elements as partially shown in fig2 providing heat input to the boiler 4 coils through adjustment of fuel and air as a function of the feedwater mass flow control elements for controlling fuel and air delivery to the boiler combustion system are actuated by the servo 6 a mechanical force booster maintenance of the precise balance of boiler firing through fuel and air control in proportion to the feedwater flow to the boiler coils establishes boiler operation for a wide range of steam temperature pressure and flow rate as shown above the precise adjustment provided by this invention prevents overfiring and subsequent 34 drying out 34 of the boiler coils resulting in substantially increased coil life and greatly enhanced boiler operation an alternate embodiment reference fig5 and 7 provides similar adjustment of the fuel air ratio as a function of feedwater mass flow through rotary displacement of the input shaft of servo mechanism 6 reference fig2 and 5 since many elements of this alternate embodiment are similar to those of the preferred embodiment similar numbers utilizing the prefix 1 are used in the fig6 and 7 whenever new elements are introduced the alternate embodiment controller assembly 102 comprises a body 114 having upper and lower chambers 113 and 111 respectively in fluid communication through an internal port 129 and flow control ports 117 piston 124 is contained internal the upper chamber 13 for reciprocal motion therein centrally located on the piston 124 is a flow control assembly 120 ref fig6 and an actuating yoke 122 for operating.