AAVCs are devices used in chemical waste systems to prevent the siphonage of trap seals. These devices do not relieve back pressure; they only allow air to enter the system. They are designed to be used for individual fixtures or for a horizontal branch serving multiple fixtures. When the devices are installed in a building, there shall be at least 1 open vent terminal to relieve positive pressure which extends to the atmosphere outside of the building serving the same building drain on which these devices are installed. These devices shall not be installed in an area with a constant air pressure differential greater than +/-0.3 inches (7.6 mm) WC.