The Virginia Mechanical Code discusses natural ventilation options in Section 402. And has an image to accompany it (shown below).
@simonxi you mentioned today another requirement for residential specifically that would allow natural ventilation as an option for when there is a specific exhaust configuration? Can you confirm what code section that is in relation to? Also, for residential projects, we would use the Residential code, correct? Could you refer me to the natural ventilation requirements in that code section?
@Kenneth Shultz According to sections 403.1, 403.3.2.1 of VMC states that a mechanical exhaust air system, supply system, or a combination of both shall be used for Group R-2, R-3, and R-4 occupancies which are three stories or less in height above grade plane. Section 403.3.1.2 of VMC also states that the outdoor air introduced into space by an exhaust system shall be considered as contributing to the outdoor airflow required by that space by table 403.3.1.1. Talking about controls, the exception to section 403.3.2.1 states that the system is not required to operate continuously if the system is equipped with a control that enables it to operate for not less than 1 hour of each 4-hour period. The average outdoor airflow over a 4-hour period shall not be less than the value obtained from the following equation
Q = 0.01 * A + 7.5 * (N + 1) Q : Outdoor airflow rate, cfm A : Floor area, Sq. Ft. N : Number of bedroomsIn Virginia Residential Code, the natural ventilation requirement is mentioned in section R303.1. It states the same conditions as described in the Virginia Mechanical Code for natural ventilation. According to section M1505.4.3 of the Virginia Residential Code, the whole house ventilation system shall provide a continuous outside airflow rate obtained from the aforementioned equation or table M1505.4.3(1). An exception to this code states that the fan shall be equipped with a control that enables it to operate not less than 25% of each 4-hour segment and the ventilation rate mentioned in table M1505.4.3(1) multiplied by a factor determined from another table M1505.4.3(2). Adding both tables here for reference.