Fired heaters secondary combustion

If you try to operate a furnace, fired heater, or boiler with too little combustion air to starve the burners of oxygen to smother or bog down the firebox, then you will likely cause afterburn or secondary combustion in the stack, you will not be able to operate on automatic temperature control, and may even destroy the equipment altogether. 

The air supply for a gas-fired, natural-draft heater consists of two portions primary air and secondary air. (In some newer burners, tertiary air is used to control nitrogen oxide emissions.) The primary air is educted or sucked into the burner through a venturi by the rapidly flowing fuel gas. The air is well mixed with the gas prior to combustion. Hence the name premix burner. A Bunsen burner is an example of a premix burner. 

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