Infiltration and natural ventilation model for whole building energy simulation of residential buildings

The infiltration term in the building energy balance equation is one of the least understood and most difficult to model. For many residential buildings, which have an energy performance dominated by the envelope, it can be one of the most important terms. There are numerous airflow models;
however, these are not combined with whole building energy simulation programs that are in common use in North America. This paper describes a simple multizone nodal airflow model
integrated with the SUNREL whole building energy simulation program. The required inputs for infiltration are taken from blower door test results and the geometry of the openings for
natural ventilation. The flow exponents and coefficients for infiltration and natural ventilation can be input or left to the default values. Control of the natural ventilation openings can be controlled with a time schedule and the indoor/outdoor temperature difference.
The mass flow rate equations are written in terms of the pressure at the base of each zone. The pressure on each surface is a combination of stack and wind effects added to the zone
base pressure. The resulting set of mass balance equations are solved using a Newton-Raphson iterative method with a variable relaxation coefficient. The relaxation coefficient is adjusted with each iteration depending on the speed of the convergence. The iterations are stopped when the mass balance in each zone converges to a specified tolerance. The model exhibits good numerical behavior, with no singularities and only a few instances of nonconvergence in some building
simulations with no thermal mass and large leakage areas. The infiltration model compares favorably to the LBL infiltration model. The natural ventilation model also compares well with another natural ventilation model and with measured results. The current infiltration model and whole building simulation program form the basis of a new residential energy-auditing tool.