This paper is devoted to analysis of influence of buildings thermal properties on heating systems’ maximal heat consumption. Paper analyses possibilities to reduce installed heating loads in cold climate. In winter time the average outdoor air temperatures fluctuation is ±10. In some cases the outdoor temperature can rapidly drop below heating system design temperature for the short time period. In that case it is difficult to insure design indoor air temperature. In the scope of this paper the dependency between heat boiler capacity and level of thermal mass of building’s envelope was evaluated. The paper also presents approach for practical use of thermal mass in design of heating systems’ maximal capacity. The existing criteria of building thermal mass – heat inertia – could be used for evaluation of heating systems’ maximal capacity for winter time. The average temperature of coldest 5 days with probability of 0,92 cannot be used as heating design temperature for timber-frame building envelopes in Latvian climatic conditions.