We have conducted field monitoring to collect data with which to assess the effects of proper maintenance on the performance of HVAC units. For ASHRAE (Research Project 1274-TRP), we conducted a field performance assessment of commercial HVAC package equipment to quantify the benefits of proper service. This involved making detailed baseline measurements pertaining to the performance of a sample of HVAC package units, performing servicing on a subset of this sample (e.g., refrigerant charging, refrigerant pipe insulation, testing and repairs), and then making a new set of performance measurements on the serviced units. In practice, the field measurement procedures were developed over time in cooperation with the ASHRAE project advisory committee. An initial set of procedures was prepared and field tested. Based on an analysis of results from this field testing, the final set of procedures was prepared. These procedures addressed the measuring of energy use, airflows, and refrigerant temperatures and pressures for all stages of the compressors. The procedures then were applied to conduct field monitoring of 184 rooftop HVAC units located in five cities: Sacramento, Los Angeles, Baltimore, Chicago, and Houston.