Wouldn't ya just love to see a flow chart of the algorithms used by some of the lambda controlled boilers? On-Off control of the fan is going to be a hard technique to generalize about because the 'Off' portion of such a cycle is so dependent on the natural draft of the individual chimney involved. Huge variations there. Even with any one chimney, weather and being already 'warmed up' can make big a difference. Might make a barometric draft control even more pertinent.