Abstract: The fuel supply to a boiler (11) is controlled by a fuel valve (17) driven by a motor (18) in response to measured steam pressure at (12) in relation to a demanded pressure. The motor (18) adjusts at the same time a damper (36) for supplying combustion air at (13). Air supply at (13) can also be controlled in response to the pressure drop across a register (38) as measured by airlines (51). The invention provides an over-riding control in response to too much or too little oxygen in the exhaust gases in the stack (15). The oxygen level is measured by a detector (26), and if it is outside limits set at (29) a motor (41) operates a valve (42) connected between the airlines (51). A timer with a contact T1 ensures that the control in response to oxygen level is only operative for a small part of each of a number of repeated 10 second cycles.

Abstract: This invention is a control for a burner enabling it to operate efficiently with very little excess air but providing a main control for the fuel and for the combustion air in response to demand, and an auxiliary control in response to the amount of free oxygen in the products of combustion. The latter quantity can be measured by a zirconia cell, and there are means for controlling its temperature accurately so that the reading is reliable. The controls in response to demand and excess air respectively are applied alternately in a cycle with a period of no control to enable the effect of an adjustment to be established before further control is applied.