It is known that the external design parameters are quite important for designing heating, ventilation and air conditioning systems, because they determine design discharge of the building: cold (in summer) and heat (in winter). Thus they substantially influence the capital and maintenance expenditure. Until recently in many countries the external design parameters are determined quite equally, exactly by arbitrarily given provision (probability of non-exceedance). Such way of determination of the external design parameters is the one in heating facilities and ventilating equipment that does not have any system-reasonable analytic methodology. The author of this paper, using the number of researches, suggests the appropriate method, referring to outside design temperature in the frost-free season. At a later date there will be proposed another methods for design moisture content – in summer and design temperature – in winter. Analytic dependences, that come from the study, take into account all the complex of characteristics: system, technological, climatological, hygienic, static, dynamic and also probabilistic. In particular it is proved, that rather essential parameter, that generally is not taken in account in computation, is outside air consumption in the compound with recirculated air. Therefore this method requires sufficient skills of the specialist, who must be well acquainted with related fields of knowledge, technics, appropriate terminology. The new method although requires the following discussion and probable improvement, but it allows determining the external design parameters more reasonable and less complicated. If it finds a broad response, it will be recommended for design standard of microclimate systems in different countries.