When determining the common vent size, which factor is critical?

The total area of all flue collars is critical when determining the common vent size because it represents the cumulative outlet size for all appliances being vented through a single vent system. The vent must be sized appropriately to ensure that all flue gases produced by the appliances can exit efficiently and without restriction.

If the vent is undersized relative to the total area of the flue collars, this can lead to inadequate venting performance, which may cause backdrafting or poor combustion efficiency. Ensuring that the total area is considered allows for proper calculations that account for the potential gas flow requirements of all appliances connected to the vent, ensuring safety and effectiveness in the venting system.

While factors such as the size of the smallest appliance, the height of the chimney, and the individual venting capacity of each appliance can play a role in overall venting efficiency, it is the cumulative area of the flue collars that most directly impacts the necessary size of the common vent to accommodate the collective output of the appliances.