Fault Detection and Diagnostics for Rooftop Air Conditioners

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5 ECONOMIC ASSESSMENTS
Since the primary consequences of faults in HVAC systems are economic, FDD
systems must be assessed based upon economic considerations. In order to performed
economic assessments, the following factors should be considered:
1. RTU systems are relatively inexpensive, around $750-1000 per ton for installed
equipment. A 6-ton RTU only costs about $5000. As a result, it is difficult to
persuade RTU manufacturers or customers to spend an additional $300 for an
automated FDD.
2. Utility costs are not very significant compared to other costs for a business. For
the northern California McDonalds sites, the monthly utility costs for a 6-ton
RTU are about $100. For the southern California Walgreens site, the monthly
costs are around $600.
3. Service costs are very high, $115 for each visit plus $60/hour ($50~75/hour) for
service. For example, a three-hour service visit costs $295.
In order to perform economic assessments, the following assumptions were
employed:
1. Service is required whenever the RTU cannot maintain comfort due to the
presence of one or more faults. This assumption implies that there is a high cost
associated with not maintaining comfort.
2. For a given building over a given time period, the cooling load is independent
of whether an RTU fault exists.
3. A technician will only detect and diagnose severe and obvious faults. In the
absence of preventative maintenance, technicians would typically be called to