On the effects of decoupling airflow and heat balance in building simulation models

Several thermal building simulators also allow coupled modeling of bulk air movements using airflow network models.However, solving the combined flow and thermal problem can be problematic, both in the context of traditional building simulators and for modern environments, where both airflow and thermal models are formulated as sets of differential-algebraic equations (DAE). For variable-time-step DAE-basedsimulators, difficult coupled problems often lead to small time steps and slow simulations. In this paper, we will investigate the differences between full coupling, where all equations aresolved simultaneously, and so-called ping-pong coupling, where airflows from the previous time step are used. The latter approach is sometimes used in traditional simulators to simplify tool integration. Here, it will be investigated in the context of a variable-time-step DAE solver. The general motivationfor DAE-based building simulation is discussed and bidirectional airflow network models in this setting are presented in some detail. Results of the decoupling experiments are presented for a set of test cases.