In wood chip boilers, CO emissions can be reduced by improving the control over the combustion process. To minimize emissions and optimize efficiency, the excess air ratio, the primary and secondary air distribution systems, and the fuel feed rates must be adjusted to maintain stable combustion. An extensive series of wood combustion tests was performed in a grate automatic 180 kWth boiler. The level of influence exerted by wood parameters, such as the origin, particle size, and moisture content, and combustion parameters, such as the excess air ratio, air distribution, and boiler load, were studied. The most important characteristic of wood with regard to CO emissions is the moisture content. The combustion tests with both types of wood chips and sawdust show that the CO emissions and excess air ratio decline when the fuel power increases. The direct relationship between the excess air ratio and the level of power facilitates the optimization that minimizes the CO emissions. The control system must be based on a fluctuating O-2 concentration in the exhaust gas by varying the excess air ratio relative to the power demand. The excess air ratio needed to implement the optimal conditions should be between 2.0 and 2.5 at 100 kWth, between 1.9 and 2.1 at 135 kWth, and between 1.7 and 1.9 at nominal power.