Adding a vent to a chimney fireplace
The ventilation system of a chimney is its flue. In most cases, the flue provides enough ventilation for a masonry fueled with wood or for zero-clearance fireplaces, but installing a direct-vent or natural-vent gas insert will never do you any harm.
Natural and direct ventilation systems that act through the fireplace are effective because they vent horizontally.rclma But when placing a new fireplace into an old firebox, the vent can be done through the flue. An horizontal vent is more efficient than a vertical one. You can also think about avoiding the masonry and building the vent directly through the wall.