The paper describes simulated load calculation made in an attempt to establish appropriate outdoor conditions for use in heating design by the use of the periodic steady state method. Conditions for Model-based Computation The models were prepared assuming ordinary office rooms in buildings located in Tokyo. Models were made to represent rooms facing north, east, south, west and all the four directions. Using these models, computations were made for nine seasons from December 1960 to March 1969. Determination of Load Sequence At the outset, load computation was made. Based on the computed loads, sequential numbers were given to all days in the descending order of volume of daily max. loads. The load on the 27th day from the top of the list (corresponding to the last day in the top 2.5% group of days) was adopted as "Design Heating Load." Assumption of Design Outdoor Conditions Daily heat loads on days in top group were summed up and averaged and the number of days involved were counted repetitively in order to see how many days from the top of the list must be considered if their average heat loads are to be equated to "Design Heating Load." According to "Climatic Pattern Classification," all the selected days were sorted into "Fine," "Cloudy" and other patterns. Thereafter, the weather data were processed to obtain every hourly and pattern-sorted average data. From these average data, values applicable to a period during 10:00 AM on the previous day through 9:00 AM on the given day were selected, combined and assumed as design outdoor conditions. In both fine-day and cloudy-day patterns, those values were found to exemplify inherent features of typical fine and cloudy days. In each pattern, the temperatures and humidities thus obtained turned out larger than the TAC-established values. Computation for Design Next, max. loads were computed by the periodic steady state calculation method using the aforesaid design outdoor conditions and the computed data was compared with the said design heating loads. Table 12 shows comparison of those two types of loads. Also shown for reference purpose are values computed using TAC 2.5% (in terms of both temperatures and humidities) in which solar radiation is neglected. Obviously, different outdoor conditions must be applied to rooms having different orientations. Use of TAC-established values which are on the safe side in every respect is likely to result in over-estimated design loads. In order to establish design outdoor conditions which are more appropriate for the building conditions, efforts must be made to broaden the scope of studies to cover a greater range of building requirements.