Product 1290F is a cheap true spherical fire damper intended and competent for point-of-origin hearth containment where round ductwork passes as a result of metal stud drywall partitions or masonry walls which have a hearth resistance rating of as many as two hours and constructing codes need a fireplace damper. https://dampers-for-industrial-se79001.targetblogs.com/37468464/heavy-duty-stainless-steel-damper-manufacturers-for-dummies