Two strategies for maximizing rooftop HVAC unit efficiency
RTUs are too often overlooked as energy-saving opportunities.
Some energy efficiency opportunities are easy to spot—in efficient and outdated lighting will fade, flicker and burn out, while refrigeration units show their age with inconsistent cooling and a noisy compressor—but deficiencies in certain types of heating, ventilation and air conditioning (HVAC) equipment aren’t as obvious.
“Rooftop units (RTUs), in particular, are routinely overlooked as energy-saving opportunities,” says Reid Arkinstall, program manager at BC Hydro. “Left unattended in harsh weather, they will become increasingly inefficient and break down over time.”
An RTU is an outdoor air-handling unit that provides HVAC to the building below. RTUs are also known as packaged systems because all of their components—including coils that release and absorb heat, compressors and fans—are housed in one unit and stored out of the way, i.e. on the rooftop.
RTUs connect directly into a building’s central ventilation system to circulate air. They are most commonly used on low-rise commercial buildings, such as restaurants, warehouses and offices.
“Approximately 20% of commercial customers in British Columbia have at least one RTU,” says Arkinstall. “Given the number of units and the tendency for them to be neglected, the energy and cost saving potential for business owners is significant.”
He suggests two strategies. The first is preventative maintenance, bringing in a qualified commercial HVAC specialist to inspect RTUs four times per year.
“Regularly having your RTU serviced by an HVAC professional is the best way to avoid costly, unforeseen repairs and to keep it in the best condition possible,” says Arkinstall. “As the saying goes, an ounce of prevention is worth a pound of cure.”
The second strategy is to retrofit an existing rooftop HVAC unit with advanced controls.
“Most RTUs run at a constant volume,” Arkinstall explains. “When they are switched on, they blow air through the building, no matter the occupancy level or time of day. They are two-speed—either on or off. Advanced control technology, on the other hand, provides variable speed control, so the RTU only operates at the volume needed. Sensors can monitor the level of carbon dioxide (CO2) of the indoor air, to see how stale it is, as a measurement of occupancy.”
The average cost of adding a variable-frequency drive (VFD) to an RTU’s two-speed fan is $4,000, but the average electrical savings yielded per year are $2,000, ensuring a return on investment (ROI) in two years.
This article was supplied by BC Hydro, which offers financial incentives to businesses for RTU energy savings. For more information, visit www.bchydro.com/news/conservation.html.
This article ran in the July 2019 issue of Energy Manager Canada magazine.