The ComfortLink control system offers diagnostic information.
Rooftop units are useful for so many hvac design situations; it's a shame they can be such a pain in the you-know-what to maintain and service.

Manufacturers are always looking for ways to make installing, maintaining, and servicing their units more convenient. One of the best ways to get new design ideas is to talk to contractors and techs, to find out what they want and issues facing them on a daily basis.

With field input from close to 1,500 hvac installers and servicers, Carrier Corp.'s latest rooftop unit offering has features that enhance its safety and serviceability, according to Ted Cherubin, senior product manager for the Centurion Rooftop product line.

"It was built with safety in mind," he said. "When we design a product, safety relates to more than service technicians; it includes installers, end users, all the way through to the people who do nothing but change filters" (filter modules have no sharp edges and feature slide-out accessibility).

It is interesting to note how often safety and serviceability go hand in hand.

The unit features a slide-out blower assembly.


In the focus groups, "Techs and installers stressed safety," said Cherubin. One way to address this is to have more options (disconnect switches, smoke detectors, and a 115-V convenience outlet) installed at the factory.

Not only does this reduce the amount of time an installation crew spends on the roof, it also precludes other hazardous scenarios, like having a 150-ft extension cord running up the side of a building amd possibly running through puddles, or some other potentially hazardous situation. Instead, they can have a 115-V outlet within the unit.

Installation of rooftop units is often a tricky proposition, frequently requiring helicopters or cranes to lift a 2,000-lb unit on any given day or time of day. That probably won't change in the foreseeable future, but Carrier made sure the new units are built for easier rigging and durability during the lift.

Before performing service and maintenance, field techs often have to remove access panels. "If I have to go up and do maintenance -- if I have to take the panels off and they catch wind -- they're gone," said Cherubin. "I have seen them, two- to three-feet wide -- they're projectiles!" That's why the new line features hinged, tool-less access panels that are secured with built-in panel stabilizers.

The engineering team also designed the unit so that the compressor is as low to the base of the unit as possible. "Say lightening strikes and the compressor scrambles," said Cherubin; with the compressor as low as possible, "the technician doesn't have to strain his back" trying to remove the compressor from a high shelf, as on older units.

Then there are blower motor hazards. "A lot of stores require blowers to run 24/7," explained Cherubin. "The motors run every single minute, which can cause them to wear out quickly." Many techs don't feel safe crawling into or around a unit on the roof, he said. That's why the new rooftop line features a slide-out blower assembly. "You don't have to worry about climbing in to check the bearing or replace a belt," he said. Access is through a hinged panel, again with built-in panel stabilizers.

There are also risks with belt-drive motors: Over time, the belts break. "When a belt breaks on a 1,000-rpm unit, there is damage to other components, and possibly even to the servicer," Cherubin said. That's why the new units have belt guards built in.

The units also offer full access to the condensate pan and both sides of evaporator and condenser coils for routine cleaning.

The new rooftop unit features hinged access panels.


The brains of the unit is the ComfortLinkTMcontrol system. "Service techs give this high marks," said Cherubin. It not only saves on the amount of time they have to spend on the roof, it also helps reduce the number of tools they have to carry up.

"Normally during the troubleshooting process, you screw on the gauges to take pressure-temperature readings and get ready with your electrical meter," Cherubin said. "Now all the pressures, temperatures, and diagnostic readings are in front of you on a large, scrolling display screen. Techs also don't have to risk, for example, the high side blowing its refrigerant charge when you're taking a reading or jumping/troubleshooting electrical components," he said. It's provided as part of the controls.

The control system can also provide information about IAQ and can control building CO2 levels through built-in demand control ventilation capabilities. And there are other diagnostic functions, such as the number of cycles on each compressor, run time, plus any existing alarms and the history of those alarms. The control system can also help technicians do remote troubleshooting with a Navigator handheld tool, or through a PC via the Carrier Comfort Network.

A cutaway view of the Centurion.


The Centurion 48/50 HG Series is available in 12.5-, 15-, 18-, 20-, and 25-nominal-ton capacities. They use the same DP/DR and TJ/HJ roof curbs used on Carrier units for more than 20 years, said Ron Woodcock, manager of Carrier Commercial Systems and Services (CSS) Large Rooftop Product Business.

Model 48HG provides electric cooling and gas heating; Model 50HG provides electric cooling and heating. The net cooling capacity for the HG Series ranges from 156,000 to 280,000 Btuh with an EER ranging from 10.4 to 11.6, depending on the model.

For installation in corrosive urban, industrial, or coastal/marine locations, the company offers E-Coating, which bonds an impermeable epoxy coating to the specially prepared fin coil surface.

The units also offer independent, dual- or triple-refrigerant circuits with thermal expansion valves (TXVs) for optimum part-load efficiency. "The more stages we offer, the better the comfort control for the occupants and better overall system operating efficiencies," said Cherubin. The design is said to eliminate the need for hot gas bypass modifications.

The unit also features demand-controlled ventilation to meet ASHRAE IAQ requirements. It's an occupant-based measurement; the amount of fresh air needed is determined according to the amount of CO2 a sensor picks up from within the conditioned space. The sensor takes readings between indoor and outdoor air and compensates, said Cherubin.

The rooftop line also complies with UL and ULC requirements. In addition, the units have undergone abuse tests, operating in extreme ambient conditions and situations "that may happen once in a lifetime," Cherubin said.

For more information, contact the company at (e-mail); 800-227-7427; or visit (website).