Hourly fire resistance rating. The hourly rating comes from the UL555 Fire Endurance Test and indicates how long a damper will block a fire. There are two ratings for fire dampers in the United States: 11/2-hr and 3-hr. Any fire-resistant partition with less than a 3-hr rating requires a 11/2-hr rated fire damper and any fire-resistant partition with a 3-hr or more rating requires a 3-hr rated fire damper. These requirements are based on recommendations made by the National Fire Protection Association (NFPA).
Dynamic rating for airflow and pressure. Fire dampers have two distinct rating classifications: dynamic and static. Static fire dampers have not been tested for closure under airflow, and therefore, they can only be applied in HVAC systems that are designed to shut down in the event of a fire. Dynamic fire dampers, on the other hand, have been tested for closure under airflow and differential pressure. Based on the new 2002 UL test methods, the minimum rating for all dynamic fire dampers is 2,000 fpm velocity and a pressure rating of 4 in. w.g. Higher ratings are established in increments of 1,000 fpm and in 2 in. w.g. for elevated HVAC duct requirements.