Building control systems are an integral component of a new building. From simple thermostatic controls and manual light switches to complex building automation systems, controls are an integral part of building health, safety and comfort. They also are a key component of a building’s energy efficiency.
Economizers reduce cooling energy use dramatically, but often are found to be inoperable. A Public Interest Energy Research Program (PIER) report titled, Integrated Design of Small Commercial HVAC Systems, Element 4, found a number of problems with package rooftop equipment. Economizers show a high rate of failure in the study. Of the units equipped with economizers, 64 percent were not operating correctly. Failure modes included dampers that were stuck or inoperable (38 percent), sensor or control failure (46 percent), and poor operation (16 percent). The average energy impact of inoperable economizers is about 37 percent of the annual cooling energy.
Refrigerant charge. A total of 46 percent of the units tested were improperly charged, resulting in reductions in cooling capacity and/or unit efficiency. The average energy impact of refrigerant charge problems was about 5 percent of the annual cooling energy.
Low airflow. Low airflow was also a common problem. Overall, 39 percent of the units tested had very low airflow rates (< 300 cfm/ton). The average flow rate of all units tested was 325 cfm/ton, which is about 20 percent less than the flow rates generally used to rate unit efficiency. Reduced airflow results in reduced unit efficiency and cooling capacity. The annual energy impact of low airflow is about 7 percent of the annual cooling energy.
Cycling fans. System fans were found to be cycling on and off with a call for heating or cooling in 38 percent of the units tested. The supply of continuous fresh air during occupied hours relies on continuous operation of the HVAC unit supply fan.
Unoccupied fan operation. Fans were also observed to run continuously during unoccupied periods in 30 percent of the systems observed. While this practice improves the ventilation of the space, it represents an opportunity to save energy through thermostat setback and fan cycling during unoccupied periods.
Simultaneous heating and cooling. Adjacent rooftop units controlled by independent thermostats were observed to provide simultaneous heating and cooling to a space in 8 percent of the units monitored in the study. This was largely due to occupant errors in the set up and use of the thermostats, and poor thermostat placement during construction.
No outdoor air. A physical inspection revealed that about 8 percent of the units were not capable of supplying any outdoor air to the spaces served. In some cases, outdoor air intakes were not provided or were sealed off at the unit. In other instances, outdoor air dampers were stuck shut, preventing outdoor air intake.
Acceptance testing is a way of assuring that targeted building systems were designed, constructed and started up to the intent of the Standards. Acceptance tests can help identify many common problems such as the economizer problems described above.