The methodology used to assess the field performance of gas-fired systems for heating multiple-unit apartment buildings is presented. The major elements of the strategy are the design of field tests, data taking, and analysis and comparison of relative performance. Three different types of gas-fired heating systems, namely highefficiency warm air systems, zone-adjustable modular systems, and integrated hydronic boilers, were installed on each floor of a three-story apartment building located in Montreal(45.4 N). The heating systems were monitored between January and April 1988. In this report, a few typical results, such as steady-state efficiencY, energy consumption characteristics, and seasonal efficiency, are given. Aside from the performance of heating systems, the result of interest to date is the length of time a heating system must be monitored to get a reasonable estimate of seasonal efficiency. Some guidelines in this regard are given.