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New efficiency standards for commercial air conditioners and furnaces — including rooftop units (RTUs) — were announced by the U.S. Department of Energy (DOE) in 2015. The standards were to be implemented in two phases, with the first phase taking effect on January 1, 2018. By that date, OEMs were required to increase the minimum efficiency of their RTUs by about 13%. The second phase will take place on Jan. 1, 2023, and require an additional 15% increase in efficiency.

RTUs are ubiquitous in the U.S., cooling about half of the total commercial floor space in applications ranging from office buildings and restaurants to schools and retail spaces. Given that the two efficiency increases were not that far apart, RTU manufacturers were proactive, with many already offering products that meet the 2023 standard.

First Phase

Meeting DOE’s 2018 deadline wasn’t too difficult for most OEMs, many of whom had already incorporated the necessary energy-efficiency features into their RTUs. For example, Carrier already had RTUs in their high and ultra-high series that met and/or exceeded the 2018 efficiency standard when it took effect.

“For the models that did not meet the 2018 standard, Carrier integrated our popular Staged Air Volume (SAV)™ indoor fan system, which automatically adjusts the indoor fan speed to match the required cooling stage to meet the new requirements,” said Ted Cherubin, senior product manager of rooftop units at Carrier. This SAV was previously used in the high and ultra-high-efficiency models successfully.”

Johnson Controls also didn’t have to make any changes to the products directly to meet the 2018 standard; instead, the company had to change the models that were offered, in order to be in compliance.

“For our RTUs above 6 tons, that meant that we no longer offered standard efficiency models with constant volume (CV) airflow control,” said Laura Meyerrose, commercial product manager of ducted systems at Johnson Controls. “Instead, our systems utilized IntelliSpeed™, which is variable frequency drive (VFD)-equipped multi-speed indoor air configuration. Using VFDs to drive fans results in reduced fan energy consumption up to 87% when operating at 40% system airflow, offering substantial energy savings over a constant volume fan.”

Moving to modulating supply fans for all its comfort cooling applications allowed Daikin Applied to meet the first-phase requirements. And by incorporating aspects of VAV control, the IEER on all of the company’s RTUs exceeded the 2018 minimum, said Matt Dodds, applied rooftop product manager at Daikin Applied.

Second Phase

In order to meet the additional efficiency increase required by the 2023 deadline, OEMs typically had to add a few more features and controls to their RTUs. At Daikin Applied, most changes involved adding modulating condenser fans based on refrigeration head pressure, said Dodds, who noted that their Rebel, Rebel Applied, and RoofPak products already meet the 2023 standard.

“By slowing down the condenser fans to maintain the optimum refrigerant condenser pressure, compressor and condenser fans operate as efficiently as possible,” he said. “Also, while it’s not required to comply with 2023 standards, we added inverter compressors to all of our rooftop products for additional part-load efficiency.”

Meeting the 2023 deadline may also result in some significant differences in the controls, added Dodds, as equipment will have VFDs that operate condenser fans and compressors, which adds control sequences that were historically reserved for the highest efficiency rooftop units. In some cases, manufacturers may need to increase the size of heat exchangers to achieve the new minimum standards, which could result in additional weight and larger footprints.

HIGHLY ENGINEERED: Carrier was able to work within its existing rooftop cabinet sizes and increase efficiencies, through highly engineered heat transfer, compression, and moving air devices. (Courtesy of Carrier)

Cherubin agrees that it was a challenge to achieve the mandated energy efficiency without expanding the footprint of their RTUs. But Carrier was able to work within its existing rooftop cabinet sizes and increase efficiencies, he said, through highly engineered heat transfer, compression, and moving air devices, which are all controlled by innovative controls.

“Our bigger challenge has been executing the amount of work needed to redesign our units to meet the 2023 standard while dealing with outside issues,” he said. “The COVID-19 pandemic has created several ongoing issues, such as team distancing, remote work force, sample component delays, and logistic issues, which have made this work more difficult. Additionally, sequencing the design changes required for the new UL standard 60335-2-40 with the efficiency update has also been a challenge.”

Carrier currently offers multi-efficiency RTU models to cover the standard, high, and ultra-high efficiency segments, which is the same way the company handled the efficiency requirements for 2018, said Cherubin.

“Our current and popular WeatherExpert® rooftop units already meet and exceed the 2023 efficiency standard,” he said. “These include our Stage Air Volume (SAV) and Variable Air Volume (VAV)™ models. In addition to these units, our rooftop models with our SystemVu™ controls provide optimum control operation of the unit with added diagnostics, free cooling, and fresh air management economizers.”

Overall, the second phase of the DOE’s efficiency standard is not expected to affect the installation, cabinet size, or the basic technology of RTUs. The primary difference will be in the types of indoor air configurations that OEMs will be able to offer, said Meyerrose, so there will be a difference in operation with the majority of systems running on VFD-equipped fans.

“Within most of our product lines, we carry various models at tiered efficiency levels: standard efficiency, high efficiency and, in some cases, ultra-high efficiency,” she said. “In order to meet the higher minimum efficiency requirement of the DOE 2023 standard, we didn’t have to create new product lines; instead, we modified existing lines. Johnson Controls’ Select, Choice, and Premier RTUs, among others, were designed with the DOE efficiency standards in mind, so many met the 2023 standards by 2019.”

Refrigerants

The impending phaseout of HFCs will eventually affect the refrigerant used in RTUs, but for now, most OEMs are still offering R-410A in their 2023 DOE-compliant products. That’s because the 2023 efficiency deadline precedes municipalities adopting the building codes needed to apply alternative refrigerants such as A2Ls (e.g., R-32, R-454B) in buildings, said Dodds.

“This means we need R-410A models to cover the gap, which is why many of our existing R-410A products already comply with the 2023 standards,” he said. “Due to refrigerant- and oil-system design differences, it’s not easy to convert R-410A products to low-GWP alternatives.”

That said, Daikin, whose North American companies include Goodman Mfg. Co., Daikin North America LLC, and Daikin Applied Americas Inc., has announced its intention to replace R-410A with R-32 in its ducted and ductless light residential, light commercial, and applied products.

“We’re currently focused on the 2023 DOE transition, in which products will utilize R-410A,” said Meyerrose. “In parallel, our teams are also continuing the development of our low-GWP product portfolio, which will utilize R-454B. R-454B cannot be dropped in or retrofitted into existing products. However, R-454B’s similar operating characteristics to R-410A are helping to ensure a smooth transition, as several of the critical system components (compressors, etc.) are very similar, if not identical, to today’s current R-410A components.”

Carrier will also be utilizing R-454B as its primary lower-GWP solution to replace R-410A in all of its ducted residential and light commercial packaged RTU solutions sold in North America. According to Cherubin, Carrier feels that with its GWP of 466 — one-fifth that of R-410A — R-454B is the best solution for this type of equipment. However, the company has not yet decided on the model series or whether it will be tied to the 2023 DOE efficiency mandate, he said.

“This is mainly because building codes must be updated to allow the installation of these products, and there are very few states and municipalities that have completed the required code updates,” he said. “Carrier is actively engaged in the building code process to help drive the necessary changes and will make product introductions with the new refrigerant accordingly.”

The refrigerant transition, combined with DOE’s energy-efficiency standards, are giving rooftop OEMs more than enough to think about these days. But, they always have an eye on what else the future may hold for them.

“Although it isn’t yet at the Federal level, states across America, along with other regions and countries, will continue to set electrification regulations in an effort to reduce greenhouse gases,” said Meyerrose. “I anticipate electrification policies will continue to impact the HVAC industry and will likely be a target at the federal level in coming years.”

Which means more regulations will likely be affecting RTUs in the future.