Here are some pics of my modified garn, o2 controlled. I abandoned the combustion air inlet, fabricated primary and secondary inlets that terminate in the same locations in the firebox. On the other end they are ducted to 4 inch primary and secondary air dampers with belimo modulating actuators. to start from the beginning, the o2 sensor relays feedback to the fluegas analyser converting a signal input to the cybocon ce model free adaptive controller, that calculates and sends a output signal to the belimo actuators/dampers that creates new input for the o2 sensor to transmit. this happens once per second. The desired result is to maintain a setpoint of 7%o2 residual in the fluestream. This is basically how most all o2 controlled boilers operate, just not done with a garn before. It took alot of trial and error, and hundreds of burning hours programming the controller to get robust predictable results. I ran out of picture space, but will post graphs of o2 trending of a stock burn and a o2 controlled burn. I have other mods as well, a tank mix loop that homoginizes the water temp as long as the inducer is on piped with a dirtcal. The garn supply water temp is mixed with a modulating 3 way valve behind the boiler that is next to the flatplate hx in the garn barn that i built this spring by myself.