According to the U.S. Department of Energy (DOE), packaged rooftop units (RTUs) are used in 46 percent (2.1 million) of all commercial buildings, serving more than 60 percent (39 billion square feet) of the commercial building floor space in the U.S. Many of these units are older and inefficient, wasting between $1,000-$3,700 per unit each year, depending on the building size and type. They may also be costly to maintain and less able to adequately control space temperature and humidity.
Replacing these older RTUs can be expensive, though, and building owners and managers may be hesitant to make such large investments in an uncertain economy. For this market segment, the answer may be for contractors to retrofit these units with advanced controls that can help save energy costs as well as improve comfort.
One control system that is intended to reduce the electrical consumption of RTUs is Enerfit, an indoor fan variable-speed retrofit designed for single-zone commercial units. Enerfit consists of a variable-frequency drive (VFD), a controller, and a set of precision sensors, which, together, fully modulate the supply fan to provide the proper dew point temperature of the air delivered to the conditioned space in cooling mode.
The cooling mode is extremely important in most commercial buildings, said Ron Patch, president, Enerfit LLC, Chattanooga, Tennessee, because internal heat gains require HVAC systems to run in cooling mode about 50 percent of the time in Climate Zones 5-7, and 60-70 percent of the time in Climate Zones 1-4 (see Figure 1, page 14).
“In the cooling mode, Enerfit’s patented control methodology dramatically reduces fan energy, significantly reduces the relative humidity in humid climates, and reduces the building cooling load due to less fan heat delivered to the space.”
In heating, ventilation, and economizer modes, Enerfit indexes the fan speed to a preset speed depending on the specific unit characteristics of the HVAC unit on which it is installed. With the addition of the new EnerTouch module, Enerfit can be accessed over the Internet through smartphones, tablets, and computers using HTML only, no JAVA, Flash, or Silverlight required.
While Enerfit can save a significant amount of energy, it can only be used on RTUs that have a three-phase fan motor and serve only one control zone. That means the minimum size is 6 ton, but there is no upper limit in unit size, as long as the unit is used in a single-zone application, Patch said. Any brand and type of unit (including geothermal) can be retrofitted; however, if the unit casing and condenser coils are beyond repair, the unit should not be retrofitted.
“The age of the unit is not as important as the condition of the unit,” said Patch. “The larger the tonnage of the unit, the better; the more indoor fan operating hours, the better; and of course, the higher the energy costs, the faster the payback.”
Regarding the latter, Patch noted RTUs retrofitted with Enerfit usually experience paybacks in the range of one to three years, before utility incentives. But, determining whether Enerfit is a good investment should be done on a case-by-case basis. If the contractor and building owner/manager decide to move forward with the retrofit, installation is straightforward and can be done in one to two hours. “The latest version of Enerfit requires between five and 11 low-voltage field-wiring terminations, and the unit-specific monitoring and sensing capabilities come pre-wired and pre-programmed out of the box.”
Besides saving energy, building owners/managers have reported that, once Enerfit was installed, they experienced improved comfort, lower HVAC unit noise, and reduced drafts from excess air movement, said Patch. “We are especially pleased to get these positive comments because, as most folks in the HVAC business know, getting an actual compliment about building comfort is unusual. Generally, in our business, no news is good news.”
A Catalyst to Savings
Another RTU retrofit option is Transformative Wave’s Catalyst, which is designed for constant-volume HVAC systems. This complete retrofit kit — which involves the addition of multiple sensors, a pre-wired external enclosure, a variable-frequency drive, direct control of the economizer actuator, an energy measurement sensor, wireless communication, and IT components — upgrades existing HVAC equipment into dynamic part-load systems that match the energy use to the true needs of the occupied space, said Danny Miller, president, Transformative Wave, Kent, Washington.
“The Catalyst uses unique logic and sequences to ensure proper ventilation, maximize the use of outside air for free cooling beyond standard economizer logic, and reduce fan energy use,” said Miller. “It has also been tested and validated by the DOE and shown to offer HVAC energy savings.”
In terms of payback, typical Catalyst customers see a return on investment (ROI) for target applications in one to three years, depending on utility costs and available incentives. In markets with high utility costs and generous incentive programs, customers will see a quicker ROI and lower out-of-pocket costs, said Miller.
The verification of savings is provided by the eIQ Platform, which is included in every Catalyst project. This wireless rooftop network allows each RTU to be monitored and controlled over the Web in real time, from any browser, smartphone, or tablet. The eIQ collects and stores approximately 40 points of RTU data in one-minute interval histories, and that data can be used to document the difference in energy use between Catalyst operation and standard operation.
Miller noted that Catalyst works best when installed on RTUs that have run times in excess of 4,400 hours per year. Equipment that runs longer will see even greater savings and a quicker ROI.
For superior performance, three-phase indoor fan motor must be at least 1 horsepower (hp) in size. Units that are larger than 7.5 ton and have indoor fan motors of 2 hp or greater make some of the best retrofit candidates.
Projects with a mixture of small and large equipment can benefit if the equipment mix tends toward the larger sizes (12.5-plus ton), if run times are extended and if there are not a lot of smaller units and split systems in the mix. While this last situation may have potential, it would need to be evaluated on a case-by-case basis.
Age is not a factor when considering a Catalyst retrofit, as it has been installed on units as old as 27 years, said Miller. “We do not exclude units based on age. If the customer has plans to replace a unit within the next 18 months, it probably does not make sense to retrofit it; however, Catalyst can be transferred from an older unit to a replacement unit at minimal cost.”
Interest in Catalyst continues to grow as HVAC and controls contractors gain appreciation for the technology that keeps RTUs operational and in an optimized state, said Miller. “As our company name implies, we are committed to creating game-changing technologies that will create a wave that transforms the efficiency landscape.”
More Comfort, Less Energy
Bes-Tech’s RTU retrofit control kit, Digi-RTU, is designed to improve the energy efficiency and demand requirements of a rooftop unit, as well as solve the humidity, common noise, and frequent on-off issues by modulating the capacity of an RTU to match the cooling or heating space requirements. In addition, it reduces operation and maintenance costs and works with all existing building and rooftop controls and energy management devices, said Bruce Geary, vice president of sales and marketing, Bes-Tech Inc., Omaha, Nebraska.
The installation of the Digi-RTU results in significant reductions in kilowatt hours and kilowatts, said Geary, and the kit includes supply fan management, compressor management, economizer control, demand control ventilation that meets ASHRAE Standard 61.2 and California Title 24, fault detection, and a demand response/load limit factor between 50-100 percent.
“Each Digi-RTU kit can be installed within four hours, and all installers must be certified during a two-day class at Bes-Tech’s training center in Omaha,” Geary said.
Digi-RTU may be used in all types of HVAC equipment, from rooftop units (3-30 ton) to split systems to water- and air-source heat pumps, said Geary. “It can be used with single- or multi-zone applications and in equipment with up to four cooling and two heating stages. It can also work with existing thermostats or building management systems. Because of its compressor management, Digi-RTU can be used in all climate zones. The only time it should not be installed is if the unit is poorly maintained.”
With all the aging RTUs currently in service, demand will likely grow for contractors who can optimize equipment performance through cost-effective, energy-saving solutions. Variable-speed controls that can be retrofitted easily in the field may be the solution that many building owners and managers are seeking.
Publication date: 9/8/2014