Detailed thermal analysis is usually performed exclusively at design time since it is a computationally intensive task. In this paper, we introduce a novel methodology for fast, yet accurate, thermal analysis. The introduced methodology is software supported by a new open source tool that enables hierarchical thermal analysis with adaptive levels of granularity. Experimental results prove the efficiency of our approach since it leads to average reduction of the execution overhead up to 70% with a penalty in accuracy ranging between 2% and 8%.