Boiler optimization is included in on-line control of parallel boilers by multiplying the total heat per unit time which must be supplied to all parallel boilers by the percentage of the total heat which should be supplied to each boiler by primary and secondary fuels in order to substantially maximize energy efficiency. The result of such multiplication is the heat per unit time which should be supplied to each boiler by the primary and secondary fuels. The fuel and air supplied to each boiler is controlled so as to supply the thus determined heat per unit time which not only results in maintenance of a desired header pressure but also results in substantially maximizing energy efficiency of the parallel boilers even where multiple fuels are combusted to supply heat.
