Premix furnace for heating an occupied space while producing lower NOx emissions and methods of mixing air and fuel delivered to a premix burner and of improving combustion stability. A mixing device may be located within an inlet tube, may have a flat surface that is perpendicular to the direction of fuel flow, or may have two surfaces held at substantially opposite angles to induce swirl. A mixing device may be attached to the fuel injector, may be made from a piece of sheet metal, and may have bends and a hole for attachment to the fuel injector. A fluidic diode in the inlet tube may improve combustion stability and may include a hollow frustum or a frustoconical portion, a cylinder concentric with the inlet tube, or a combination thereof. Some embodiments include refractory insulation lining the combustion chamber or may adjust for elevation or fuel characteristics.