Therefore is the combustion air split into two streams, one passes through the grate for fixed carbon combustion, the other stream is injected above the grate level to burn the volatiles. A big task to solve by a control algorithm is balancing the correct distribution of the primary and secondary air streams; however, these types of boilers do not deal with this issue. The most modern boilers are equipped with manual possibility to set the primary/secondary air ratio, but this is not actively controlled, even if it would be a marginal cost in relation to the overall price of the boiler