In contrast to critical systems, one of the primary consequences of faults in HVAC systems is economic rather than safety-related. Therefore, FDD systems applied to HVAC systems must be assessed based upon economic considerations. However, existing research in this application has mainly focused on technique development and validation. This paper addresses the economics of FDD application to HVAC systems. Two major aspects of savings associated with FDD application to rooftop air conditioning units (RTU) are investigated qualitatively and quantitatively: service and operating cost savings. Automated FDD reduces service costs due to reduced preventive maintenance inspections, fault prevention, lower cost fault detection and diagnosis (FDD), better scheduling of multiple service activities, and better scheduling of service to low season. Operating cost savings consist of utility cost and equipment life savings. A methodology was presented for evaluating the cost savings for application of automated FDD to RTUs. Application of the methodology to a number of sites in California sites showed that $108/ton-year, around 70% of the original service costs, could be eliminated, and the operating cost savings range from $20 to $180/ton-year, which depends on the location and application. The savings are significant and payback period for an FDD technique presented by Li and Braun (2004) that relies on low-cost sensors is less than one year.
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