One of the key concerns of building facility managers is whether their HVAC systems can maintain building comfort on even the warmest days. Often, the path of least resistance is the one they are most likely to tread - embracing the all-too-common philosophy of “If it ain’t broke, don’t fix it.” For HVAC systems in general, however, and rooftop units (RTUs) in particular, this may be very bad advice.
In fact, according to many studies, most packaged RTU air conditioners and heat pumps in commercial buildings are operating inefficiently, due primarily to inadequate equipment maintenance. RTUs rarely receive regular preventative maintenance, and service calls are often an emergency response to a major system failure that severely impacts occupant comfort, much to the dismay of employees and customers. Even with maintenance service contracts, technicians typically only perform basic routine qualitative assessments, so impending equipment failures often go unrecognized, as do non-emergency faults that wind up causing significant energy waste.
When it Comes to the RTU Problem, How Big is Big?
To assess the magnitude of the RTU maintenance problem, the New Buildings Institute (NBI) conducted a study in 2004 for the Northwest Power and Conservation Council [NPCC]. This comprehensive report combined the results of four field studies comprising a total of 503 rooftop RTU units, at 181 commercial buildings sites, in five states. Summarizing briefly the report’s findings, 64% of the economizers had failed or required adjustment, 46% of the units had refrigerant charge levels that were out of range, airflow was found to be out-of-range on an average of 42% of the units and thermostats had problems in 58% of the units. That's a lot of problems!
Some of the thermostat problems identified in the report included:
- Inadequate thermostats that enabled only single-stage cooling
- Fan cycling during occupied periods or continuous fan operation during unoccupied periods
- No nighttime setback programmed
- Issues that prevented economizers from working at all
Most RTUs had more than one problem. In fact, 64% of the units had two or more of these problems. While we can hope that this situation has since improved, the overarching message this report conveys is that the clear majority of HVAC systems are not operating very efficiently. The good news is that many of the most common problems can be corrected with proper preventive maintenance.
Fixing the Problems: What Preventive Maintenance Do You Need?
Now that your awareness has been raised, what should you do to ensure that RTUs are operating at near top efficiency? Here are some of the most common problems that should be checked at least annually.
- Both the condenser and evaporator coils are prime suspects and should be cleaned regularly. Not only will this improve RTU efficiency and cooling capacity, it will also reduce the risk of fungal, bacterial and microbial growth in coils, ducts and pipes.
- Where included in the HVAC system, economizers are another very common source of problems. Economizer dampers may not open or close fully, or may be stuck in an intermediate position. Damper failures can be diagnosed by setting the economizer service switch to 100% and verifying that the outside air damper opens fully and the return air damper closes fully. Conversely, with the service switch set to 0%, the outside air damper should close fully and the return air damper open fully. Failure to operate correctly may be caused by a failure of the economizer actuator, maladjustment or slipping of the damper linkages or outside or mixed air temperature (dry or wet bulb) sensor malfunction.
- Less than optimum programming of economizer controls is another common problem. For example, in the temperature range of approximately 30°F to 55°F, outside and inside air is mixed to maintain the desired room temperature without additional heating or cooling. At higher outdoor temperatures, typically in the range of about 55°F to 75°F (depending on the humidity level), outdoor air still provides some cooling capacity but not enough to completely satisfy the load, so the economizer outside air damper should still be fully open, but supplemental mechanical cooling is now required. At higher outside temperatures, the outside damper should close, but not fully since some level of fresh air must always be mixed with the return air.
A couple of other things that should also be included in an RTU preventive maintenance check list are compressor refrigerant charge level (too high or too low) and proper air flow level. These items should be checked at least annually to maintain full RTU efficiency and capacity. For further information about RTU fault detection and diagnosis see, for example, Rooftop HVAC Fault Detection and Diagnostics: Capabilities of New Technologies for Energy Reduction and the references cited therein.
 Cowan, N. “Review of Recent Commercial Roof Top Unit Field Studies In the Pacific Northwest and California”. New Buildings Institute (NBI) for Northwest Power and Conservation Council [NPCC], October 2004. www.newbuildings.org.
 Heinemeier, Kristin (WCEC) and Mark Cherniack (NBI). 2012. Rooftop HVAC Fault Detection and Diagnostics: Capabilities of New Technologies for Energy Reduction.