Developments in energy conversion technology and multi-carrier energy systems have led to higher flexibility of energy systems, thus improving the reliability of supply and decreasing the energy costs. Usually in previous studies on reliability evaluation of Multi-Energy Source Building (MESB), the required energy for supplying the loads is considered to be fixed during each time interval and the ability of the system to supply the loads is evaluated on the basis of this assumption. In this paper, due to the high amount of the thermal loads in a building's energy portfolio, the flexibility of these loads is addressed and an analytical method is presented to model the dynamic behavior of thermal loads in reliability analyses of MCEBs. The proposed method is based on Markov chain concepts integrating thermodynamic equations. In addition, in order to evaluate the validity of the proposed method, a Monte-Carlo simulation method is employed. The methods are simulated on test systems and the results are presented and discussed. (C) 2016 Elsevier B.V. All rights reserved.