This is a replacement burner head designed for 79, 85, AFS, and AFM Series Furnaces. This burner has an enclosure for the orifice and small port holes specific to the BTU's of the furnaces it's installed on.

The different models and/or BTU ranges of furnaces use different burner heads. These burners differ by the size of the top gas port holes or the deflector that runs through the throat of the burner. If the wrong one is used, it may cause an ignition, sooting or flame sense problem.