A Look at the Evolving Rooftop Unit Market
Trends and evolution in DOAS, motors, and controls get things started for RTUs
When Carole King and Gerry Goffin wrote “Up On The Roof” in 1962 — recorded for hits by The Drifters and James Taylor, among others — they envisioned the roof as a sort of placid getaway. In HVAC, the world of rooftop equipment doesn’t quite resemble an oasis of calm up above the bustle, but that’s a good thing.
Some increasingly common capabilities and customer demands are trickling upward to the equipment in this area, while other progress continues in terms of rooftop unit (RTU) performance and better, cheaper maintenance. In at least one case, technology seems to be making a true leap in terms of what kind of equipment one might find (or install) up there.
A tour of rooftop trends begins with the natural pattern of technological advancements getting less expensive as they gain acceptance — which, in turn, leads to yet more acceptance.
Modulation is enjoying that phenomenon now, according to Eric Newburg, director of commercial product management, Commercial and Applied Ducted Systems, Johnson Controls Inc.
“Modulating gas heat, modulating compressors, and modulating hot gas reheat … the advancement of controls has made modulation more cost-effective,” he said.
Dedicated outdoor air systems (DOAS) have increased their usage in general and their name recognition over the last decade, but Chris Stocker, commercial rooftop product manager at Daikin Applied, sees one consequence.
NO RE-ENTRY: Daikin describes its DOAS RTU inverter compressor technology as having a small refrigerant system,preventing the unit from shutting down and therefore allowing moisture to return to the airstream.
“Most buildings’ largest humidity load comes from the ventilation air brought into the occupied space,” said Stocker. “With the rise in popularity of DOAS, rooftops are now the primary source of building dehumidification.”
Most equipment-related savings for owners tend to come as increases in performance efficiency. Newberg noted an exception that those minding the facility budgets will likely welcome all the same.
“There is growing desire for airflow systems on rooftop units to eliminate belts, to reduce the constant adjustment and replacement requirements,” Newberg said.
As a result, he sees direct-drive fan systems growing in popularity.
Another issue comes from more of a ripple effect following an earlier trend. As variable-frequency drives (VFDs) become more common in RTUs, that presence is shedding light on possible compatibility and sizing issues with power configurations serving the equipment, according to Newburg.
“VFDs are generally sized using balanced legs in a Y power configuration,” he said. “When delta power configurations are used, VFDs must be sized accordingly. When rooftop units are designed, it is based on balanced power.”
Thus, he concluded, delta power installation might lead to VFD failures.
CONTROLS AT THE GATEWAY
The mention above of controls as the key factor in extending the use of modulation was only the beginning. Stocker explained how inverter compressors in a modern RTU like his company’s Rebel® can keep the refrigeration circuit running in order to keep moisture from getting reintroduced into the space because a compressor has turned off with a wet coil.
“Advanced DOAS control in smart rooftops can aid further in dehumidification by independently controlling the evaporator, removing moisture down to a set dew point,” said Stocker.
“This further allows the use of free heating from the hot gas reheat coil to independently control the discharge to the desired dry bulb, resulting in a perfectly cooled space and optimum occupant comfort.”
And of course, practically no aspect of life is immune from the creeping connectivity expectations of smart controls and remote management. Stocker mentioned that Rebel owners can peruse hundreds of data points on their phones, from cfm introduced to the leaving coil temperature of the evaporator.
Newberg pointed to this as another category where costs come down and usage goes up.
“Rooftop units are joining the community of internet-connected devices,” he said.
To that end, he also expects the monitoring of performance, logging usage details, and staying abreast of any reports/alarms to gain more mainstream momentum.
(RTU-to-phone isn’t the only avenue getting widened for better connectivity these days. Newberg also noted that his company’s Verasys control system now provides predesigned system application programming to integrate different HVAC units.)
VRF TAKES A LEAP
According to the manufacturer, the Toshiba Carrier VRF Rooftop is one of the first RTU product lines specifically designed using VRF technology. It is designed to allow multiple rooftop units to be connected to one condensing unit and additional VRF fan coil units. Part of the idea’s appeal is to use existing roof curbs and ductwork, avoiding the need to run new refrigerant piping into the building.
Economizer capabilities, especially when used alongside CO sensors, are something the company touts as helping owners to meet their particular building codes and standard compliance. The manufacturer offers other options, such as electric heat and a choice of horizontal or vertical discharge.
The unit is available in 3-, 4-, and 5-ton capacities with an eye toward application for schools, shopping centers, places of worship, and buildings with structure load requirements.
Beyond this design, Carrier continues to offer a line of other outdoor packaged units, with customers choosing from gas, electric, hybrid, electric heating only, and no cooling. Those choices are accompanied by the company’s 62X DOAS unit, which contains high-capacity DX coils and scroll compressors.
STREAMLINE THE DESIGN
Elsewhere in this category of equipment, Johnson Controls Inc. is promoting is NexusPremier™ 25- to 50-ton commercial RTUs. According to company officials, the equipment is meant to achieve efficiencies 50 percent above what is required (depending on local code).
Before the unit reaches the roof, the line endeavors to streamline the design process by offering tailored BIM tools, improved submittals, and simplified specifications.
Once the unit is in place, startup wizards and what the company describes as an array of user-adaptable parameters serve to expedite startup and commissioning. The Smart Equipment tool allows users to manage multiple RTUs and integrate with a number of the company’s controllers and BAS components.
On the connectivity side, the Navigator® product service app paves the way for accessing technical literature and a direct connection to technical support via a unique QR code on each RTU as of this spring. As an option, customers can employ the Mobile Access Portal for additional monitoring that may make for fewer trips to the roof to diagnose issues.
Finally, Daikin’s Rebel RTU line, touched on earlier, includes some connectivity appeal but focuses heavily on performance. The manufacturer describes the line as providing a 41 percent energy savings over a year’s operation in cooling mode (ASHRAE 90.1-2013 minimum) while producing a 20.6 IEER.
At the core of these capabilities, the Rebel relies on its inverter scroll compressor to deliver a high level of temperature/humidity control, efficiency, and resulting cost savings.
Options include CORE heat exchangers and a rotating energy recovery wheel to further boost efficiency.
Publication date: 5/27/2019