Abstract: A combination duct/extinguishing system includes a duct and plural sprinkler head assembly and mounting systems on the duct. Each sprinkler head assembly and mounting system includes port structure mounted on the duct defining a passageway into an area internally of the duct, sprinkler head structure having one end associated with a corresponding port structure, and another end associated with an extinguishing media, and collar structure for removable engaging the port structure and the sprinkler head structure to fixedly hold the sprinkler head in the area internally of the duct. Preferably, the collar structure includes two halves hingedly joined, each having wall structure defining a groove for receiving a portion of the port structure and the sprinkler head structure.