Final answer:
An additional circuit vent is required when plumbing fixture distances exceed code limits or when multiple fixtures' layout demands more venting for proper drainage system functioning.
Step-by-step explanation:
An additional circuit vent must be installed when the plumbing fixture layout is such that the distances from the vent to the fixtures exceed those allowed by local plumbing codes, or when multiple fixtures are connected in a way that requires additional venting to maintain the proper function of the drainage system. Circuit venting is a method used to protect the trap seals in the drainage system by allowing air to enter the system, preventing siphonage and back-pressure, especially when there are multiple fixtures on a floor that may affect each other's traps when they drain simultaneously.
Typically, the need for an additional circuit vent arises during large-scale installations or complex fixture layouts where a single vent cannot handle the demand, or the layout requires a specific circuit vent system due to the distances and number of fixtures involved. The circuit vent is generally installed between the last fixture and where the drainage pipe connects to the stack, which allows proper venting for all the fixtures on the circuit.