They permit airflow to go in a single way and to prevent it in the opposite way. They're made from corrosion resistant extruded aluminum. Product 0540V Vast Frame Static Curtain Fireplace Damper is UL permitted for use exactly where making codes demand the defense of HVAC ductwork penetrations in vertical https://beaujzqfs.blogolize.com/a-review-of-heavy-duty-stainless-steel-damper-manufacturers-76002961