P2-1. Rooftop Air Conditioining > Outcomes

Technical Outcomes:

• The project successfully demonstrated a fault detection and diagnosis (FDD) system for rooftop air conditioners that can be the extended to families of manufacturer's rooftop units. FDD for a single unit can be generalized to fault detection in families of equipment using temperature measurements and parametric values based on published information. A hybrid model of polynomial and GRNN was found to work best. Several modeling approaches were evaluated, including polynomial functions, general regression neural networks (GRNN), back propagation, and radial basis functions in their ability to detect anomalies.
  
  
• Significant improvements were made in methods for detecting single faults. A new steady-state detector was incorporated, which has better robustness for filtering transient data and thus improves overall FDD performance. A new modeling approach was used for predicting normal operation variables. A novel fault detection classifier, called the normalized distance method, was developed that eliminates probability calculations and requires very little computer memory. Overall performance models were created, which act as virtual sensors to estimate some system performance variables that cannot be economically measured.
  
  
• Multiple-fault FDD is also technically feasible and the methods are applicable to packaged rooftop air-conditioners with fixed orifice and thermal expansion valves. A new method capable of detecting at least three simultaneous faults was developed. The new model was tested using laboratory and field data and can be used on units with thermal expansion devices as well as fixed-orifice metering devices. Figure 1 is a screen capture of the multi-fault demonstration movie.
  
  

Figure 1 Snapshot of Multi-fault FDD Demonstration Movie

• The economics of using FDD technology shows promise. The range of net savings (total savings minus FDD system cost) over the ten year life of the unit is estimated to be between $4000 to $10,000 per RTU. Operational cost savings includes two parts, utility cost and equipment life savings, and could be significant for building owners. The utility cost savings are associated with the equipment operating more efficiently due to better maintenance. Equipment life savings are from better maintenance and less adverse operating conditions. Fault detection and diagnosis savings are possible, which includes unnecessary service and fault diagnosis savings. Unnecessary service includes regular service, such as coil cleaning, that is not justified and unnecessary repairs that are based upon incorrect fault diagnoses. Fault diagnosis savings are due to reduced technician time associated with diagnosing a problem. Greater savings are possible in hotter climates due to larger cooling requirements. The savings would be also greater for heat pumps because they operate throughout the whole year.
  
• The results will produce better indoor environments in California buildings. Usually equipment malfunctions are only detected when someone complains about a lack of cooling, heating, fresh air, or other comfort issues. Other problems simply continue unreported. Automatic FDD for rooftop equipment will call attention to problems before they lead to discomfort or health problems in a building's occupants.
  
  
• FDD can lead to more effective utilization of technician's labor. Having diagnostic results available before arriving at the site would allow technician's to plan their work time and have the right parts on the truck. Multiple trips to the site to work on the same problem could be avoided.
  
• FDD will prevent premature equipment failure. Compressor failure is the most expense repair on packaged rooftop units, and FDD technology can detect the faults that cause it as well as other problems.
  
• FDD technology will reduce unplanned outages. Using FDD to track repair histories as well as fault occurrences can assist building owners and mechanical contractors to make better "repair or replace" decisions.
  
• The results will provide energy benefits. Using FDD may improve rooftop air conditioner performance, providing better indoor conditions for less energy input. FDD may also allow a technician to rule out problems with a unit and look for problems elsewhere in the building. For example low flow across the cooling coil may be due to clogging of the evaporator coil caused by filter bypass, but it may also result for duct kinks and stuck dampers.
  
• Market penetration of FDD technology for packaged air-conditioning units will require adoption by air-conditioning manufacturers, air-conditioning component manufacturers, and building automation vendors. Market awareness of the value of built-in diagnostics will be needed before the manufacturers will offer FDD as part of the built-in controls of each unit, or as part of the fault reporting for a building automation system. In many cases, buildings using packaged air-conditioners do not have a building automation system. To provide a means of reporting the faults, software will be required that will provide an appropriate alarm or report to a person responsible for HVAC maintenance.