A stiff, variable time step transport solver for CONTAM

We describe the implementation of a new transport solver for CONTAM, a whole-building airflow and contaminant transport model developed by the National Institute of Standards and Technology. Based on CVODE, a general-purpose code for ordinary differential equations, the new solver features variable time steps, high-order integration methods, and automatic error control. These techniques can make CONTAM more accurate when simulating fast transport mechanisms such as high air change rates, sorption, and chemical reactions. We present the relevant theory, then describe the modeling decisions needed to integrate CVODE into CONTAM. Testing with two realistic building models shows that CVODE can run faster than the legacy solvers.