When sizing a common vent for more than one natural gas appliance, how should the size be determined?

Determining the size of a common vent for multiple natural gas appliances takes into account the capacity required to accommodate the combined exhaust from all appliances. The correct method involves using the largest draft control outlet in conjunction with a percentage of the additional appliances’ outlet areas.

Option C specifies that the area shall equal the largest draft control outlet plus 50% of the additional appliances' outlet areas. This method is used to ensure that the vent has adequate capacity to handle the maximum potential exhaust flow, while also accounting for the varying outputs of each appliance. By using the largest outlet as the baseline, it ensures that the vent can manage the highest single appliance's exhaust. Adding 50% of the other outlet areas provides a buffer to accommodate the cumulative exhaust from all the appliances, ensuring efficient and safe venting without risk of backdrafts or overstressing the venting system.

This approach is aligned with best practices and codes regarding vent sizing, which aim to promote safety and efficiency when multiple appliances share a common vent. Other options do not provide a suitable balance between efficiency and safety for the exhaust handling needs of multiple appliances.