system,” he said. “Whether standard, DER, or a combination of the two is chosen, it’s important that retrofits incorporate compliant solutions specific to a building’s challenges to help facility executives accomplish specific cost savings and air quality goals.”
Another factor to take into account when retrofits are being considered is whether specified equipment meets existing and future energy standards, such as the DOE 2023 standard and updated ASHRAE 90.1 standard, said Carter. New efficiency rebates and LEED requirements can also drive equipment specification.
Project development engineer Daikin Applied
Once it has been determined that upgrading an RTU to improve energy efficiency makes sense, there are several retrofit options to consider. One is to convert fixed-speed condenser fans to variable-speed or multiple stage indoor blowers, but this should only be done after the indoor airflow requirements of the occupied building are considered, said Padhmanabhan. Converting to variable-speed fans will provide the greatest ROI because of the ease of installation and high compatibility with most applications, he noted.
“Another simple measure is to retrofit fixed-speed compressors with intermediate discharge valves (IDVs) or equivalent technology to improve the performance during part loads,” he said. “IDV technology eliminates overcompression during lower pressure ratios (low ambient) when it is not needed. In some cases, fixed-speed compressors can also be converted to variable-speed compression systems, which can be more complicated but will have the highest energy saving potential.”
If it’s within budget and possible to work into the existing space, Carter said that variable air volume (VAV) is the top airflow control choice, because it saves fan power costs and also provides better occupant comfort. He noted that VAV controls also offer significant energy savings by using a VFD and pressure transducer to maintain static pressure in the supply duct.
“Discrete fan control is also an option for constant air volume (CAV) systems that stage fan compressors to improve energy efficiency as well as comfort and performance,” he said. “This type of single-zone airflow control is ideal for spaces like warehouses. Unlike the typical single-zone system operation, continuous reset single-zone (CRSZ) control is an option that operates with the minimum fan speed needed to maximize energy savings and comfort. This can also support passive dehumidification.”
Depending on the location, free cooling and other economizer controls can also be utilized to provide energy-efficient cooling, said Carter. This works best in areas where nighttime temperatures fall below about 55°F, he noted, as an economizer can bring in outdoor air to cool a space without the use of compressors or the unit’s full cooling capacity.
Cazee believes that without a doubt, the biggest efficiency gains are made through control upgrades, adding that being able to modulate fan speeds and optimize run times delivers significant efficiency and cost benefits.
“While it all varies based on the size of the units and ability to modulate the capacity, getting the controls dialed in and optimizing the equipment to efficiently manage part load conditions, such as installing VFDs, is where you’re going to see the most energy savings,” he said. “Typically, a retrofit will result in energy savings between 5% and 15%. To achieve higher savings, it will generally be necessary to replace and upgrade to newer technologies, where the capital investment can be justified with the ROI.”
Still, if repairing a piece of equipment requires more than 60% of the replacement cost and the maintenance has been deferred, it’s likely time to replace, added Cazee.
Besides energy savings, RTU retrofits can result in better occupant comfort and IAQ, as well as stretching the useful life of equipment with the proper upgrades and maintenance, said Cazee. For example, if a rooftop unit is seven to eight years old, making updates such as installing VFDs could extend the life of that equipment and delay replacement, he noted.
Other retrofits may not result in energy savings but could be useful in improving occupant comfort. For example, demand control ventilation can be added to help bring fresh air into buildings by increasing the level of outdoor air intake, and ultraviolet lights are also very popular for retrofits to improve IAQ, said Carter.
“These systems can kill or inactivate mold, mildew, dander, bacteria, and viruses by using UV-C light to damage the nucleic acids and proteins of pathogens,” he said. “These devices can be mounted in ductwork or packaged units, helping them stay clean and run more efficiently. They can also help combat the mold and bacteria often found in high-condensation areas of system components like the indoor coil.”
End users are definitely placing more importance on IAQ, which can be improved with upgrades, said Hueckel. This can include bringing in additional fresh, clean air indoors, which improves the health and safety standards for the building occupants. With winter just around the corner, IAQ will be even more vital in helping to address the challenges of flu season.
“IoT is another benefit that can be realized through upgrading,” he said. “Helping buildings and systems stay connected and run more efficiently is a key focus area. Our compressors can include embedded sensors for predictive diagnostics and remote detection to enhance system protection, improve fault and oil detection. When combined with our multisystem thermostats, our technology manages schedule changes and notifies you when a room temperature is over- or undercooled.”
Smart controls such as fault detection and diagnostics (FDD) can also help facility executives get the most out of their rooftop units, said Carter. FDD technology uses multiple sensors to continuously monitor refrigeration circuit temperatures and pressures, economizer operation, and outdoor humidity and temperatures. FDD algorithms then process the collected data and deliver detailed alerts if and when issues arise, helping facility executives ensure equipment is always performing as specified.
“By monitoring these critical variables, facility executives can respond to issues before they first arise, taking a proactive rather than reactive approach to HVAC management and maximizing the lifecycle of a commercial RTU,” said Carter.
Click graph to enlarge
SMART CONTROLS: In the graph shown here, Johnson Controls demonstrates how smart controls such as fault detection and diagnostics (FDD) can help facility executives extend the lives of their rooftop units. (Courtesy of Johnson Controls)
While there are many benefits associated with retrofitting rooftops, convincing building owners and managers to invest in these upgrades can sometimes be a challenge. One of the best ways to overcome that hurdle is to use an energy simulation program that provides before and after scenarios, said Padhmanabhan.
“These are compelling tools for contractors to use that clearly show building owners the potential energy savings that can be achieved,” he said. “Sophisticated versions of these energy simulation tools, such as EnergyPlus, can also quantify the building’s IAQ and
Manufacturers such as Johnson Controls also offer free energy saving calculators and other tools to help contractors and facility managers determine ROI and energy savings of various retrofits, based on the location of the application, said Carter.
Contractors can also help educate building owners about the tax deductions available for commercial buildings as part of an upgrade process, said Hueckel. For example, according to Section 179D of the Internal Revenue Code, deductions are allowed for energy-efficient commercial buildings, and those deductions were made permanent under the Consolidated Appropriations Act of 2021.
“A tax deduction of up to $1.80 per square foot is available to owners or designers of commercial buildings or systems that demonstrate a 50% reduction in energy usage accomplished solely through improvements to the heating, cooling, ventilation, hot water, and interior lighting systems,” he said. “Partial deductions of up to $.60 per square foot can be taken for qualifying measures. These deductions are available for buildings or systems placed in service after Dec. 31, 2017.”
In the end, it usually comes down to dollars and cents, said Cazee. “If end users are in a market with higher utility rates, which tend to be states along either coast, it can be easier to justify retrofits due to the corresponding energy savings. In many areas, it is also possible to take advantage of performance contracting and guaranteed energy savings programs that use longer-term cost savings to fund infrastructure upgrades and retrofit work. And given that many organizations now have sustainability and carbon-reduction goals, pairing energy savings with anticipated emissions reductions can make a strong argument for whether or not to move forward with a retrofit project.”