The developments in boiler design necessitated by the employment of high steam pressure and temperature have increased the sensitivity of the boiler unit to variations in firing rate. As a result the duties of the operator have become more back-breaking, and reliance on the skill of the individual renders the performance of the plant dependent on the human element. Recognition of the limitations imposed on plant efficiency by the intervention of the boiler operator has led to many plants being equipped with automatic devices to execute routine readjustments in fuel and air supply and feed-water flow. This proposal describes a heat-operated system of automatic boiler control, though the principles stated apply equally to systems using other types of motive power. The equipment described will be reliable in operation and apart from the improved technical performance over manual control it will also increase the overall efficiency of boiler plants to be equipped.