Development of Economic Impact Models for RTU Economizer Faults

Stuck outdoor-air dampers can lead to significant energy waste when undiagnosed for extended periods of time. This is especially true for packaged (rooftop) air conditioners where preventative maintenance may not be frequent or only reserved for emergencies. Automated fault detection and diagnosis (AFDD) tools for outdoor-air dampers and economizers have been proposed in the past to reduce the effort and cost for this kind of maintenance and are even required by some new building standards (CA - Title 24 2013). While qualitatively, the effects of stuck damper faults are understood, much less has been written about these faults’ impacts on cooling cycle performance and actual operating costs. An investigation of incorrect outdoor-air fraction on cooling capacity, efficiency, sensible heat ratio (SHR), and run-time is presented. An evaluation of the commanded damper position based on economizer controller logic is used to capture impacts of stuck damper faults at the full range of position and under different ambient conditions. Using experimental data, these models are validated for a 4-ton rooftop air-conditioner (RTU) with integrated economizer. The combined effect of these impacts are analyzed based on air-side virtual sensors outputs and modified version of an economic performance degradation index (EPDI) first proposed by Li and Braun (2006). This performance index estimates fault impacts on operation costs as well as the added equipment costs due to the need to operate the air-conditioner longer. These economic performance impact outputs can be used in an optimal maintenance scheduling tool in future work.