How is outside combustion air typically opened for ventilation?

Outside combustion air is typically opened for ventilation by creating direct openings or utilizing vertical ducts. This method is essential for ensuring that the combustion process has access to sufficient fresh air, which is necessary for efficient and safe operation of gas appliances. Direct openings allow air to enter from the outside environment directly into the area where the gas appliance is located, promoting better combustion efficiency and reducing the risk of harmful gases accumulating indoors.

Vertical ducts also serve as pathways for ventilation air to enter the appliance, facilitating a smooth flow of air that is critical for maintaining the right air-to-gas mixture. Using vertical ducts can help in areas where horizontal space is limited or when appliances are installed in locations that require more structured air distribution.

In contrast, options that involve horizontal ducts or entrapment in flues are less effective or applicable for ensuring adequate ventilation for combustion air. Furthermore, using a combustible screen is not a safe or acceptable method for opening outside combustion air, as it does not fulfill the necessary safety and efficiency criteria. Therefore, the approach of using direct openings or vertical ducts efficiently provides the required external combustion air, supporting the safe and effective operation of gas-fueled equipment.