The vast majority of furnaces at present in use employ the high pressure atomizing type gun burner. Proper combustion is the main prerequisite for good furnace efficiency and to achieve good combustion it is first necessary to ensure that there is good air-fuel mixing. In the high pressure atomizing type burner, mixing is accomplished by pumping the fuel oil (generally No. 2 fuel oil) to a precision atomizing nozzle through which it is sprayed into the combustion chamber. The nozzle is designed and dimensioned to deliver the fuel at a specified rate and at a given pressure which is generally about 700 kpa. The fine oil spray mixes with air that is directed towards the nozzle by a fan through an air mixing tube. The air is deflected near the nozzle, creating turbulence which is conducive to good air-oil mixing. Ignition is generally accomplished by an electric spark.