Specifically designed for use with Norseman Add-on Furnaces, model 1,500 or 2,500, this damper prevents chimney back drafts. It also aids in starting the stove to get the draft going faster. Installation is fast and easy. Prevents back draft from returning to the furnace plenum. Highly recommended for proper furnace installation. Spring activated for proper functioning. Easy installation. Complete assembly installs into 8 in. heat duct. Exterior foam gasket forms a tight seal within heat duct. Dimensions 8D x 7-3 4W x 3H, weight 8.