Control method and apparatus for optimizing the operation of a natural draft combustion zone through which a conduit containing a process fluid to be heated passes, by decreasing the supply of combustion air until one or more of the following predetermined limiting conditions is reached: maximum CO in the flue gas, minimum O2 in flue gas, minimum draft in the combustion zone, maximum temperature of the outer surface of said conduit, and increase in the rate of fuel addition above a minimum amount. When a limiting condition is reached, the supply of combustion air is increased until the limiting condition is no longer present, and the cycle then repeated.
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