The Aprilaire 6618 is a round ventilation damper of the following size:
The Aprilaire 6618 - 18" Automatic Ventilation Damper is a motorized damper with durable 24 gauge construction. This automatic vent damper features a position indicator for easy troubleshooting. This Aprilaire damper has a 2 wire design for easy installation.
The Aprilaire 6618 specifications: