A line diagram shows the closed-loop steam supply / firing-rate control loop: boiler steam pressure -> steam pressure transmitter/sensor -> master controller (follows steam pressure) -> fuel valve and forced-draught fan speed/damper -> furnace -> steam chest -> back to sensor forming the feedback loop; with a separate air/fuel ratio controller that measures fuel flow and air flow, and an oxygen (O2) analyser in the funnel providing a trimming/bias signal.

Operation: The object is to supply steam at the required pressure and to burn fuel efficiently with the correct air/fuel ratio. The master (steam pressure) controller senses the boiler steam pressure against set point. When the user takes steam, pressure falls; the master controller increases the firing-rate demand, which opens the fuel valve and increases the forced draught (fan speed/air damper) together, raising the heat input. When steam demand falls, pressure rises and the controller reduces firing rate. The air/fuel ratio controller ensures the correct proportion of air to fuel is supplied for the firing rate and, using the O2 analyser, trims the air so that combustion stays within the optimum band (avoiding too much excess air, which wastes heat, or too little, which causes soot and danger). A flame safeguard system supervises the flame during light-up, purge and operation.

• Low water level cut-out shuts the fuel supply if boiler water falls dangerously low.
• Flame failure/safeguard: if no flame is confirmed (UV/photo-e/e cell) within a set time, fuel is tripped off, and a purge is initiated before any re-light.
• Air failure: interlocks ensure fuel is only admitted when the forced-draught fan is running and purge is complete; loss of combustion air flow cuts fuel.
• High steam pressure cut-out and/or safety relief valves prevent over-pressure.
• Pre-purge and post-purge sequences clear any accumulated unburnt gas before and after firing; a lock-out prevents re-light after flame failure until manual reset.
• Loss of the control air/electrical supply trips the system to the safe condition; fuel valves fail closed on power/air loss.
• High/low gas or oil pressure trips and an air/fuel ratio guard prevent dangerous mixtures.

The master controller is a proportional-plus-integral (or PI/PID) controller that compares the measured steam pressure with the set point. The error (set point - measured) generates an output which positions the firing-rate demand - raising it when pressure is low and lowering it when pressure is high. This firing-rate demand simultaneously drives the fuel valve and the air supply (fan speed/damper) through a cam/programmed relationship so both follow the demanded load. The air/fuel ratio is then adjusted by a secondary trim loop: the O2 analyser in the exhaust gives a signal to the air controller, which trims (increases or decreases) the air flow with respect to fuel to maintain the air/fuel ratio in the optimum efficiency band, regardless of firing rate. Thus steam pressure sets the firing rate, and air flow is slaved to fuel flow with an oxygen trim to keep combustion correct at every load.