When sizing either a vent or a chimney which will vent more than one natural gas appliance, which of the following could be used in determining the size of the common vent?

The correct choice provides a comprehensive guideline for sizing a common vent for multiple natural gas appliances. It incorporates a key principle of venting systems, which is to ensure that the common vent has sufficient capacity to handle the combined exhaust from all connected appliances.

By specifying that the area of the common vent must equal the area of either the largest draft control device outlet or the largest flue outlet, the option ensures that the vent can accommodate the flow of gases from the most demanding appliance. The addition of 50% of the sum of the outlet areas of the additional appliances recognizes that these appliances also contribute to the total venting demand. This method ensures a balance between effective venting and not oversizing, which could lead to issues such as inefficient operation or difficulty in maintaining proper draft.

The other options either simplify the sizing process too much or do not adequately account for the combined effects of the various appliances on the venting system, which could compromise performance and safety. The incorporation of a percentage of additional outlets is critical for ensuring that all appliances function efficiently and safely within the common venting system.