Disclosed herein is a method of controlling the air to fuel ratio in a burner containing a venturi assembly. The venturi includes an air inlet, a primary fuel inlet with a converging section, a throat portion downstream from the converging section, a diverging section downstream from the throat portion, an outlet, and a secondary gas inlet disposed downstream from the converging section and upstream from the outlet. The method comprises introducing fuel into the fuel inlet, receiving air through the air inlet by inspiration, and feeding a gas through the secondary gas inlet, the flow rate and content of the gas fed through the secondary gas inlet being selected to result in a desired air to fuel ratio through the outlet. A method of firing a heater, a burner, a furnace and firing control systems also are disclosed.